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

This was pointed out in PR #93940.

Author: sdesmalen-arm

llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

@@ -431,8 +431,16 @@ 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.hasFPARMv8() &&
+                                 !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

llvm/test/CodeGen/AArch64/arm64-redzone.ll

@@ -16,3 +16,16 @@ define i32 @foo(i32 %a, i32 %b) nounwind ssp {
   %tmp2 = load i32, ptr %x, align 4
   ret i32 %tmp2
 }
+
+; We disable red-zone if NEON is available because copies of Q-regs
+; require a spill/fill and dynamic allocation. But we only need to do
+; this when FP registers are enabled.
+define void @bar(fp128 %f) "target-features"="-fp-armv8" {
+; CHECK-LABEL: bar:
+; CHECK: // %bb.0:
+; CHECK-NEXT: stp x0, x1, [sp, #-16]
+; CHECK-NEXT: ret
+  %ptr = alloca fp128
+  store fp128 %f, ptr %ptr
+  ret void
+}