[PATCH] D27315: [ARM] Fix for 64-bit CAS expansion on ARM32 with -O0

[Oleg Ranevskyy via Phabricator via llvm-commits]

iid_iunknown created this revision.
iid_iunknown added a reviewer: t.p.northover.
iid_iunknown added subscribers: asl, llvm-commits.
iid_iunknown set the repository for this revision to rL LLVM.
Herald added subscribers: rengolin, aemerson.

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.


Repository:
  rL LLVM

https://reviews.llvm.org/D27315

Files:
  lib/Target/ARM/ARMExpandPseudoInsts.cpp
  test/CodeGen/ARM/cmpxchg-O0.ll


Index: test/CodeGen/ARM/cmpxchg-O0.ll
===================================================================
--- test/CodeGen/ARM/cmpxchg-O0.ll
+++ test/CodeGen/ARM/cmpxchg-O0.ll
@@ -69,9 +69,9 @@
 ; CHECK: [[RETRY:.LBB[0-9]+_[0-9]+]]:
 ; CHECK:     ldrexd [[OLDLO:r[0-9]+]], [[OLDHI:r[0-9]+]], [r0]
 ; CHECK:     cmp [[OLDLO]], r6
-; CHECK:     sbcs{{(\.w)?}} [[STATUS:r[0-9]+]], [[OLDHI]], r7
+; CHECK:     cmpeq [[OLDHI]], r7
 ; CHECK:     bne [[DONE:.LBB[0-9]+_[0-9]+]]
-; CHECK:     strexd [[STATUS]], r4, r5, [r0]
+; CHECK:     strexd [[STATUS:r[0-9]+]], r4, r5, [r0]
 ; CHECK:     cmp{{(\.w)?}} [[STATUS]], #0
 ; CHECK:     bne [[RETRY]]
 ; CHECK: [[DONE]]:
@@ -87,9 +87,9 @@
 ; CHECK: [[RETRY:.LBB[0-9]+_[0-9]+]]:
 ; CHECK:     ldrexd [[OLDLO:r[0-9]+]], [[OLDHI:r[0-9]+]], [r0]
 ; CHECK:     cmp [[OLDLO]], {{r[0-9]+}}
-; CHECK:     sbcs{{(\.w)?}} [[STATUS:r[0-9]+]], [[OLDHI]], {{r[0-9]+}}
+; CHECK:     cmpeq [[OLDHI]], {{r[0-9]+}}
 ; CHECK:     bne [[DONE:.LBB[0-9]+_[0-9]+]]
-; CHECK:     strexd [[STATUS]], {{r[0-9]+}}, {{r[0-9]+}}, [r0]
+; CHECK:     strexd [[STATUS:r[0-9]+]], {{r[0-9]+}}, {{r[0-9]+}}, [r0]
 ; CHECK:     cmp{{(\.w)?}} [[STATUS]], #0
 ; CHECK:     bne [[RETRY]]
 ; CHECK: [[DONE]]:
Index: lib/Target/ARM/ARMExpandPseudoInsts.cpp
===================================================================
--- lib/Target/ARM/ARMExpandPseudoInsts.cpp
+++ lib/Target/ARM/ARMExpandPseudoInsts.cpp
@@ -935,13 +935,10 @@
                      .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)
+  MIB = BuildMI(LoadCmpBB, DL, TII->get(CMPrr))
             .addReg(DestHi, getKillRegState(Dest.isDead()))
-            .addReg(DesiredHi, getKillRegState(Desired.isDead()));
-  AddDefaultPred(MIB);
-  MIB.addReg(ARM::CPSR, RegState::Kill);
+            .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))


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