AArch64: fix 128-bit cmpxchg at -O0 (again, again).

This time the issue is fortunately just a simple mistake rather than a horrible
design spectre. I thought SUBS/SBCS provided sufficient NZCV flags for
comparing two 64-bit values, but they don't.

The fix is slightly clunkier in AArch64 because we can't use conditional
execution to emit a pair of CMPs. Traditionally an "icmp ne i128" would map to
an EOR/EOR/ORR/CBNZ, but that uses more registers so it's easier to go with a
CSET/CINC/CBNZ combination. Slightly less efficient, but this is -O0 anyway.

Thanks to Anton Korobeynikov for pointing out the issue.

llvm-svn: 288418

1 files changed, 14 insertions, 6 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64ExpandPseudoInsts.cpp b/llvm/lib/Target/AArch64/AArch64ExpandPseudoInsts.cpp
index a708a171cfc..94b208a8e7d 100644
--- a/llvm/lib/Target/AArch64/AArch64ExpandPseudoInsts.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ExpandPseudoInsts.cpp
@@ -712,13 +712,21 @@ bool AArch64ExpandPseudo::expandCMP_SWAP_128(
       .addReg(DestLo.getReg(), getKillRegState(DestLo.isDead()))
       .addOperand(DesiredLo)
       .addImm(0);
-  BuildMI(LoadCmpBB, DL, TII->get(AArch64::SBCSXr), AArch64::XZR)
+  BuildMI(LoadCmpBB, DL, TII->get(AArch64::CSINCWr), StatusReg)
+    .addUse(AArch64::WZR)
+    .addUse(AArch64::WZR)
+    .addImm(AArch64CC::EQ);
+  BuildMI(LoadCmpBB, DL, TII->get(AArch64::SUBSXrs), AArch64::XZR)
       .addReg(DestHi.getReg(), getKillRegState(DestHi.isDead()))
-      .addOperand(DesiredHi);
-  BuildMI(LoadCmpBB, DL, TII->get(AArch64::Bcc))
-      .addImm(AArch64CC::NE)
-      .addMBB(DoneBB)
-      .addReg(AArch64::NZCV, RegState::Implicit | RegState::Kill);
+      .addOperand(DesiredHi)
+      .addImm(0);
+  BuildMI(LoadCmpBB, DL, TII->get(AArch64::CSINCWr), StatusReg)
+      .addUse(StatusReg, RegState::Kill)
+      .addUse(StatusReg, RegState::Kill)
+      .addImm(AArch64CC::EQ);
+  BuildMI(LoadCmpBB, DL, TII->get(AArch64::CBNZW))
+      .addUse(StatusReg, RegState::Kill)
+      .addMBB(DoneBB);
   LoadCmpBB->addSuccessor(DoneBB);
   LoadCmpBB->addSuccessor(StoreBB);