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Clean up

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This commit is contained in:
Robin Kruppe 2017-10-28 16:43:08 +02:00
parent 0a843df264
commit ce4664956f
1 changed files with 26 additions and 32 deletions
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58
src/librustc_trans/mir/rvalue.rs
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@ -878,18 +878,11 @@ fn cast_float_to_int(bcx: &Builder,
// we're rounding towards zero, we just get float_ty::MAX (which is always an integer).
// This already happens today with u128::MAX = 2^128 - 1 > f32::MAX.
fn compute_clamp_bounds<F: Float>(signed: bool, int_ty: Type) -> (u128, u128) {
let f_min = if signed {
let rounded_min = F::from_i128_r(int_min(signed, int_ty), Round::TowardZero);
assert_eq!(rounded_min.status, Status::OK);
rounded_min.value
} else {
F::ZERO
};
let rounded_min = F::from_i128_r(int_min(signed, int_ty), Round::TowardZero);
assert_eq!(rounded_min.status, Status::OK);
let rounded_max = F::from_u128_r(int_max(signed, int_ty), Round::TowardZero);
assert!(rounded_max.value.is_finite());
(f_min.to_bits(), rounded_max.value.to_bits())
(rounded_min.value.to_bits(), rounded_max.value.to_bits())
}
fn int_max(signed: bool, int_ty: Type) -> u128 {
let shift_amount = 128 - int_ty.int_width();
@ -906,11 +899,6 @@ fn cast_float_to_int(bcx: &Builder,
0
}
}
let (f_min, f_max) = match float_ty.float_width() {
32 => compute_clamp_bounds::<ieee::Single>(signed, int_ty),
64 => compute_clamp_bounds::<ieee::Double>(signed, int_ty),
n => bug!("unsupported float width {}", n),
};
let float_bits_to_llval = |bits| {
let bits_llval = match float_ty.float_width() {
32 => C_u32(bcx.ccx, bits as u32),
@ -919,6 +907,11 @@ fn cast_float_to_int(bcx: &Builder,
};
consts::bitcast(bits_llval, float_ty)
};
let (f_min, f_max) = match float_ty.float_width() {
32 => compute_clamp_bounds::<ieee::Single>(signed, int_ty),
64 => compute_clamp_bounds::<ieee::Double>(signed, int_ty),
n => bug!("unsupported float width {}", n),
};
let f_min = float_bits_to_llval(f_min);
let f_max = float_bits_to_llval(f_max);
// To implement saturation, we perform the following steps:
@ -935,37 +928,38 @@ fn cast_float_to_int(bcx: &Builder,
// undef does not introduce any non-determinism either.
// More importantly, the above procedure correctly implements saturating conversion.
// Proof (sketch):
// If x is NaN, 0 is trivially returned.
// If x is NaN, 0 is returned by definition.
// Otherwise, x is finite or infinite and thus can be compared with f_min and f_max.
// This yields three cases to consider:
// (1) if x in [f_min, f_max], the result of fpto[su]i is returned, which agrees with
// saturating conversion for inputs in that range.
// (2) if x > f_max, then x is larger than int_ty::MAX. This holds even if f_max is rounded
// (i.e., if f_max < int_ty::MAX) because in those cases, nextUp(f_max) is already larger
// than int_ty::MAX. Because x is larger than int_ty::MAX, the return value is correct.
// than int_ty::MAX. Because x is larger than int_ty::MAX, the return value of int_ty::MAX
// is correct.
// (3) if x < f_min, then x is smaller than int_ty::MIN. As shown earlier, f_min exactly equals
// int_ty::MIN and therefore the return value of int_ty::MIN is immediately correct.
// int_ty::MIN and therefore the return value of int_ty::MIN is correct.
// QED.
// Step 1 was already performed above.
// Step 2: We use two comparisons and two selects, with s1 being the result:
// %less = fcmp ult %x, %f_min
// Step 2: We use two comparisons and two selects, with %s1 being the result:
// %less_or_nan = fcmp ult %x, %f_min
// %greater = fcmp olt %x, %f_max
// %s0 = select %less, int_ty::MIN, %fptosi_result
// %s0 = select %less_or_nan, int_ty::MIN, %fptosi_result
// %s1 = select %greater, int_ty::MAX, %s0
// Note that %less uses an *unordered* comparison. This comparison is true if the operands are
// not comparable (i.e., if x is NaN). The unordered comparison ensures that s1 becomes
// int_ty::MIN if x is NaN.
// Performance note: It can be lowered to a flipped comparison and a negation (and the negation
// can be merged into the select), so it not necessarily any more expensive than a ordered
// ("normal") comparison. Whether these optimizations will be performed is ultimately up to the
// backend but at least x86 does that.
let less = bcx.fcmp(llvm::RealULT, x, f_min);
// Note that %less_or_nan uses an *unordered* comparison. This comparison is true if the
// operands are not comparable (i.e., if x is NaN). The unordered comparison ensures that s1
// becomes int_ty::MIN if x is NaN.
// Performance note: Unordered comparison can be lowered to a "flipped" comparison and a
// negation, and the negation can be merged into the select. Therefore, it not necessarily any
// more expensive than a ordered ("normal") comparison. Whether these optimizations will be
// performed is ultimately up to the backend, but at least x86 does perform them.
let less_or_nan = bcx.fcmp(llvm::RealULT, x, f_min);
let greater = bcx.fcmp(llvm::RealOGT, x, f_max);
let int_max = C_big_integral(int_ty, int_max(signed, int_ty) as u128);
let int_max = C_big_integral(int_ty, int_max(signed, int_ty));
let int_min = C_big_integral(int_ty, int_min(signed, int_ty) as u128);
let s0 = bcx.select(less, int_min, fptosui_result);
let s0 = bcx.select(less_or_nan, int_min, fptosui_result);
let s1 = bcx.select(greater, int_max, s0);
// Step 3: NaN replacement.
@ -973,7 +967,7 @@ fn cast_float_to_int(bcx: &Builder,
// Therefore we only need to execute this step for signed integer types.
if signed {
// LLVM has no isNaN predicate, so we use (x == x) instead
bcx.select(bcx.fcmp(llvm::RealOEQ, x, x), s1, C_big_integral(int_ty, 0))
bcx.select(bcx.fcmp(llvm::RealOEQ, x, x), s1, C_uint(int_ty, 0))
} else {
s1
}