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Auto merge of #77972 - Mark-Simulacrum:side-effect-loop, r=nagisa

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Prevent miscompilation in trivial loop {}

Ideally, we would want to handle a broader set of cases to fully fix the
underlying bug here. That is currently relatively expensive at compile and
runtime, so we don't do that for now.

Performance results indicate this is not a major regression, if at all, so it should be safe to land.

cc #28728
This commit is contained in:
bors 2020-10-16 14:06:37 +00:00
commit a78a62fc99
5 changed files with 42 additions and 9 deletions
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20
compiler/rustc_codegen_ssa/src/mir/block.rs
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@ -163,7 +163,7 @@ impl<'a, 'tcx> TerminatorCodegenHelper<'tcx> {
target <= self.bb
&& target.start_location().is_predecessor_of(self.bb.start_location(), mir)
}) {
bx.sideeffect();
bx.sideeffect(false);
}
}
}
@ -964,7 +964,23 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
}
mir::TerminatorKind::Goto { target } => {
helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
if bb == target {
// This is an unconditional branch back to this same basic
// block. That means we have something like a `loop {}`
// statement. Currently LLVM miscompiles this because it
// assumes forward progress. We want to prevent this in all
// cases, but that has a fairly high cost to compile times
// currently. Instead, try to handle this specific case
// which comes up commonly in practice (e.g., in embedded
// code).
//
// The `true` here means we insert side effects regardless
// of -Zinsert-sideeffect being passed on unconditional
// branching to the same basic block.
bx.sideeffect(true);
} else {
helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
}
helper.funclet_br(self, &mut bx, target);
}