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clarify const_prop ICE protection comment

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This commit is contained in:
Ralf Jung 2019-10-19 11:46:56 +02:00
parent e5b8c118a3
commit 282403e6bd
1 changed files with 10 additions and 8 deletions
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18
src/librustc_mir/transform/const_prop.rs
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@ -518,12 +518,13 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
} }
} }
// Work around: avoid ICE in miri. // Work around: avoid ICE in miri. FIXME(wesleywiser)
// FIXME(wesleywiser) we don't currently handle the case where we try to make a ref // The Miri engine ICEs when taking a reference to an uninitialized unsized
// from a function argument that hasn't been assigned to in this function. The main // local. There's nothing it can do here: taking a reference needs an allocation
// issue is if an arg is a fat-pointer, miri `expects()` to be able to read the value // which needs to know the size. Normally that's okay as during execution
// of that pointer to get size info. However, since this is `ConstProp`, that argument // (e.g. for CTFE) it can never happen. But here in const_prop
// doesn't actually have a backing value and so this causes an ICE. // we leave function arguments uninitialized, so if one of these is unsized
// and has a reference taken, we get an ICE.
Rvalue::Ref(_, _, Place { base: PlaceBase::Local(local), projection: box [] }) => { Rvalue::Ref(_, _, Place { base: PlaceBase::Local(local), projection: box [] }) => {
trace!("checking Ref({:?})", place); trace!("checking Ref({:?})", place);
let alive = let alive =
@ -531,14 +532,15 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
true true
} else { false }; } else { false };
// local 0 is the return place; locals 1..=arg_count are the arguments.
if local.as_usize() <= self.ecx.frame().body.arg_count && !alive { if local.as_usize() <= self.ecx.frame().body.arg_count && !alive {
trace!("skipping Ref({:?})", place); trace!("skipping Ref({:?})", place);
return None; return None;
} }
} }
// Work around: avoid extra unnecessary locals. // Work around: avoid extra unnecessary locals. FIXME(wesleywiser)
// FIXME(wesleywiser): const eval will turn this into a `const Scalar(<ZST>)` that // Const eval will turn this into a `const Scalar(<ZST>)` that
// `SimplifyLocals` doesn't know it can remove. // `SimplifyLocals` doesn't know it can remove.
Rvalue::Aggregate(_, operands) if operands.len() == 0 => { Rvalue::Aggregate(_, operands) if operands.len() == 0 => {
return None; return None;