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float to/from bits and classify: update comments regarding non-conformant hardware

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This commit is contained in:
Ralf Jung 2024-08-03 11:51:16 +02:00
parent 355a307a87
commit 368a4c6808
7 changed files with 107 additions and 683 deletions
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110
library/core/src/num/f128.rs
View File

@ -290,7 +290,7 @@ impl f128 {
#[inline]
#[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
pub(crate) const fn abs_private(self) -> f128 {
// SAFETY: This transmutation is fine. Probably. For the reasons std is using it.
// SAFETY: This transmutation is fine just like in `to_bits`/`from_bits`.
unsafe {
mem::transmute::<u128, f128>(mem::transmute::<f128, u128>(self) & !Self::SIGN_MASK)
}
@ -439,22 +439,12 @@ impl f128 {
#[unstable(feature = "f128", issue = "116909")]
#[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
pub const fn classify(self) -> FpCategory {
// Other float types cannot use a bitwise classify because they may suffer a variety
// of errors if the backend chooses to cast to different float types (x87). `f128` cannot
// fit into any other float types so this is not a concern, and we rely on bit patterns.
// Other float types suffer from various platform bugs that violate the usual IEEE semantics
// and also make bitwise classification not always work reliably. However, `f128` cannot fit
// into any other float types so this is not a concern, and we can rely on bit patterns.
// SAFETY: POD bitcast, same as in `to_bits`.
let bits = unsafe { mem::transmute::<f128, u128>(self) };
Self::classify_bits(bits)
}
/// This operates on bits, and only bits, so it can ignore concerns about weird FPUs.
/// FIXME(jubilee): In a just world, this would be the entire impl for classify,
/// plus a transmute. We do not live in a just world, but we can make it more so.
#[inline]
#[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
const fn classify_bits(b: u128) -> FpCategory {
match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
let bits = self.to_bits();
match (bits & Self::MAN_MASK, bits & Self::EXP_MASK) {
(0, Self::EXP_MASK) => FpCategory::Infinite,
(_, Self::EXP_MASK) => FpCategory::Nan,
(0, 0) => FpCategory::Zero,
@ -922,48 +912,7 @@ impl f128 {
#[must_use = "this returns the result of the operation, without modifying the original"]
pub const fn to_bits(self) -> u128 {
// SAFETY: `u128` is a plain old datatype so we can always transmute to it.
// ...sorta.
//
// It turns out that at runtime, it is possible for a floating point number
// to be subject to a floating point mode that alters nonzero subnormal numbers
// to zero on reads and writes, aka "denormals are zero" and "flush to zero".
//
// And, of course evaluating to a NaN value is fairly nondeterministic.
// More precisely: when NaN should be returned is knowable, but which NaN?
// So far that's defined by a combination of LLVM and the CPU, not Rust.
// This function, however, allows observing the bitstring of a NaN,
// thus introspection on CTFE.
//
// In order to preserve, at least for the moment, const-to-runtime equivalence,
// we reject any of these possible situations from happening.
#[inline]
#[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
const fn ct_f128_to_u128(ct: f128) -> u128 {
// FIXME(f16_f128): we should use `.classify()` like `f32` and `f64`, but that
// is not available on all platforms (needs `netf2` and `unordtf2`). So classify
// the bits instead.
// SAFETY: this is a POD transmutation
let bits = unsafe { mem::transmute::<f128, u128>(ct) };
match f128::classify_bits(bits) {
FpCategory::Nan => {
panic!("const-eval error: cannot use f128::to_bits on a NaN")
}
FpCategory::Subnormal => {
panic!("const-eval error: cannot use f128::to_bits on a subnormal number")
}
FpCategory::Infinite | FpCategory::Normal | FpCategory::Zero => bits,
}
}
#[inline(always)] // See https://github.com/rust-lang/compiler-builtins/issues/491
fn rt_f128_to_u128(x: f128) -> u128 {
// SAFETY: `u128` is a plain old datatype so we can always... uh...
// ...look, just pretend you forgot what you just read.
// Stability concerns.
unsafe { mem::transmute(x) }
}
intrinsics::const_eval_select((self,), ct_f128_to_u128, rt_f128_to_u128)
unsafe { mem::transmute(self) }
}
/// Raw transmutation from `u128`.
@ -1011,49 +960,8 @@ impl f128 {
#[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
pub const fn from_bits(v: u128) -> Self {
// It turns out the safety issues with sNaN were overblown! Hooray!
// SAFETY: `u128` is a plain old datatype so we can always transmute from it
// ...sorta.
//
// It turns out that at runtime, it is possible for a floating point number
// to be subject to floating point modes that alter nonzero subnormal numbers
// to zero on reads and writes, aka "denormals are zero" and "flush to zero".
// This is not a problem usually, but at least one tier2 platform for Rust
// actually exhibits this behavior by default: thumbv7neon
// aka "the Neon FPU in AArch32 state"
//
// And, of course evaluating to a NaN value is fairly nondeterministic.
// More precisely: when NaN should be returned is knowable, but which NaN?
// So far that's defined by a combination of LLVM and the CPU, not Rust.
// This function, however, allows observing the bitstring of a NaN,
// thus introspection on CTFE.
//
// In order to preserve, at least for the moment, const-to-runtime equivalence,
// reject any of these possible situations from happening.
#[inline]
#[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
const fn ct_u128_to_f128(ct: u128) -> f128 {
match f128::classify_bits(ct) {
FpCategory::Subnormal => {
panic!("const-eval error: cannot use f128::from_bits on a subnormal number")
}
FpCategory::Nan => {
panic!("const-eval error: cannot use f128::from_bits on NaN")
}
FpCategory::Infinite | FpCategory::Normal | FpCategory::Zero => {
// SAFETY: It's not a frumious number
unsafe { mem::transmute::<u128, f128>(ct) }
}
}
}
#[inline(always)] // See https://github.com/rust-lang/compiler-builtins/issues/491
fn rt_u128_to_f128(x: u128) -> f128 {
// SAFETY: `u128` is a plain old datatype so we can always... uh...
// ...look, just pretend you forgot what you just read.
// Stability concerns.
unsafe { mem::transmute(x) }
}
intrinsics::const_eval_select((v,), ct_u128_to_f128, rt_u128_to_f128)
// SAFETY: `u128` is a plain old datatype so we can always transmute from it.
unsafe { mem::transmute(v) }
}
/// Returns the memory representation of this floating point number as a byte array in

163
library/core/src/num/f16.rs
View File

@ -284,7 +284,7 @@ impl f16 {
#[inline]
#[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
pub(crate) const fn abs_private(self) -> f16 {
// SAFETY: This transmutation is fine. Probably. For the reasons std is using it.
// SAFETY: This transmutation is fine just like in `to_bits`/`from_bits`.
unsafe { mem::transmute::<u16, f16>(mem::transmute::<f16, u16>(self) & !Self::SIGN_MASK) }
}
@ -426,15 +426,15 @@ impl f16 {
pub const fn classify(self) -> FpCategory {
// A previous implementation for f32/f64 tried to only use bitmask-based checks,
// using `to_bits` to transmute the float to its bit repr and match on that.
// Unfortunately, floating point numbers can be much worse than that.
// This also needs to not result in recursive evaluations of `to_bits`.
// If we only cared about being "technically" correct, that's an entirely legit
// implementation.
//
// Platforms without native support generally convert to `f32` to perform operations,
// and most of these platforms correctly round back to `f16` after each operation.
// However, some platforms have bugs where they keep the excess `f32` precision (e.g.
// WASM, see llvm/llvm-project#96437). This implementation makes a best-effort attempt
// to account for that excess precision.
// Unfortunately, there are platforms out there that do not correctly implement the IEEE
// float semantics Rust relies on: some hardware flushes denormals to zero, and some
// platforms convert to `f32` to perform operations without properly rounding back (e.g.
// WASM, see llvm/llvm-project#96437). These are platforms bugs, and Rust will misbehave on
// such platforms, but we can at least try to make things seem as sane as possible by being
// careful here.
if self.is_infinite() {
// Thus, a value may compare unequal to infinity, despite having a "full" exponent mask.
FpCategory::Infinite
@ -446,49 +446,20 @@ impl f16 {
// as correctness requires avoiding equality tests that may be Subnormal == -0.0
// because it may be wrong under "denormals are zero" and "flush to zero" modes.
// Most of std's targets don't use those, but they are used for thumbv7neon.
// So, this does use bitpattern matching for the rest.
// SAFETY: f16 to u16 is fine. Usually.
// If classify has gotten this far, the value is definitely in one of these categories.
unsafe { f16::partial_classify(self) }
}
}
/// This doesn't actually return a right answer for NaN on purpose,
/// seeing as how it cannot correctly discern between a floating point NaN,
/// and some normal floating point numbers truncated from an x87 FPU.
///
/// # Safety
///
/// This requires making sure you call this function for values it answers correctly on,
/// otherwise it returns a wrong answer. This is not important for memory safety per se,
/// but getting floats correct is important for not accidentally leaking const eval
/// runtime-deviating logic which may or may not be acceptable.
#[inline]
#[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
const unsafe fn partial_classify(self) -> FpCategory {
// SAFETY: The caller is not asking questions for which this will tell lies.
let b = unsafe { mem::transmute::<f16, u16>(self) };
match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
(0, Self::EXP_MASK) => FpCategory::Infinite,
(0, 0) => FpCategory::Zero,
(_, 0) => FpCategory::Subnormal,
_ => FpCategory::Normal,
}
}
/// This operates on bits, and only bits, so it can ignore concerns about weird FPUs.
/// FIXME(jubilee): In a just world, this would be the entire impl for classify,
/// plus a transmute. We do not live in a just world, but we can make it more so.
#[inline]
#[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
const fn classify_bits(b: u16) -> FpCategory {
match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
(0, Self::EXP_MASK) => FpCategory::Infinite,
(_, Self::EXP_MASK) => FpCategory::Nan,
(0, 0) => FpCategory::Zero,
(_, 0) => FpCategory::Subnormal,
_ => FpCategory::Normal,
// So, this does use bitpattern matching for the rest. On x87, due to the incorrect
// float codegen on this hardware, this doesn't actually return a right answer for NaN
// because it cannot correctly discern between a floating point NaN, and some normal
// floating point numbers truncated from an x87 FPU -- but we took care of NaN above, so
// we are fine.
// FIXME(jubilee): This probably could at least answer things correctly for Infinity,
// like the f64 version does, but I need to run more checks on how things go on x86.
// I fear losing mantissa data that would have answered that differently.
let b = self.to_bits();
match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
(0, 0) => FpCategory::Zero,
(_, 0) => FpCategory::Subnormal,
_ => FpCategory::Normal,
}
}
}
@ -952,48 +923,7 @@ impl f16 {
#[must_use = "this returns the result of the operation, without modifying the original"]
pub const fn to_bits(self) -> u16 {
// SAFETY: `u16` is a plain old datatype so we can always transmute to it.
// ...sorta.
//
// It turns out that at runtime, it is possible for a floating point number
// to be subject to a floating point mode that alters nonzero subnormal numbers
// to zero on reads and writes, aka "denormals are zero" and "flush to zero".
//
// And, of course evaluating to a NaN value is fairly nondeterministic.
// More precisely: when NaN should be returned is knowable, but which NaN?
// So far that's defined by a combination of LLVM and the CPU, not Rust.
// This function, however, allows observing the bitstring of a NaN,
// thus introspection on CTFE.
//
// In order to preserve, at least for the moment, const-to-runtime equivalence,
// we reject any of these possible situations from happening.
#[inline]
#[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
const fn ct_f16_to_u16(ct: f16) -> u16 {
// FIXME(f16_f128): we should use `.classify()` like `f32` and `f64`, but we don't yet
// want to rely on that on all platforms because it is nondeterministic (e.g. x86 has
// convention discrepancies calling intrinsics). So just classify the bits instead.
// SAFETY: this is a POD transmutation
let bits = unsafe { mem::transmute::<f16, u16>(ct) };
match f16::classify_bits(bits) {
FpCategory::Nan => {
panic!("const-eval error: cannot use f16::to_bits on a NaN")
}
FpCategory::Subnormal => {
panic!("const-eval error: cannot use f16::to_bits on a subnormal number")
}
FpCategory::Infinite | FpCategory::Normal | FpCategory::Zero => bits,
}
}
#[inline(always)] // See https://github.com/rust-lang/compiler-builtins/issues/491
fn rt_f16_to_u16(x: f16) -> u16 {
// SAFETY: `u16` is a plain old datatype so we can always... uh...
// ...look, just pretend you forgot what you just read.
// Stability concerns.
unsafe { mem::transmute(x) }
}
intrinsics::const_eval_select((self,), ct_f16_to_u16, rt_f16_to_u16)
unsafe { mem::transmute(self) }
}
/// Raw transmutation from `u16`.
@ -1040,49 +970,8 @@ impl f16 {
#[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
pub const fn from_bits(v: u16) -> Self {
// It turns out the safety issues with sNaN were overblown! Hooray!
// SAFETY: `u16` is a plain old datatype so we can always transmute from it
// ...sorta.
//
// It turns out that at runtime, it is possible for a floating point number
// to be subject to floating point modes that alter nonzero subnormal numbers
// to zero on reads and writes, aka "denormals are zero" and "flush to zero".
// This is not a problem usually, but at least one tier2 platform for Rust
// actually exhibits this behavior by default: thumbv7neon
// aka "the Neon FPU in AArch32 state"
//
// And, of course evaluating to a NaN value is fairly nondeterministic.
// More precisely: when NaN should be returned is knowable, but which NaN?
// So far that's defined by a combination of LLVM and the CPU, not Rust.
// This function, however, allows observing the bitstring of a NaN,
// thus introspection on CTFE.
//
// In order to preserve, at least for the moment, const-to-runtime equivalence,
// reject any of these possible situations from happening.
#[inline]
#[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
const fn ct_u16_to_f16(ct: u16) -> f16 {
match f16::classify_bits(ct) {
FpCategory::Subnormal => {
panic!("const-eval error: cannot use f16::from_bits on a subnormal number")
}
FpCategory::Nan => {
panic!("const-eval error: cannot use f16::from_bits on NaN")
}
FpCategory::Infinite | FpCategory::Normal | FpCategory::Zero => {
// SAFETY: It's not a frumious number
unsafe { mem::transmute::<u16, f16>(ct) }
}
}
}
#[inline(always)] // See https://github.com/rust-lang/compiler-builtins/issues/491
fn rt_u16_to_f16(x: u16) -> f16 {
// SAFETY: `u16` is a plain old datatype so we can always... uh...
// ...look, just pretend you forgot what you just read.
// Stability concerns.
unsafe { mem::transmute(x) }
}
intrinsics::const_eval_select((v,), ct_u16_to_f16, rt_u16_to_f16)
// SAFETY: `u16` is a plain old datatype so we can always transmute from it.
unsafe { mem::transmute(v) }
}
/// Returns the memory representation of this floating point number as a byte array in

169
library/core/src/num/f32.rs
View File

@ -529,7 +529,7 @@ impl f32 {
#[inline]
#[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
pub(crate) const fn abs_private(self) -> f32 {
// SAFETY: This transmutation is fine. Probably. For the reasons std is using it.
// SAFETY: This transmutation is fine just like in `to_bits`/`from_bits`.
unsafe { mem::transmute::<u32, f32>(mem::transmute::<f32, u32>(self) & !Self::SIGN_MASK) }
}
@ -654,18 +654,20 @@ impl f32 {
pub const fn classify(self) -> FpCategory {
// A previous implementation tried to only use bitmask-based checks,
// using f32::to_bits to transmute the float to its bit repr and match on that.
// Unfortunately, floating point numbers can be much worse than that.
// This also needs to not result in recursive evaluations of f64::to_bits.
// If we only cared about being "technically" correct, that's an entirely legit
// implementation.
//
// Unfortunately, there is hardware out there that does not correctly implement the IEEE
// float semantics Rust relies on: x87 uses a too-large mantissa and exponent, and some
// hardware flushes subnormals to zero. These are platforms bugs, and Rust will misbehave on
// such hardware, but we can at least try to make things seem as sane as possible by being
// careful here.
//
// On some processors, in some cases, LLVM will "helpfully" lower floating point ops,
// in spite of a request for them using f32 and f64, to things like x87 operations.
// These have an f64's mantissa, but can have a larger than normal exponent.
// FIXME(jubilee): Using x87 operations is never necessary in order to function
// on x86 processors for Rust-to-Rust calls, so this issue should not happen.
// Code generation should be adjusted to use non-C calling conventions, avoiding this.
//
if self.is_infinite() {
// Thus, a value may compare unequal to infinity, despite having a "full" exponent mask.
// A value may compare unequal to infinity, despite having a "full" exponent mask.
FpCategory::Infinite
} else if self.is_nan() {
// And it may not be NaN, as it can simply be an "overextended" finite value.
@ -675,48 +677,20 @@ impl f32 {
// as correctness requires avoiding equality tests that may be Subnormal == -0.0
// because it may be wrong under "denormals are zero" and "flush to zero" modes.
// Most of std's targets don't use those, but they are used for thumbv7neon.
// So, this does use bitpattern matching for the rest.
// SAFETY: f32 to u32 is fine. Usually.
// If classify has gotten this far, the value is definitely in one of these categories.
unsafe { f32::partial_classify(self) }
}
}
// This doesn't actually return a right answer for NaN on purpose,
// seeing as how it cannot correctly discern between a floating point NaN,
// and some normal floating point numbers truncated from an x87 FPU.
// FIXME(jubilee): This probably could at least answer things correctly for Infinity,
// like the f64 version does, but I need to run more checks on how things go on x86.
// I fear losing mantissa data that would have answered that differently.
//
// # Safety
// This requires making sure you call this function for values it answers correctly on,
// otherwise it returns a wrong answer. This is not important for memory safety per se,
// but getting floats correct is important for not accidentally leaking const eval
// runtime-deviating logic which may or may not be acceptable.
#[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
const unsafe fn partial_classify(self) -> FpCategory {
// SAFETY: The caller is not asking questions for which this will tell lies.
let b = unsafe { mem::transmute::<f32, u32>(self) };
match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
(0, 0) => FpCategory::Zero,
(_, 0) => FpCategory::Subnormal,
_ => FpCategory::Normal,
}
}
// This operates on bits, and only bits, so it can ignore concerns about weird FPUs.
// FIXME(jubilee): In a just world, this would be the entire impl for classify,
// plus a transmute. We do not live in a just world, but we can make it more so.
#[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
const fn classify_bits(b: u32) -> FpCategory {
match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
(0, Self::EXP_MASK) => FpCategory::Infinite,
(_, Self::EXP_MASK) => FpCategory::Nan,
(0, 0) => FpCategory::Zero,
(_, 0) => FpCategory::Subnormal,
_ => FpCategory::Normal,
// So, this does use bitpattern matching for the rest. On x87, due to the incorrect
// float codegen on this hardware, this doesn't actually return a right answer for NaN
// because it cannot correctly discern between a floating point NaN, and some normal
// floating point numbers truncated from an x87 FPU -- but we took care of NaN above, so
// we are fine.
// FIXME(jubilee): This probably could at least answer things correctly for Infinity,
// like the f64 version does, but I need to run more checks on how things go on x86.
// I fear losing mantissa data that would have answered that differently.
let b = self.to_bits();
match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
(0, 0) => FpCategory::Zero,
(_, 0) => FpCategory::Subnormal,
_ => FpCategory::Normal,
}
}
}
@ -1143,51 +1117,7 @@ impl f32 {
#[inline]
pub const fn to_bits(self) -> u32 {
// SAFETY: `u32` is a plain old datatype so we can always transmute to it.
// ...sorta.
//
// It turns out that at runtime, it is possible for a floating point number
// to be subject to a floating point mode that alters nonzero subnormal numbers
// to zero on reads and writes, aka "denormals are zero" and "flush to zero".
// This is not a problem per se, but at least one tier2 platform for Rust
// actually exhibits this behavior by default.
//
// In addition, on x86 targets with SSE or SSE2 disabled and the x87 FPU enabled,
// i.e. not soft-float, the way Rust does parameter passing can actually alter
// a number that is "not infinity" to have the same exponent as infinity,
// in a slightly unpredictable manner.
//
// And, of course evaluating to a NaN value is fairly nondeterministic.
// More precisely: when NaN should be returned is knowable, but which NaN?
// So far that's defined by a combination of LLVM and the CPU, not Rust.
// This function, however, allows observing the bitstring of a NaN,
// thus introspection on CTFE.
//
// In order to preserve, at least for the moment, const-to-runtime equivalence,
// we reject any of these possible situations from happening.
#[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
const fn ct_f32_to_u32(ct: f32) -> u32 {
match ct.classify() {
FpCategory::Nan => {
panic!("const-eval error: cannot use f32::to_bits on a NaN")
}
FpCategory::Subnormal => {
panic!("const-eval error: cannot use f32::to_bits on a subnormal number")
}
FpCategory::Infinite | FpCategory::Normal | FpCategory::Zero => {
// SAFETY: We have a normal floating point number. Now we transmute, i.e. do a bitcopy.
unsafe { mem::transmute::<f32, u32>(ct) }
}
}
}
#[inline(always)] // See https://github.com/rust-lang/compiler-builtins/issues/491
fn rt_f32_to_u32(x: f32) -> u32 {
// SAFETY: `u32` is a plain old datatype so we can always... uh...
// ...look, just pretend you forgot what you just read.
// Stability concerns.
unsafe { mem::transmute(x) }
}
intrinsics::const_eval_select((self,), ct_f32_to_u32, rt_f32_to_u32)
unsafe { mem::transmute(self) }
}
/// Raw transmutation from `u32`.
@ -1232,53 +1162,8 @@ impl f32 {
#[inline]
pub const fn from_bits(v: u32) -> Self {
// It turns out the safety issues with sNaN were overblown! Hooray!
// SAFETY: `u32` is a plain old datatype so we can always transmute from it
// ...sorta.
//
// It turns out that at runtime, it is possible for a floating point number
// to be subject to floating point modes that alter nonzero subnormal numbers
// to zero on reads and writes, aka "denormals are zero" and "flush to zero".
// This is not a problem usually, but at least one tier2 platform for Rust
// actually exhibits this behavior by default: thumbv7neon
// aka "the Neon FPU in AArch32 state"
//
// In addition, on x86 targets with SSE or SSE2 disabled and the x87 FPU enabled,
// i.e. not soft-float, the way Rust does parameter passing can actually alter
// a number that is "not infinity" to have the same exponent as infinity,
// in a slightly unpredictable manner.
//
// And, of course evaluating to a NaN value is fairly nondeterministic.
// More precisely: when NaN should be returned is knowable, but which NaN?
// So far that's defined by a combination of LLVM and the CPU, not Rust.
// This function, however, allows observing the bitstring of a NaN,
// thus introspection on CTFE.
//
// In order to preserve, at least for the moment, const-to-runtime equivalence,
// reject any of these possible situations from happening.
#[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
const fn ct_u32_to_f32(ct: u32) -> f32 {
match f32::classify_bits(ct) {
FpCategory::Subnormal => {
panic!("const-eval error: cannot use f32::from_bits on a subnormal number")
}
FpCategory::Nan => {
panic!("const-eval error: cannot use f32::from_bits on NaN")
}
FpCategory::Infinite | FpCategory::Normal | FpCategory::Zero => {
// SAFETY: It's not a frumious number
unsafe { mem::transmute::<u32, f32>(ct) }
}
}
}
#[inline(always)] // See https://github.com/rust-lang/compiler-builtins/issues/491
fn rt_u32_to_f32(x: u32) -> f32 {
// SAFETY: `u32` is a plain old datatype so we can always... uh...
// ...look, just pretend you forgot what you just read.
// Stability concerns.
unsafe { mem::transmute(x) }
}
intrinsics::const_eval_select((v,), ct_u32_to_f32, rt_u32_to_f32)
// SAFETY: `u32` is a plain old datatype so we can always transmute from it.
unsafe { mem::transmute(v) }
}
/// Returns the memory representation of this floating point number as a byte array in

143
library/core/src/num/f64.rs
View File

@ -528,7 +528,7 @@ impl f64 {
#[inline]
#[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
pub(crate) const fn abs_private(self) -> f64 {
// SAFETY: This transmutation is fine. Probably. For the reasons std is using it.
// SAFETY: This transmutation is fine just like in `to_bits`/`from_bits`.
unsafe { mem::transmute::<u64, f64>(mem::transmute::<f64, u64>(self) & !Self::SIGN_MASK) }
}
@ -653,12 +653,14 @@ impl f64 {
pub const fn classify(self) -> FpCategory {
// A previous implementation tried to only use bitmask-based checks,
// using f64::to_bits to transmute the float to its bit repr and match on that.
// Unfortunately, floating point numbers can be much worse than that.
// This also needs to not result in recursive evaluations of f64::to_bits.
// If we only cared about being "technically" correct, that's an entirely legit
// implementation.
//
// Unfortunately, there is hardware out there that does not correctly implement the IEEE
// float semantics Rust relies on: x87 uses a too-large exponent, and some hardware flushes
// subnormals to zero. These are platforms bugs, and Rust will misbehave on such hardware,
// but we can at least try to make things seem as sane as possible by being careful here.
//
// On some processors, in some cases, LLVM will "helpfully" lower floating point ops,
// in spite of a request for them using f32 and f64, to things like x87 operations.
// These have an f64's mantissa, but can have a larger than normal exponent.
// FIXME(jubilee): Using x87 operations is never necessary in order to function
// on x86 processors for Rust-to-Rust calls, so this issue should not happen.
// Code generation should be adjusted to use non-C calling conventions, avoiding this.
@ -672,41 +674,18 @@ impl f64 {
// as correctness requires avoiding equality tests that may be Subnormal == -0.0
// because it may be wrong under "denormals are zero" and "flush to zero" modes.
// Most of std's targets don't use those, but they are used for thumbv7neon.
// So, this does use bitpattern matching for the rest.
// SAFETY: f64 to u64 is fine. Usually.
// If control flow has gotten this far, the value is definitely in one of the categories
// that f64::partial_classify can correctly analyze.
unsafe { f64::partial_classify(self) }
}
}
// This doesn't actually return a right answer for NaN on purpose,
// seeing as how it cannot correctly discern between a floating point NaN,
// and some normal floating point numbers truncated from an x87 FPU.
#[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
const unsafe fn partial_classify(self) -> FpCategory {
// SAFETY: The caller is not asking questions for which this will tell lies.
let b = unsafe { mem::transmute::<f64, u64>(self) };
match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
(0, Self::EXP_MASK) => FpCategory::Infinite,
(0, 0) => FpCategory::Zero,
(_, 0) => FpCategory::Subnormal,
_ => FpCategory::Normal,
}
}
// This operates on bits, and only bits, so it can ignore concerns about weird FPUs.
// FIXME(jubilee): In a just world, this would be the entire impl for classify,
// plus a transmute. We do not live in a just world, but we can make it more so.
#[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
const fn classify_bits(b: u64) -> FpCategory {
match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
(0, Self::EXP_MASK) => FpCategory::Infinite,
(_, Self::EXP_MASK) => FpCategory::Nan,
(0, 0) => FpCategory::Zero,
(_, 0) => FpCategory::Subnormal,
_ => FpCategory::Normal,
// So, this does use bitpattern matching for the rest. On x87, due to the incorrect
// float codegen on this hardware, this doesn't actually return a right answer for NaN
// because it cannot correctly discern between a floating point NaN, and some normal
// floating point numbers truncated from an x87 FPU -- but we took care of NaN above, so
// we are fine.
let b = self.to_bits();
match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
(0, Self::EXP_MASK) => FpCategory::Infinite,
(0, 0) => FpCategory::Zero,
(_, 0) => FpCategory::Subnormal,
_ => FpCategory::Normal,
}
}
}
@ -1134,33 +1113,7 @@ impl f64 {
#[inline]
pub const fn to_bits(self) -> u64 {
// SAFETY: `u64` is a plain old datatype so we can always transmute to it.
// ...sorta.
//
// See the SAFETY comment in f64::from_bits for more.
#[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
const fn ct_f64_to_u64(ct: f64) -> u64 {
match ct.classify() {
FpCategory::Nan => {
panic!("const-eval error: cannot use f64::to_bits on a NaN")
}
FpCategory::Subnormal => {
panic!("const-eval error: cannot use f64::to_bits on a subnormal number")
}
FpCategory::Infinite | FpCategory::Normal | FpCategory::Zero => {
// SAFETY: We have a normal floating point number. Now we transmute, i.e. do a bitcopy.
unsafe { mem::transmute::<f64, u64>(ct) }
}
}
}
#[inline(always)] // See https://github.com/rust-lang/compiler-builtins/issues/491
fn rt_f64_to_u64(rt: f64) -> u64 {
// SAFETY: `u64` is a plain old datatype so we can always... uh...
// ...look, just pretend you forgot what you just read.
// Stability concerns.
unsafe { mem::transmute::<f64, u64>(rt) }
}
intrinsics::const_eval_select((self,), ct_f64_to_u64, rt_f64_to_u64)
unsafe { mem::transmute(self) }
}
/// Raw transmutation from `u64`.
@ -1205,58 +1158,8 @@ impl f64 {
#[inline]
pub const fn from_bits(v: u64) -> Self {
// It turns out the safety issues with sNaN were overblown! Hooray!
// SAFETY: `u64` is a plain old datatype so we can always transmute from it
// ...sorta.
//
// It turns out that at runtime, it is possible for a floating point number
// to be subject to floating point modes that alter nonzero subnormal numbers
// to zero on reads and writes, aka "denormals are zero" and "flush to zero".
// This is not a problem usually, but at least one tier2 platform for Rust
// actually exhibits an FTZ behavior by default: thumbv7neon
// aka "the Neon FPU in AArch32 state"
//
// Even with this, not all instructions exhibit the FTZ behaviors on thumbv7neon,
// so this should load the same bits if LLVM emits the "correct" instructions,
// but LLVM sometimes makes interesting choices about float optimization,
// and other FPUs may do similar. Thus, it is wise to indulge luxuriously in caution.
//
// In addition, on x86 targets with SSE or SSE2 disabled and the x87 FPU enabled,
// i.e. not soft-float, the way Rust does parameter passing can actually alter
// a number that is "not infinity" to have the same exponent as infinity,
// in a slightly unpredictable manner.
//
// And, of course evaluating to a NaN value is fairly nondeterministic.
// More precisely: when NaN should be returned is knowable, but which NaN?
// So far that's defined by a combination of LLVM and the CPU, not Rust.
// This function, however, allows observing the bitstring of a NaN,
// thus introspection on CTFE.
//
// In order to preserve, at least for the moment, const-to-runtime equivalence,
// reject any of these possible situations from happening.
#[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
const fn ct_u64_to_f64(ct: u64) -> f64 {
match f64::classify_bits(ct) {
FpCategory::Subnormal => {
panic!("const-eval error: cannot use f64::from_bits on a subnormal number")
}
FpCategory::Nan => {
panic!("const-eval error: cannot use f64::from_bits on NaN")
}
FpCategory::Infinite | FpCategory::Normal | FpCategory::Zero => {
// SAFETY: It's not a frumious number
unsafe { mem::transmute::<u64, f64>(ct) }
}
}
}
#[inline(always)] // See https://github.com/rust-lang/compiler-builtins/issues/491
fn rt_u64_to_f64(rt: u64) -> f64 {
// SAFETY: `u64` is a plain old datatype so we can always... uh...
// ...look, just pretend you forgot what you just read.
// Stability concerns.
unsafe { mem::transmute::<u64, f64>(rt) }
}
intrinsics::const_eval_select((v,), ct_u64_to_f64, rt_u64_to_f64)
// SAFETY: `u64` is a plain old datatype so we can always transmute from it.
unsafe { mem::transmute(v) }
}
/// Returns the memory representation of this floating point number as a byte array in

22
tests/ui/consts/const-float-bits-conv.rs
View File

@ -38,6 +38,17 @@ fn f32() {
const_assert!(f32::from_bits(0x44a72000), 1337.0);
const_assert!(f32::from_ne_bytes(0x44a72000u32.to_ne_bytes()), 1337.0);
const_assert!(f32::from_bits(0xc1640000), -14.25);
// Check that NaNs roundtrip their bits regardless of signalingness
// 0xA is 0b1010; 0x5 is 0b0101 -- so these two together clobbers all the mantissa bits
// ...actually, let's just check that these break. :D
const MASKED_NAN1: u32 = f32::NAN.to_bits() ^ 0x002A_AAAA;
const MASKED_NAN2: u32 = f32::NAN.to_bits() ^ 0x0055_5555;
const_assert!(f32::from_bits(MASKED_NAN1).is_nan());
const_assert!(f32::from_bits(MASKED_NAN1).is_nan());
const_assert!(f32::from_bits(MASKED_NAN1).to_bits(), MASKED_NAN1);
const_assert!(f32::from_bits(MASKED_NAN2).to_bits(), MASKED_NAN2);
}
fn f64() {
@ -55,6 +66,17 @@ fn f64() {
const_assert!(f64::from_bits(0x4094e40000000000), 1337.0);
const_assert!(f64::from_ne_bytes(0x4094e40000000000u64.to_ne_bytes()), 1337.0);
const_assert!(f64::from_bits(0xc02c800000000000), -14.25);
// Check that NaNs roundtrip their bits regardless of signalingness
// 0xA is 0b1010; 0x5 is 0b0101 -- so these two together clobbers all the mantissa bits
// ...actually, let's just check that these break. :D
const MASKED_NAN1: u64 = f64::NAN.to_bits() ^ 0x000A_AAAA_AAAA_AAAA;
const MASKED_NAN2: u64 = f64::NAN.to_bits() ^ 0x0005_5555_5555_5555;
const_assert!(f64::from_bits(MASKED_NAN1).is_nan());
const_assert!(f64::from_bits(MASKED_NAN1).is_nan());
const_assert!(f64::from_bits(MASKED_NAN1).to_bits(), MASKED_NAN1);
const_assert!(f64::from_bits(MASKED_NAN2).to_bits(), MASKED_NAN2);
}
fn main() {

68
tests/ui/consts/const-float-bits-reject-conv.rs
View File

@ -1,68 +0,0 @@
//@ compile-flags: -Zmir-opt-level=0
//@ error-pattern: cannot use f32::to_bits on a NaN
#![feature(const_float_bits_conv)]
#![feature(const_float_classify)]
// Don't promote
const fn nop<T>(x: T) -> T { x }
macro_rules! const_assert {
($a:expr) => {
{
const _: () = assert!($a);
assert!(nop($a));
}
};
($a:expr, $b:expr) => {
{
const _: () = assert!($a == $b);
assert_eq!(nop($a), nop($b));
}
};
}
fn f32() {
// Check that NaNs roundtrip their bits regardless of signalingness
// 0xA is 0b1010; 0x5 is 0b0101 -- so these two together clobbers all the mantissa bits
// ...actually, let's just check that these break. :D
const MASKED_NAN1: u32 = f32::NAN.to_bits() ^ 0x002A_AAAA;
//~^ inside
const MASKED_NAN2: u32 = f32::NAN.to_bits() ^ 0x0055_5555;
//~^ inside
// The rest of the code is dead because the constants already fail to evaluate.
const_assert!(f32::from_bits(MASKED_NAN1).is_nan());
const_assert!(f32::from_bits(MASKED_NAN1).is_nan());
// LLVM does not guarantee that loads and stores of NaNs preserve their exact bit pattern.
// In practice, this seems to only cause a problem on x86, since the most widely used calling
// convention mandates that floating point values are returned on the x87 FPU stack. See #73328.
// However, during CTFE we still preserve bit patterns (though that is not a guarantee).
const_assert!(f32::from_bits(MASKED_NAN1).to_bits(), MASKED_NAN1);
const_assert!(f32::from_bits(MASKED_NAN2).to_bits(), MASKED_NAN2);
}
fn f64() {
// Check that NaNs roundtrip their bits regardless of signalingness
// 0xA is 0b1010; 0x5 is 0b0101 -- so these two together clobbers all the mantissa bits
// ...actually, let's just check that these break. :D
const MASKED_NAN1: u64 = f64::NAN.to_bits() ^ 0x000A_AAAA_AAAA_AAAA;
//~^ inside
const MASKED_NAN2: u64 = f64::NAN.to_bits() ^ 0x0005_5555_5555_5555;
//~^ inside
// The rest of the code is dead because the constants already fail to evaluate.
const_assert!(f64::from_bits(MASKED_NAN1).is_nan());
const_assert!(f64::from_bits(MASKED_NAN1).is_nan());
// See comment above.
const_assert!(f64::from_bits(MASKED_NAN1).to_bits(), MASKED_NAN1);
const_assert!(f64::from_bits(MASKED_NAN2).to_bits(), MASKED_NAN2);
}
fn main() {
f32();
f64();
}

115
tests/ui/consts/const-float-bits-reject-conv.stderr
View File

@ -1,115 +0,0 @@
error[E0080]: evaluation of constant value failed
--> $SRC_DIR/core/src/num/f32.rs:LL:COL
|
= note: the evaluated program panicked at 'const-eval error: cannot use f32::to_bits on a NaN', $SRC_DIR/core/src/num/f32.rs:LL:COL
|
note: inside `core::f32::<impl f32>::to_bits::ct_f32_to_u32`
--> $SRC_DIR/core/src/num/f32.rs:LL:COL
note: inside `core::f32::<impl f32>::to_bits`
--> $SRC_DIR/core/src/num/f32.rs:LL:COL
note: inside `f32::MASKED_NAN1`
--> $DIR/const-float-bits-reject-conv.rs:28:30
|
LL | const MASKED_NAN1: u32 = f32::NAN.to_bits() ^ 0x002A_AAAA;
| ^^^^^^^^^^^^^^^^^^
= note: this error originates in the macro `$crate::panic::panic_2021` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
error[E0080]: evaluation of constant value failed
--> $SRC_DIR/core/src/num/f32.rs:LL:COL
|
= note: the evaluated program panicked at 'const-eval error: cannot use f32::to_bits on a NaN', $SRC_DIR/core/src/num/f32.rs:LL:COL
|
note: inside `core::f32::<impl f32>::to_bits::ct_f32_to_u32`
--> $SRC_DIR/core/src/num/f32.rs:LL:COL
note: inside `core::f32::<impl f32>::to_bits`
--> $SRC_DIR/core/src/num/f32.rs:LL:COL
note: inside `f32::MASKED_NAN2`
--> $DIR/const-float-bits-reject-conv.rs:30:30
|
LL | const MASKED_NAN2: u32 = f32::NAN.to_bits() ^ 0x0055_5555;
| ^^^^^^^^^^^^^^^^^^
= note: this error originates in the macro `$crate::panic::panic_2021` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
note: erroneous constant encountered
--> $DIR/const-float-bits-reject-conv.rs:35:34
|
LL | const_assert!(f32::from_bits(MASKED_NAN1).is_nan());
| ^^^^^^^^^^^
note: erroneous constant encountered
--> $DIR/const-float-bits-reject-conv.rs:36:34
|
LL | const_assert!(f32::from_bits(MASKED_NAN1).is_nan());
| ^^^^^^^^^^^
note: erroneous constant encountered
--> $DIR/const-float-bits-reject-conv.rs:42:34
|
LL | const_assert!(f32::from_bits(MASKED_NAN1).to_bits(), MASKED_NAN1);
| ^^^^^^^^^^^
note: erroneous constant encountered
--> $DIR/const-float-bits-reject-conv.rs:43:34
|
LL | const_assert!(f32::from_bits(MASKED_NAN2).to_bits(), MASKED_NAN2);
| ^^^^^^^^^^^
error[E0080]: evaluation of constant value failed
--> $SRC_DIR/core/src/num/f64.rs:LL:COL
|
= note: the evaluated program panicked at 'const-eval error: cannot use f64::to_bits on a NaN', $SRC_DIR/core/src/num/f64.rs:LL:COL
|
note: inside `core::f64::<impl f64>::to_bits::ct_f64_to_u64`
--> $SRC_DIR/core/src/num/f64.rs:LL:COL
note: inside `core::f64::<impl f64>::to_bits`
--> $SRC_DIR/core/src/num/f64.rs:LL:COL
note: inside `f64::MASKED_NAN1`
--> $DIR/const-float-bits-reject-conv.rs:50:30
|
LL | const MASKED_NAN1: u64 = f64::NAN.to_bits() ^ 0x000A_AAAA_AAAA_AAAA;
| ^^^^^^^^^^^^^^^^^^
= note: this error originates in the macro `$crate::panic::panic_2021` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
error[E0080]: evaluation of constant value failed
--> $SRC_DIR/core/src/num/f64.rs:LL:COL
|
= note: the evaluated program panicked at 'const-eval error: cannot use f64::to_bits on a NaN', $SRC_DIR/core/src/num/f64.rs:LL:COL
|
note: inside `core::f64::<impl f64>::to_bits::ct_f64_to_u64`
--> $SRC_DIR/core/src/num/f64.rs:LL:COL
note: inside `core::f64::<impl f64>::to_bits`
--> $SRC_DIR/core/src/num/f64.rs:LL:COL
note: inside `f64::MASKED_NAN2`
--> $DIR/const-float-bits-reject-conv.rs:52:30
|
LL | const MASKED_NAN2: u64 = f64::NAN.to_bits() ^ 0x0005_5555_5555_5555;
| ^^^^^^^^^^^^^^^^^^
= note: this error originates in the macro `$crate::panic::panic_2021` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
note: erroneous constant encountered
--> $DIR/const-float-bits-reject-conv.rs:57:34
|
LL | const_assert!(f64::from_bits(MASKED_NAN1).is_nan());
| ^^^^^^^^^^^
note: erroneous constant encountered
--> $DIR/const-float-bits-reject-conv.rs:58:34
|
LL | const_assert!(f64::from_bits(MASKED_NAN1).is_nan());
| ^^^^^^^^^^^
note: erroneous constant encountered
--> $DIR/const-float-bits-reject-conv.rs:61:34
|
LL | const_assert!(f64::from_bits(MASKED_NAN1).to_bits(), MASKED_NAN1);
| ^^^^^^^^^^^
note: erroneous constant encountered
--> $DIR/const-float-bits-reject-conv.rs:62:34
|
LL | const_assert!(f64::from_bits(MASKED_NAN2).to_bits(), MASKED_NAN2);
| ^^^^^^^^^^^
error: aborting due to 4 previous errors
For more information about this error, try `rustc --explain E0080`.