[ARM] Fix for 64-bit CAS expansion on ARM32 with -O0

Summary:
This patch fixes comparison of 64-bit atomic with its expected value in CMP_SWAP_64 expansion.

Currently, the low words are compared with CMP, while the high words are compared with SBC. SBC expects the carry flag to be set if CMP detects a difference. CMP might leave the carry unset for unequal arguments though if the first one is >= than the second. This might cause the comparison logic to detect false equality.

Example of the broken C++ code:
```
std::atomic<long long> at(2);

long long ll = 1;
std::atomic_compare_exchange_strong(&at, &ll, 3);
```
Even though the atomic `at` and the expected value `ll` are not equal and `atomic_compare_exchange_strong` returns `false`, `at` is changed to 3.

The patch replaces SBC with CMPEQ.

Reviewers: t.p.northover

Subscribers: aemerson, rengolin, llvm-commits, asl

Differential Revision: https://reviews.llvm.org/D27315

llvm-svn: 288433

1 files changed, 4 insertions, 7 deletions

diff --git a/llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp b/llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp
index e2c386348ed..95fcc8dcb45 100644
--- a/llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp
+++ b/llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp
@@ -935,13 +935,10 @@ bool ARMExpandPseudo::ExpandCMP_SWAP_64(MachineBasicBlock &MBB,
                      .addReg(DestLo, getKillRegState(Dest.isDead()))
                      .addReg(DesiredLo, getKillRegState(Desired.isDead())));
 
-  unsigned SBCrr = IsThumb ? ARM::t2SBCrr : ARM::SBCrr;
-  MIB = BuildMI(LoadCmpBB, DL, TII->get(SBCrr))
-            .addReg(StatusReg, RegState::Define | RegState::Dead)
-            .addReg(DestHi, getKillRegState(Dest.isDead()))
-            .addReg(DesiredHi, getKillRegState(Desired.isDead()));
-  AddDefaultPred(MIB);
-  MIB.addReg(ARM::CPSR, RegState::Kill);
+  BuildMI(LoadCmpBB, DL, TII->get(CMPrr))
+      .addReg(DestHi, getKillRegState(Dest.isDead()))
+      .addReg(DesiredHi, getKillRegState(Desired.isDead()))
+      .addImm(ARMCC::EQ).addReg(ARM::CPSR, RegState::Kill);
 
   unsigned Bcc = IsThumb ? ARM::tBcc : ARM::Bcc;
   BuildMI(LoadCmpBB, DL, TII->get(Bcc))