[llvm] [AArch64] Disable red-zone when lowering Q-reg copy through memory. (PR #94962)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-aarch64

Author: Sander de Smalen (sdesmalen-arm)

<details>
<summary>Changes</summary>

This was pointed out in PR #<!-- -->93940.

---
Full diff: https://github.com/llvm/llvm-project/pull/94962.diff


1 Files Affected:

- (modified) llvm/lib/Target/AArch64/AArch64FrameLowering.cpp (+8-1) 


``````````diff
diff --git a/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp b/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
index cd532671f5018..65e3bbf4e9c35 100644
--- a/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
@@ -431,8 +431,15 @@ bool AArch64FrameLowering::canUseRedZone(const MachineFunction &MF) const {
   const AArch64FunctionInfo *AFI = MF.getInfo<AArch64FunctionInfo>();
   uint64_t NumBytes = AFI->getLocalStackSize();
 
+  // If neither NEON or SVE are available, a COPY from one Q-reg to
+  // another requires a spill -> reload sequence. We can do that
+  // using a pre-decrementing store/post-decrementing load, but
+  // if we do so, we can't use the Red Zone.
+  bool LowerQRegCopyThroughMem =
+      !Subtarget.isNeonAvailable() && !Subtarget.hasSVE();
+
   return !(MFI.hasCalls() || hasFP(MF) || NumBytes > RedZoneSize ||
-           getSVEStackSize(MF));
+           getSVEStackSize(MF) || LowerQRegCopyThroughMem);
 }
 
 /// hasFP - Return true if the specified function should have a dedicated frame

``````````

</details>


https://github.com/llvm/llvm-project/pull/94962