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

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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
This commit is contained in:
Tim Northover 2016-12-01 21:31:59 +00:00
parent d9082699df
commit bde2af05d4
2 changed files with 22 additions and 10 deletions
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20
lib/Target/AArch64/AArch64ExpandPseudoInsts.cpp
View File

@ -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);

12
test/CodeGen/AArch64/cmpxchg-O0.ll
View File

@ -65,8 +65,10 @@ define { i128, i1 } @test_cmpxchg_128(i128* %addr, i128 %desired, i128 %new) nou
; CHECK: [[RETRY:.LBB[0-9]+_[0-9]+]]:
; CHECK: ldaxp [[OLD_LO:x[0-9]+]], [[OLD_HI:x[0-9]+]], [x0]
; CHECK: cmp [[OLD_LO]], x2
; CHECK: sbcs xzr, [[OLD_HI]], x3
; CHECK: b.ne [[DONE:.LBB[0-9]+_[0-9]+]]
; CHECK: cset [[CMP_TMP:w[0-9]+]], ne
; CHECK: cmp [[OLD_HI]], x3
; CHECK: cinc [[CMP:w[0-9]+]], [[CMP_TMP]], ne
; CHECK: cbnz [[CMP]], [[DONE:.LBB[0-9]+_[0-9]+]]
; CHECK: stlxp [[STATUS:w[0-9]+]], x4, x5, [x0]
; CHECK: cbnz [[STATUS]], [[RETRY]]
; CHECK: [[DONE]]:
@ -88,8 +90,10 @@ define {i128, i1} @test_cmpxchg_128_unsplit(i128* %addr) {
; CHECK: [[RETRY:.LBB[0-9]+_[0-9]+]]:
; CHECK: ldaxp [[OLD_LO:x[0-9]+]], [[OLD_HI:x[0-9]+]], [x0]
; CHECK: cmp [[OLD_LO]], [[DESIRED_LO]]
; CHECK: sbcs xzr, [[OLD_HI]], [[DESIRED_HI]]
; CHECK: b.ne [[DONE:.LBB[0-9]+_[0-9]+]]
; CHECK: cset [[CMP_TMP:w[0-9]+]], ne
; CHECK: cmp [[OLD_HI]], [[DESIRED_HI]]
; CHECK: cinc [[CMP:w[0-9]+]], [[CMP_TMP]], ne
; CHECK: cbnz [[CMP]], [[DONE:.LBB[0-9]+_[0-9]+]]
; CHECK: stlxp [[STATUS:w[0-9]+]], [[NEW_LO]], [[NEW_HI]], [x0]
; CHECK: cbnz [[STATUS]], [[RETRY]]
; CHECK: [[DONE]]: