[llvm-bugs] [Bug 42064] New: Optimized 128-bit atomic compare-and-swap can generate incorrect code on msvc target

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42064

            Bug ID: 42064
           Summary: Optimized 128-bit atomic compare-and-swap can generate
                    incorrect code on msvc target
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: cerickson at curedev.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

Created attachment 22047
  --> https://bugs.llvm.org/attachment.cgi?id=22047&action=edit
Example source code

When compiling the attached source with:

        clang --target=x86_64-pc-windows-msvc -march=sandybridge -O2

it appears that the generated code tries to use rbx as both an input for
cmpxchg16b and as a stack pointer:

        mov     rbx, rsp
        ...
        movabs  rax, 1393743441367457520
        mov     rdx, rax
        mov     rcx, rax
        mov     rbx, rax
        lock            cmpxchg16b      xmmword ptr [r8]
        vxorps  xmm0, xmm0, xmm0
        vmovaps xmmword ptr [rbx + 128], xmm0 // rbx is garbage at this point

The example C++ code is a bit silly because it is as reduced as I could get it
and still have the problem occur. The same issue occurs whether using
_InterlockedCompareExchange128() or __sync_val_compare_and_swap().

For convenience, here is the example on godbolt: https://godbolt.org/z/C64_VH

I ran into this issue in clang 7.0.0 but it seems to still be there in trunk
(according to godbolt).

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