Added MIR constant propagation of Scalars into function call arguments #71697
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@@ -787,6 +787,7 @@ impl<'tcx> Visitor<'tcx> for CanConstProp { | |
| | NonMutatingUse(NonMutatingUseContext::Inspect) | ||
| | NonMutatingUse(NonMutatingUseContext::Projection) | ||
| | NonUse(_) => {} | ||
| // FIXME(felix91gr): explain the reasoning behind this | ||
| MutatingUse(MutatingUseContext::Projection) => { | ||
| if self.local_kinds[local] != LocalKind::Temp { | ||
| self.can_const_prop[local] = ConstPropMode::NoPropagation; | ||
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@@ -969,13 +970,58 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> { | |
| | TerminatorKind::GeneratorDrop | ||
| | TerminatorKind::FalseEdges { .. } | ||
| | TerminatorKind::FalseUnwind { .. } => {} | ||
| // Every argument in our function calls can be const propagated. | ||
| TerminatorKind::Call { ref mut args, .. } => { | ||
| let mir_opt_level = self.tcx.sess.opts.debugging_opts.mir_opt_level; | ||
| // Constant Propagation into function call arguments is gated | ||
| // under mir-opt-level 2, because LLVM codegen gives performance | ||
| // regressions with it. | ||
| if mir_opt_level >= 2 { | ||
| for opr in args { | ||
| /* | ||
| The following code would appear to be incomplete, because | ||
| the function `Operand::place()` returns `None` if the | ||
| `Operand` is of the variant `Operand::Constant`. In this | ||
| context however, that variant will never appear. This is why: | ||
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There was a problem hiding this comment. The fn main() {
foo(42);
}
fn foo(i: i32) {}But that's already "propagated" so it's fine and we don't need to care about it. There was a problem hiding this comment. It took me a while to realize what you meant. But indeed! Those are already "propagated": https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=9bfca57ccf6c252f7e601ed58beb7895 Should I add something to the comment? There was a problem hiding this comment. Yea, I think the comment should be adjusted. It's not that an |
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| When constructing the MIR, all function call arguments are | ||
| copied into `Locals` of `LocalKind::Temp`. At least, all arguments | ||
| that are not unsized (Less than 0.1% are unsized. See #71170 | ||
| to learn more about those). | ||
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| This means that, conversely, all `Operands` found as function call | ||
| arguments are of the variant `Operand::Copy`. This allows us to | ||
| simplify our handling of `Operands` in this case. | ||
| */ | ||
| if let Some(l) = opr.place().and_then(|p| p.as_local()) { | ||
| if let Some(value) = self.get_const(l) { | ||
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| if self.should_const_prop(value) { | ||
| // FIXME(felix91gr): this code only handles `Scalar` cases. | ||
| // For now, we're not handling `ScalarPair` cases because | ||
| // doing so here would require a lot of code duplication. | ||
| // We should hopefully generalize `Operand` handling into a fn, | ||
| // and use it to do const-prop here and everywhere else | ||
| // where it makes sense. | ||
| if let interpret::Operand::Immediate( | ||
| interpret::Immediate::Scalar( | ||
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| interpret::ScalarMaybeUndef::Scalar(scalar), | ||
| ), | ||
| ) = *value | ||
| { | ||
| *opr = self.operand_from_scalar( | ||
| scalar, | ||
| value.layout.ty, | ||
| source_info.span, | ||
| ); | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| // We remove all Locals which are restricted in propagation to their containing blocks. | ||
| for local in self.locals_of_current_block.iter() { | ||
| Self::remove_const(&mut self.ecx, local); | ||
| } | ||
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