https://github.com/guy-david created
https://github.com/llvm/llvm-project/pull/136792
Note: the patch is probably amending the wrong piece of code, I've tried to add
it to `buildThisParam` but hit an assertion because of a missing translation
unit context.
Clang, unlike GCC, does not transform the following example into a 128-bit load
and store:
```c++
class vector4f
{
private:
float _elements[4];
public:
explicit __attribute__((noinline)) vector4f(float const *src)
{
_elements[0] = src[0];
_elements[1] = src[1];
_elements[2] = src[2];
_elements[3] = src[3];
}
};
```
That's because `src` might overlap with `_elements`.
However, according to the standard in 11.10.4.2 under [class.cdtor]:
> "During the construction of an object, if the value of the object or any
> of its subobjects is accessed through a glvalue that is not obtained,
> directly or indirectly, from the constructor’s this pointer, the value
> of the object or subobject thus obtained is unspecified."
which sounds like `restrict`.
Relevant GCC chain-mail:
https://gcc.gnu.org/pipermail/gcc-patches/2018-May/498812.html.
>From ba3d52c74add481bfe35a448963424dc448c0fad Mon Sep 17 00:00:00 2001
From: Guy David <guyd...@gmail.com>
Date: Wed, 23 Apr 2025 02:24:41 +0300
Subject: [PATCH] [Clang] Add `noalias` to `this` pointer in C++ constructors
---
clang/lib/CodeGen/CodeGenFunction.cpp | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp
b/clang/lib/CodeGen/CodeGenFunction.cpp
index 4d29ceace646f..2da48cea7849d 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -1567,8 +1567,10 @@ void CodeGenFunction::GenerateCode(GlobalDecl GD,
llvm::Function *Fn,
PGO.assignRegionCounters(GD, CurFn);
if (isa<CXXDestructorDecl>(FD))
EmitDestructorBody(Args);
- else if (isa<CXXConstructorDecl>(FD))
+ else if (isa<CXXConstructorDecl>(FD)) {
+ Fn->addParamAttr(0, llvm::Attribute::NoAlias);
EmitConstructorBody(Args);
+ }
else if (getLangOpts().CUDA &&
!getLangOpts().CUDAIsDevice &&
FD->hasAttr<CUDAGlobalAttr>())
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