[AArch64] Expand bcmp() for small block lengths

Patch D56593 by @courbet results in calls to `bcmp()` in some cases, should
the target support the it.  Unless `TTI::MemCmpExpansionOptions()`
is overridden by the target.

In a proprietary benchmark we see a performance drop of about 12% on PNG
compression before this patch, though it passes all tests.

This patch mirrors X86 for AArch64 and initializes
`TTI::MemCmpExpansionOptions()` to then expand calls to `bcmp()` when
appropriate.  No tuning of the parameters was performed, but, at this point,
it's enough to recover the performance drop above.

This problem also exists on ARM.  Once a consensus is reached for AArch64, we
can work to fix ARM as well.

Authors:
- Evandro Menezes (@evandro) <e.menezes@samsung.com>
- Brian Rzycki (@brzycki) <b.rzycki@samsung.com>

Differential revision: https://reviews.llvm.org/D64805

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@367898 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -629,6 +629,10 @@ AArch64TargetLowering::AArch64TargetLowering(const TargetMachine &TM,
 
   MaxStoresPerMemmoveOptSize = MaxStoresPerMemmove = 4;
 
+  MaxLoadsPerMemcmpOptSize = 4;
+  MaxLoadsPerMemcmp = Subtarget->requiresStrictAlign()
+                      ? MaxLoadsPerMemcmpOptSize : 8;
+
   setStackPointerRegisterToSaveRestore(AArch64::SP);
 
   setSchedulingPreference(Sched::Hybrid);

lib/Target/AArch64/AArch64TargetTransformInfo.cpp

@@ -618,6 +618,19 @@ int AArch64TTIImpl::getCmpSelInstrCost(unsigned Opcode, Type *ValTy,
   return BaseT::getCmpSelInstrCost(Opcode, ValTy, CondTy, I);
 }
 
+AArch64TTIImpl::TTI::MemCmpExpansionOptions
+AArch64TTIImpl::enableMemCmpExpansion(bool OptSize, bool IsZeroCmp) const {
+  TTI::MemCmpExpansionOptions Options;
+  Options.AllowOverlappingLoads = !ST->requiresStrictAlign();
+  Options.MaxNumLoads = TLI->getMaxExpandSizeMemcmp(OptSize);
+  Options.NumLoadsPerBlock = Options.MaxNumLoads;
+  // TODO: Though vector loads usually perform well on AArch64, in some targets
+  // they may wake up the FP unit, which raises the power consumption.  Perhaps
+  // they could be used with no holds barred (-O3).
+  Options.LoadSizes = {8, 4, 2, 1};
+  return Options;
+}
+
 int AArch64TTIImpl::getMemoryOpCost(unsigned Opcode, Type *Ty,
                                     unsigned Alignment, unsigned AddressSpace,
                                     const Instruction *I) {

lib/Target/AArch64/AArch64TargetTransformInfo.h

@@ -130,6 +130,9 @@ public:
   int getCmpSelInstrCost(unsigned Opcode, Type *ValTy, Type *CondTy,
                          const Instruction *I = nullptr);
 
+  TTI::MemCmpExpansionOptions enableMemCmpExpansion(bool OptSize,
+                                                    bool IsZeroCmp) const;
+
   int getMemoryOpCost(unsigned Opcode, Type *Src, unsigned Alignment,
                       unsigned AddressSpace, const Instruction *I = nullptr);
 

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

@@ -0,0 +1,44 @@
+; RUN: llc -O2 < %s -mtriple=aarch64-linux-gnu                     | FileCheck %s --check-prefixes=CHECK,CHECKN
+; RUN: llc -O2 < %s -mtriple=aarch64-linux-gnu -mattr=strict-align | FileCheck %s --check-prefixes=CHECK,CHECKS
+
+declare i32 @bcmp(i8*, i8*, i64) nounwind readonly
+declare i32 @memcmp(i8*, i8*, i64) nounwind readonly
+
+define i1 @bcmp_b2(i8* %s1, i8* %s2) {
+entry:
+  %bcmp = call i32 @bcmp(i8* %s1, i8* %s2, i64 15)
+  %ret = icmp eq i32 %bcmp, 0
+  ret i1 %ret
+
+; CHECK-LABEL: bcmp_b2:
+; CHECK-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
+}
+
+define i1 @bcmp_bs(i8* %s1, i8* %s2) optsize {
+entry:
+  %memcmp = call i32 @memcmp(i8* %s1, i8* %s2, i64 31)
+  %ret = icmp eq i32 %memcmp, 0
+  ret i1 %ret
+
+; CHECK-LABEL: bcmp_bs:
+; CHECKN-NOT:  bl memcmp
+; CHECKN:      ldp  x
+; CHECKN-NEXT: ldp  x
+; CHECKN-NEXT: ldr  x
+; CHECKN-NEXT: ldr  x
+; CHECKN-NEXT: ldur x
+; CHECKN-NEXT: ldur x
+; CHECKS:      bl memcmp
+}