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Rollup merge of #100239 - RalfJung:const-prop-uninit, r=oli-obk

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remove an ineffective check in const_prop

Based on https://github.com/rust-lang/rust/pull/100043, only the last two commits are new.

ConstProp has a special check when reading from a local that prevents reading uninit locals. However, if that local flows into `force_allocation`, then no check fires and evaluation proceeds. So this check is not really effective at preventing accesses to uninit locals.

With https://github.com/rust-lang/rust/pull/100043, `read_immediate` and friends always fail when reading uninit locals, so I don't see why ConstProp would need a separate check. Thus I propose we remove it. This is needed to be able to do https://github.com/rust-lang/rust/pull/100085.
This commit is contained in:
Dylan DPC 2022-08-29 16:49:40 +05:30 committed by GitHub
commit 3ea5456366
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8 changed files with 64 additions and 58 deletions
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3
compiler/rustc_const_eval/src/interpret/eval_context.rs
View File

@ -187,9 +187,6 @@ pub enum LocalValue<Prov: Provenance = AllocId> {
impl<'tcx, Prov: Provenance + 'static> LocalState<'tcx, Prov> {
/// Read the local's value or error if the local is not yet live or not live anymore.
///
/// Note: This may only be invoked from the `Machine::access_local` hook and not from
/// anywhere else. You may be invalidating machine invariants if you do!
#[inline]
pub fn access(&self) -> InterpResult<'tcx, &Operand<Prov>> {
match &self.value {

17
compiler/rustc_const_eval/src/interpret/machine.rs
View File

@ -215,23 +215,12 @@ pub trait Machine<'mir, 'tcx>: Sized {
right: &ImmTy<'tcx, Self::Provenance>,
) -> InterpResult<'tcx, (Scalar<Self::Provenance>, bool, Ty<'tcx>)>;
/// Called to read the specified `local` from the `frame`.
/// Since reading a ZST is not actually accessing memory or locals, this is never invoked
/// for ZST reads.
#[inline]
fn access_local<'a>(
frame: &'a Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>,
local: mir::Local,
) -> InterpResult<'tcx, &'a Operand<Self::Provenance>>
where
'tcx: 'mir,
{
frame.locals[local].access()
}
/// Called to write the specified `local` from the `frame`.
/// Since writing a ZST is not actually accessing memory or locals, this is never invoked
/// for ZST reads.
///
/// Due to borrow checker trouble, we indicate the `frame` as an index rather than an `&mut
/// Frame`.
#[inline]
fn access_local_mut<'a>(
ecx: &'a mut InterpCx<'mir, 'tcx, Self>,

9
compiler/rustc_const_eval/src/interpret/operand.rs
View File

@ -444,7 +444,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
}
}
/// Read from a local. Will not actually access the local if reading from a ZST.
/// Read from a local.
/// Will not access memory, instead an indirect `Operand` is returned.
///
/// This is public because it is used by [priroda](https://github.com/oli-obk/priroda) to get an
@ -456,12 +456,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
layout: Option<TyAndLayout<'tcx>>,
) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
let layout = self.layout_of_local(frame, local, layout)?;
let op = if layout.is_zst() {
// Bypass `access_local` (helps in ConstProp)
Operand::Immediate(Immediate::Uninit)
} else {
*M::access_local(frame, local)?
};
let op = *frame.locals[local].access()?;
Ok(OpTy { op, layout, align: Some(layout.align.abi) })
}

2
compiler/rustc_const_eval/src/interpret/place.rs
View File

@ -642,7 +642,7 @@ where
// avoid force_allocation.
let src = match self.read_immediate_raw(src)? {
Ok(src_val) => {
assert!(!src.layout.is_unsized(), "cannot have unsized immediates");
assert!(!src.layout.is_unsized(), "cannot copy unsized immediates");
assert!(
!dest.layout.is_unsized(),
"the src is sized, so the dest must also be sized"

3
compiler/rustc_const_eval/src/interpret/projection.rs
View File

@ -100,6 +100,8 @@ where
// This makes several assumptions about what layouts we will encounter; we match what
// codegen does as good as we can (see `extract_field` in `rustc_codegen_ssa/src/mir/operand.rs`).
let field_val: Immediate<_> = match (*base, base.layout.abi) {
// if the entire value is uninit, then so is the field (can happen in ConstProp)
(Immediate::Uninit, _) => Immediate::Uninit,
// the field contains no information, can be left uninit
_ if field_layout.is_zst() => Immediate::Uninit,
// the field covers the entire type
@ -124,6 +126,7 @@ where
b_val
})
}
// everything else is a bug
_ => span_bug!(
self.cur_span(),
"invalid field access on immediate {}, layout {:#?}",

56
compiler/rustc_mir_transform/src/const_prop.rs
View File

@ -243,24 +243,6 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx>
throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
}
fn access_local<'a>(
frame: &'a Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>,
local: Local,
) -> InterpResult<'tcx, &'a interpret::Operand<Self::Provenance>> {
let l = &frame.locals[local];
if matches!(
l.value,
LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit))
) {
// For us "uninit" means "we don't know its value, might be initiailized or not".
// So stop here.
throw_machine_stop_str!("tried to access alocal with unknown value ")
}
l.access()
}
fn access_local_mut<'a>(
ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
frame: usize,
@ -431,7 +413,13 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
let op = match self.ecx.eval_place_to_op(place, None) {
Ok(op) => op,
Ok(op) => {
if matches!(*op, interpret::Operand::Immediate(Immediate::Uninit)) {
// Make sure nobody accidentally uses this value.
return None;
}
op
}
Err(e) => {
trace!("get_const failed: {}", e);
return None;
@ -643,6 +631,14 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
if rvalue.needs_subst() {
return None;
}
if !rvalue
.ty(&self.ecx.frame().body.local_decls, *self.ecx.tcx)
.is_sized(self.ecx.tcx, self.param_env)
{
// the interpreter doesn't support unsized locals (only unsized arguments),
// but rustc does (in a kinda broken way), so we have to skip them here
return None;
}
if self.tcx.sess.mir_opt_level() >= 4 {
self.eval_rvalue_with_identities(rvalue, place)
@ -660,18 +656,20 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
self.use_ecx(|this| match rvalue {
Rvalue::BinaryOp(op, box (left, right))
| Rvalue::CheckedBinaryOp(op, box (left, right)) => {
let l = this.ecx.eval_operand(left, None);
let r = this.ecx.eval_operand(right, None);
let l = this.ecx.eval_operand(left, None).and_then(|x| this.ecx.read_immediate(&x));
let r =
this.ecx.eval_operand(right, None).and_then(|x| this.ecx.read_immediate(&x));
let const_arg = match (l, r) {
(Ok(ref x), Err(_)) | (Err(_), Ok(ref x)) => this.ecx.read_immediate(x)?,
(Err(e), Err(_)) => return Err(e),
(Ok(_), Ok(_)) => return this.ecx.eval_rvalue_into_place(rvalue, place),
(Ok(x), Err(_)) | (Err(_), Ok(x)) => x, // exactly one side is known
(Err(e), Err(_)) => return Err(e), // neither side is known
(Ok(_), Ok(_)) => return this.ecx.eval_rvalue_into_place(rvalue, place), // both sides are known
};
if !matches!(const_arg.layout.abi, abi::Abi::Scalar(..)) {
// We cannot handle Scalar Pair stuff.
return this.ecx.eval_rvalue_into_place(rvalue, place);
// No point in calling `eval_rvalue_into_place`, since only one side is known
throw_machine_stop_str!("cannot optimize this")
}
let arg_value = const_arg.to_scalar().to_bits(const_arg.layout.size)?;
@ -696,7 +694,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
this.ecx.write_immediate(*const_arg, &dest)
}
}
_ => this.ecx.eval_rvalue_into_place(rvalue, place),
_ => throw_machine_stop_str!("cannot optimize this"),
}
}
_ => this.ecx.eval_rvalue_into_place(rvalue, place),
@ -1073,7 +1071,11 @@ impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
if let Some(ref value) = self.eval_operand(&cond) {
trace!("assertion on {:?} should be {:?}", value, expected);
let expected = Scalar::from_bool(*expected);
let value_const = self.ecx.read_scalar(&value).unwrap();
let Ok(value_const) = self.ecx.read_scalar(&value) else {
// FIXME should be used use_ecx rather than a local match... but we have
// quite a few of these read_scalar/read_immediate that need fixing.
return
};
if expected != value_const {
// Poison all places this operand references so that further code
// doesn't use the invalid value

29
compiler/rustc_mir_transform/src/const_prop_lint.rs
View File

@ -6,6 +6,7 @@ use crate::const_prop::ConstPropMachine;
use crate::const_prop::ConstPropMode;
use crate::MirLint;
use rustc_const_eval::const_eval::ConstEvalErr;
use rustc_const_eval::interpret::Immediate;
use rustc_const_eval::interpret::{
self, InterpCx, InterpResult, LocalState, LocalValue, MemoryKind, OpTy, Scalar, StackPopCleanup,
};
@ -229,7 +230,13 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
let op = match self.ecx.eval_place_to_op(place, None) {
Ok(op) => op,
Ok(op) => {
if matches!(*op, interpret::Operand::Immediate(Immediate::Uninit)) {
// Make sure nobody accidentally uses this value.
return None;
}
op
}
Err(e) => {
trace!("get_const failed: {}", e);
return None;
@ -515,6 +522,14 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
if rvalue.needs_subst() {
return None;
}
if !rvalue
.ty(&self.ecx.frame().body.local_decls, *self.ecx.tcx)
.is_sized(self.ecx.tcx, self.param_env)
{
// the interpreter doesn't support unsized locals (only unsized arguments),
// but rustc does (in a kinda broken way), so we have to skip them here
return None;
}
self.use_ecx(source_info, |this| this.ecx.eval_rvalue_into_place(rvalue, place))
}
@ -624,7 +639,11 @@ impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
if let Some(ref value) = self.eval_operand(&cond, source_info) {
trace!("assertion on {:?} should be {:?}", value, expected);
let expected = Scalar::from_bool(*expected);
let value_const = self.ecx.read_scalar(&value).unwrap();
let Ok(value_const) = self.ecx.read_scalar(&value) else {
// FIXME should be used use_ecx rather than a local match... but we have
// quite a few of these read_scalar/read_immediate that need fixing.
return
};
if expected != value_const {
enum DbgVal<T> {
Val(T),
@ -641,9 +660,9 @@ impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
let mut eval_to_int = |op| {
// This can be `None` if the lhs wasn't const propagated and we just
// triggered the assert on the value of the rhs.
self.eval_operand(op, source_info).map_or(DbgVal::Underscore, |op| {
DbgVal::Val(self.ecx.read_immediate(&op).unwrap().to_const_int())
})
self.eval_operand(op, source_info)
.and_then(|op| self.ecx.read_immediate(&op).ok())
.map_or(DbgVal::Underscore, |op| DbgVal::Val(op.to_const_int()))
};
let msg = match msg {
AssertKind::DivisionByZero(op) => {

3
src/test/mir-opt/const_prop/mutable_variable_unprop_assign.main.ConstProp.diff
View File

@ -41,7 +41,8 @@
StorageLive(_4); // scope 2 at $DIR/mutable_variable_unprop_assign.rs:+4:9: +4:10
_4 = (_2.1: i32); // scope 2 at $DIR/mutable_variable_unprop_assign.rs:+4:13: +4:16
StorageLive(_5); // scope 3 at $DIR/mutable_variable_unprop_assign.rs:+5:9: +5:10
_5 = (_2.0: i32); // scope 3 at $DIR/mutable_variable_unprop_assign.rs:+5:13: +5:16
- _5 = (_2.0: i32); // scope 3 at $DIR/mutable_variable_unprop_assign.rs:+5:13: +5:16
+ _5 = const 1_i32; // scope 3 at $DIR/mutable_variable_unprop_assign.rs:+5:13: +5:16
nop; // scope 0 at $DIR/mutable_variable_unprop_assign.rs:+0:11: +6:2
StorageDead(_5); // scope 3 at $DIR/mutable_variable_unprop_assign.rs:+6:1: +6:2
StorageDead(_4); // scope 2 at $DIR/mutable_variable_unprop_assign.rs:+6:1: +6:2