Commit e9ac757f authored by Eli Friedman's avatar Eli Friedman
Browse files

[AArch64] Don't expand memcmp in strict align mode. 

7aecf232 fixed the bug where we would miscompile, but we still generate
a crazy amount of code. Turn off the expansion until someone implements
an appropriate heuristic.

Differential Revision: https://reviews.llvm.org/D77599
parent f596ab40

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp

+6 −1
Original line number Diff line number Diff line
@@ -629,7 +629,12 @@ int AArch64TTIImpl::getCmpSelInstrCost(unsigned Opcode, Type *ValTy, 
AArch64TTIImpl::TTI::MemCmpExpansionOptions

AArch64TTIImpl::enableMemCmpExpansion(bool OptSize, bool IsZeroCmp) const {

  TTI::MemCmpExpansionOptions Options;

  Options.AllowOverlappingLoads = !ST->requiresStrictAlign();

  if (ST->requiresStrictAlign()) {

    // TODO: Add cost modeling for strict align. Misaligned loads expand to

    // a bunch of instructions when strict align is enabled.

    return Options;

  }

  Options.AllowOverlappingLoads = true;

  Options.MaxNumLoads = TLI->getMaxExpandSizeMemcmp(OptSize);

  Options.NumLoadsPerBlock = Options.MaxNumLoads;

  // TODO: Though vector loads usually perform well on AArch64, in some targets

llvm/test/CodeGen/AArch64/bcmp-inline-small.ll

+5 −11
Original line number Diff line number Diff line
@@ -11,12 +11,12 @@ entry: 
  ret i1 %ret



; CHECK-LABEL: test_b2:

; CHECK-NOT:   bl bcmp

; CHECKN-NOT:  bl bcmp

; CHECKN:      ldr  x

; CHECKN-NEXT: ldr  x

; CHECKN-NEXT: ldur x

; CHECKN-NEXT: ldur x

; CHECKS-COUNT-30: ldrb w

; CHECKS: bl bcmp

}



define i1 @test_b2_align8(i8* align 8 %s1, i8* align 8 %s2) {
@@ -26,19 +26,13 @@ entry: 
  ret i1 %ret



; CHECK-LABEL: test_b2_align8:

; CHECK-NOT:   bl bcmp

; CHECKN-NOT:  bl bcmp

; CHECKN:      ldr  x

; CHECKN-NEXT: ldr  x

; CHECKN-NEXT: ldur x

; CHECKN-NEXT: ldur x

; CHECKS:      ldr  x

; CHECKS-NEXT: ldr  x

; CHECKS-NEXT: ldr  w

; CHECKS-NEXT: ldr  w

; CHECKS-NEXT: ldrh  w

; CHECKS-NEXT: ldrh  w

; CHECKS-NEXT: ldrb  w

; CHECKS-NEXT: ldrb  w

; TODO: Four loads should be within the limit, but the heuristic isn't implemented.

; CHECKS: bl bcmp

}



define i1 @test_bs(i8* %s1, i8* %s2) optsize {