[clang] [llvm] [clang-tools-extra] [X86] Use plain load/store instead of cmpxchg16b for atomics with AVX (PR #74275)

[James Y Knight via cfe-commits]

================
@@ -30095,12 +30102,16 @@ TargetLoweringBase::AtomicExpansionKind
 X86TargetLowering::shouldExpandAtomicStoreInIR(StoreInst *SI) const {
   Type *MemType = SI->getValueOperand()->getType();
 
-  bool NoImplicitFloatOps =
-      SI->getFunction()->hasFnAttribute(Attribute::NoImplicitFloat);
-  if (MemType->getPrimitiveSizeInBits() == 64 && !Subtarget.is64Bit() &&
-      !Subtarget.useSoftFloat() && !NoImplicitFloatOps &&
-      (Subtarget.hasSSE1() || Subtarget.hasX87()))
-    return AtomicExpansionKind::None;
+  if (!SI->getFunction()->hasFnAttribute(Attribute::NoImplicitFloat) &&
+      !Subtarget.useSoftFloat()) {
+    if (MemType->getPrimitiveSizeInBits() == 64 && !Subtarget.is64Bit() &&
+        (Subtarget.hasSSE1() || Subtarget.hasX87()))
----------------
jyknight wrote:

No, misaligned atomic ops are converted to `__atomic_*` libcall before this 
function is ever called.

https://github.com/llvm/llvm-project/pull/74275
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