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Rollup merge of #144566 - scottmcm:align-of-slice, r=oli-obk

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Simplify `align_of_val::<[T]>(…)` → `align_of::<T>()`

I spotted this while working on the inliner (rust-lang/rust#144561).  In particular, if [`Layout::for_value`](https://doc.rust-lang.org/std/alloc/struct.Layout.html#method.for_value) inlines, then it can be pretty easy to end up with an `align_of_val::<[T]>` today (demo: <https://rust.godbolt.org/z/Tesnscj4a>) where we can save at least a block, if not more, by using the version that's an rvalue and not a call.
This commit is contained in:
Stuart Cook
2025-07-29 23:50:35 +10:00
committed by GitHub
parent f8370285cf 8ef9233339
commit 826c462c32
8 changed files with 516 additions and 0 deletions
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31
compiler/rustc_mir_transform/src/instsimplify.rs
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@@ -55,6 +55,7 @@ impl<'tcx> crate::MirPass<'tcx> for InstSimplify {
let terminator = block.terminator.as_mut().unwrap();
ctx.simplify_primitive_clone(terminator, &mut block.statements);
ctx.simplify_align_of_slice_val(terminator, &mut block.statements);
ctx.simplify_intrinsic_assert(terminator);
ctx.simplify_nounwind_call(terminator);
simplify_duplicate_switch_targets(terminator);
@@ -252,6 +253,36 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
terminator.kind = TerminatorKind::Goto { target: *destination_block };
}
// Convert `align_of_val::<[T]>(ptr)` to `align_of::<T>()`, since the
// alignment of a slice doesn't actually depend on metadata at all
// and the element type is always `Sized`.
//
// This is here so it can run after inlining, where it's more useful.
// (LowerIntrinsics is done in cleanup, before the optimization passes.)
fn simplify_align_of_slice_val(
&self,
terminator: &mut Terminator<'tcx>,
statements: &mut Vec<Statement<'tcx>>,
) {
if let TerminatorKind::Call {
func, args, destination, target: Some(destination_block), ..
} = &terminator.kind
&& args.len() == 1
&& let Some((fn_def_id, generics)) = func.const_fn_def()
&& self.tcx.is_intrinsic(fn_def_id, sym::align_of_val)
&& let ty::Slice(elem_ty) = *generics.type_at(0).kind()
{
statements.push(Statement::new(
terminator.source_info,
StatementKind::Assign(Box::new((
*destination,
Rvalue::NullaryOp(NullOp::AlignOf, elem_ty),
))),
));
terminator.kind = TerminatorKind::Goto { target: *destination_block };
}
}
fn simplify_nounwind_call(&self, terminator: &mut Terminator<'tcx>) {
let TerminatorKind::Call { ref func, ref mut unwind, .. } = terminator.kind else {
return;