[llvm] [clang] Use unaligned atomic load and stores on x86 (PR #79191)

[via cfe-commits]

https://github.com/AtariDreams updated https://github.com/llvm/llvm-project/pull/79191

>From 9454e1cc48088508d4df33b7a4de47418de03348 Mon Sep 17 00:00:00 2001
From: Rose <83477269+AtariDreams at users.noreply.github.com>
Date: Tue, 23 Jan 2024 13:59:05 -0500
Subject: [PATCH] Use unaligned atomic load and stores on x86

The backend supports it now, so we can use it.
---
 clang/lib/CodeGen/CGObjC.cpp            | 5 +++--
 llvm/lib/Target/X86/X86ISelLowering.cpp | 1 +
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/clang/lib/CodeGen/CGObjC.cpp b/clang/lib/CodeGen/CGObjC.cpp
index 03fc0ec7ff54e1c..debfc84f49e4848 100644
--- a/clang/lib/CodeGen/CGObjC.cpp
+++ b/clang/lib/CodeGen/CGObjC.cpp
@@ -846,8 +846,9 @@ static void emitStructGetterCall(CodeGenFunction &CGF, ObjCIvarDecl *ivar,
 /// accesses.  They don't have to be fast, just faster than a function
 /// call and a mutex.
 static bool hasUnalignedAtomics(llvm::Triple::ArchType arch) {
-  // FIXME: Allow unaligned atomic load/store on x86.  (It is not
-  // currently supported by the backend.)
+  // x86 is the only one so far that we know support this as of now
+  if (arch == llvm::Triple::x86 || arch == llvm::Triple::x86_64)
+    return true;
   return false;
 }
 
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index e158312caffdec7..9b5128cc1361147 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -107,6 +107,7 @@ X86TargetLowering::X86TargetLowering(const X86TargetMachine &TM,
     setSchedulingPreference(Sched::RegPressure);
   const X86RegisterInfo *RegInfo = Subtarget.getRegisterInfo();
   setStackPointerRegisterToSaveRestore(RegInfo->getStackRegister());
+  setSupportsUnalignedAtomics(true);
 
   // Bypass expensive divides and use cheaper ones.
   if (TM.getOptLevel() >= CodeGenOptLevel::Default) {