Allow BackendRepr::Scalar for repr(Rust, packed) structs [MCP1007]#159033
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Allow BackendRepr::Scalar for repr(Rust, packed) structs [MCP1007]#159033scottmcm wants to merge 2 commits into
BackendRepr::Scalar for repr(Rust, packed) structs [MCP1007]#159033scottmcm wants to merge 2 commits into
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| pub unsafe fn write_packed_scalar(ptr: *mut PackedScalar, val: PackedScalar) { | ||
| // CHECK: start | ||
| // CHECK-NEXT: store i64 %val, ptr %ptr, align 2 | ||
| // CHECK-NEXT: ret void | ||
| *ptr = val; | ||
| } |
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Compare what we emit today: https://rust.godbolt.org/z/8Pv61EhbK
This one, for example, emits 2 allocas and 2 memcpys without this PR:
define void @write_packed_scalar(ptr noundef %ptr, i64 noundef %0) unnamed_addr {
start:
%1 = alloca [8 x i8], align 8
%val = alloca [8 x i8], align 2
call void @llvm.lifetime.start.p0(ptr %1)
store i64 %0, ptr %1, align 8
call void @llvm.memcpy.p0.p0.i64(ptr align 2 %val, ptr align 8 %1, i64 8, i1 false)
call void @llvm.lifetime.end.p0(ptr %1)
call void @llvm.memcpy.p0.p0.i64(ptr align 2 %ptr, ptr align 2 %val, i64 8, i1 false)
ret void
}(which LLVM of course cleans up even in -O1, but still nice to just not emit that in the first place.)
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This was referenced Jul 9, 2026
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…uct, r=joshtriplett
Better comment the struct used by `{read,write}_unaligned`
To address rust-lang#159033 (comment)
r? @RalfJung
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Rollup merge of #159157 - scottmcm:clearer-ptr-unaligned-struct, r=joshtriplett Better comment the struct used by `{read,write}_unaligned` To address #159033 (comment) r? @RalfJung
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Co-authored-by: Ralf Jung <post@ralfj.de>
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This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed. Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers. |
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Tracking issue: #158985
This only changes
Scalar(notScalarPairnorSimdVector) because I wanted to do this in small bites. It also preserves the requirement that there can't be padding, so won't apply foralign(MORE_THAN_SIZE)either. I also didn't touch unions.Note that after #158666 there are zero mentions of
default_alignin codegen, so this "just working" in codegen without any changes is IMHO expected. Codegen has long been expected to always use PlaceValue::align instead of anything from the layout when dealing with alignment.r? workingjubilee