Incorrect destructor parameter alignment on windows

[jacobly0]

struct SE {
    virtual ~SE() = default;

    virtual void oE() {}
};

struct WE : virtual SE {
    ~WE() override = default;

    void oE() override;
};

struct FT : virtual WE {
    ~FT() override;

    int m_b alignas(2 * sizeof(void*));
};

FT::~FT() {
    m_b = 0;
}

$ clang --version
clang version 15.0.7
Target: x86_64-w64-windows-gnu
Thread model: posix
InstalledDir: D:/dep/msys64/ucrt64/bin

$ clang -O1 -target x86_64-windows repro.cpp -S -emit-llvm -o -
<snip>
define dso_local void @"??1FT@@UEAA@XZ"(ptr nocapture noundef nonnull align 16 dereferenceable(32) %0) unnamed_addr #
0 align 2 {
  <snip>
  %15 = getelementptr inbounds i8, ptr %0, i64 -24
  store i32 0, ptr %15, align 16, !tbaa !7
  ret void
}
<snip>

Note that both %0 and %15 are marked with align 16 even though the offset between them is 24 bytes.

---

This is much reduced with shortened identifiers so that I can still associate it with the original code. This bug manifested as optimized builds crashing at runtime on a movaps instruction which was storing to a pointer that was not, in fact, 16 byte aligned, but which was derived from an 8-byte aligned pointer with an incorrectly marked alignment of 16 bytes and an offset that happened to be a multiple of 16 bytes.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[jacobly0]

Reduced reproducer after some investigation into the bug:

struct A { virtual ~A(); };

struct alignas(2 * sizeof(void *)) B : virtual A {
    ~B();

    void *x, *y;
};
B::~B() { y = nullptr; }

$ clang --version
clang version 17.0.0
Target: x86_64-w64-windows-gnu
Thread model: posix
InstalledDir: D:/dep/msys64/home/Admin/prg/llvm/build-Debug/bin

$ clang -O1 -target x86_64-windows repro.cpp -S -emit-llvm -o -
<snip>
; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(read, argmem: readwrite, inaccessiblemem: none) uwtable
define dso_local void @"??1B@@UEAA@XZ"(ptr nocapture noundef nonnull align 16 dereferenceable(24) %this) unnamed_addr #0 align 2 {
entry:
  %0 = getelementptr inbounds i8, ptr %this, i64 -24
  %vbtable = load ptr, ptr %0, align 16
  %1 = getelementptr inbounds i32, ptr %vbtable, i64 1
  %vbase_offs = load i32, ptr %1, align 4
  %2 = sext i32 %vbase_offs to i64
  %add.ptr = getelementptr inbounds i8, ptr %0, i64 %2
  store ptr @"??_7B@@6B@", ptr %add.ptr, align 8, !tbaa !4
  %y = getelementptr inbounds i8, ptr %this, i64 -8
  store ptr null, ptr %y, align 16, !tbaa !7
  ret void
}
<snip>

Note that both %this and %y are marked with align 16 even though the offset between them is 8 bytes.

[jacobly0]

https://reviews.llvm.org/D143233