[clang] [Clang] Add `noalias` to `this` pointer in C++ constructors (PR #136792)

[Guy David via cfe-commits]

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 <guyda96 at 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>())