[ExpandMemCmp] Honor prefer-vector-width.

[craig.topper]

LGTM

[courbet]

Thanks.

llvm/lib/Target/X86/X86TargetTransformInfo.cpp

X86TTIImpl::enableMemCmpExpansion(bool OptSize, bool IsZeroCmp) const {
   TTI::MemCmpExpansionOptions Options;
   Options.MaxNumLoads = TLI->getMaxExpandSizeMemcmp(OptSize);
   Options.NumLoadsPerBlock = 2;
   if (IsZeroCmp) {
     // Only enable vector loads for equality comparison. Right now the vector
     // version is not as fast for three way compare (see #33329).
     // TODO: enable AVX512 when the DAG is ready.
     // if (ST->hasAVX512()) Options.LoadSizes.push_back(64);
-    if (ST->hasAVX2()) Options.LoadSizes.push_back(32);
-    if (ST->hasSSE2()) Options.LoadSizes.push_back(16);
+    const unsigned PreferredWidth = ST->getPreferVectorWidth();
+    if (PreferredWidth >= 256 && ST->hasAVX2()) Options.LoadSizes.push_back(32);
+    if (PreferredWidth >= 128 && ST->hasSSE2()) Options.LoadSizes.push_back(16);
     // All GPR and vector loads can be unaligned. SIMD compare requires integer
     // vectors (SSE2/AVX2).
     Options.AllowOverlappingLoads = true;
   }
   if (ST->is64Bit()) {
     Options.LoadSizes.push_back(8);
   }
   Options.LoadSizes.push_back(4);

llvm/test/CodeGen/X86/memcmp.ll

 ; X64-AVX2-NEXT:    sete %al
 ; X64-AVX2-NEXT:    vzeroupper
 ; X64-AVX2-NEXT:    retq
   %call = tail call i32 @memcmp(i8* %x, i8* %y, i64 32) nounwind
   %cmp = icmp eq i32 %call, 0
   ret i1 %cmp
 }
 
+define i1 @length32_eq_prefer128(i8* %x, i8* %y) nounwind "prefer-vector-width"="128" {
+; X86-NOSSE-LABEL: length32_eq_prefer128:
+; X86-NOSSE:       # %bb.0:
+; X86-NOSSE-NEXT:    pushl $0
+; X86-NOSSE-NEXT:    pushl $32
+; X86-NOSSE-NEXT:    pushl {{[0-9]+}}(%esp)
+; X86-NOSSE-NEXT:    pushl {{[0-9]+}}(%esp)
+; X86-NOSSE-NEXT:    calll memcmp
+; X86-NOSSE-NEXT:    addl $16, %esp
+; X86-NOSSE-NEXT:    testl %eax, %eax
+; X86-NOSSE-NEXT:    sete %al
+; X86-NOSSE-NEXT:    retl
+;
+; X86-SSE1-LABEL: length32_eq_prefer128:
+; X86-SSE1:       # %bb.0:
+; X86-SSE1-NEXT:    pushl $0
+; X86-SSE1-NEXT:    pushl $32
+; X86-SSE1-NEXT:    pushl {{[0-9]+}}(%esp)
+; X86-SSE1-NEXT:    pushl {{[0-9]+}}(%esp)
+; X86-SSE1-NEXT:    calll memcmp
+; X86-SSE1-NEXT:    addl $16, %esp
+; X86-SSE1-NEXT:    testl %eax, %eax
+; X86-SSE1-NEXT:    sete %al
+; X86-SSE1-NEXT:    retl
+;
+; X86-SSE2-LABEL: length32_eq_prefer128:
+; X86-SSE2:       # %bb.0:
+; X86-SSE2-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; X86-SSE2-NEXT:    movl {{[0-9]+}}(%esp), %ecx
+; X86-SSE2-NEXT:    movdqu (%ecx), %xmm0
+; X86-SSE2-NEXT:    movdqu 16(%ecx), %xmm1
+; X86-SSE2-NEXT:    movdqu (%eax), %xmm2
+; X86-SSE2-NEXT:    pcmpeqb %xmm0, %xmm2
+; X86-SSE2-NEXT:    movdqu 16(%eax), %xmm0
+; X86-SSE2-NEXT:    pcmpeqb %xmm1, %xmm0
+; X86-SSE2-NEXT:    pand %xmm2, %xmm0
+; X86-SSE2-NEXT:    pmovmskb %xmm0, %eax
+; X86-SSE2-NEXT:    cmpl $65535, %eax # imm = 0xFFFF
+; X86-SSE2-NEXT:    sete %al
+; X86-SSE2-NEXT:    retl
+;
+; X64-SSE2-LABEL: length32_eq_prefer128:
+; X64-SSE2:       # %bb.0:
+; X64-SSE2-NEXT:    movdqu (%rdi), %xmm0
+; X64-SSE2-NEXT:    movdqu 16(%rdi), %xmm1
+; X64-SSE2-NEXT:    movdqu (%rsi), %xmm2
+; X64-SSE2-NEXT:    pcmpeqb %xmm0, %xmm2
+; X64-SSE2-NEXT:    movdqu 16(%rsi), %xmm0
+; X64-SSE2-NEXT:    pcmpeqb %xmm1, %xmm0
+; X64-SSE2-NEXT:    pand %xmm2, %xmm0
+; X64-SSE2-NEXT:    pmovmskb %xmm0, %eax
+; X64-SSE2-NEXT:    cmpl $65535, %eax # imm = 0xFFFF
+; X64-SSE2-NEXT:    sete %al
+; X64-SSE2-NEXT:    retq
+;
+; X64-AVX-LABEL: length32_eq_prefer128:
+; X64-AVX:       # %bb.0:
+; X64-AVX-NEXT:    vmovdqu (%rdi), %xmm0
+; X64-AVX-NEXT:    vmovdqu 16(%rdi), %xmm1
+; X64-AVX-NEXT:    vpcmpeqb 16(%rsi), %xmm1, %xmm1
+; X64-AVX-NEXT:    vpcmpeqb (%rsi), %xmm0, %xmm0
+; X64-AVX-NEXT:    vpand %xmm1, %xmm0, %xmm0
+; X64-AVX-NEXT:    vpmovmskb %xmm0, %eax
+; X64-AVX-NEXT:    cmpl $65535, %eax # imm = 0xFFFF
+; X64-AVX-NEXT:    sete %al
+; X64-AVX-NEXT:    retq
+  %call = tail call i32 @memcmp(i8* %x, i8* %y, i64 32) nounwind
+  %cmp = icmp eq i32 %call, 0
+  ret i1 %cmp
+}
+
 define i1 @length32_eq_const(i8* %X) nounwind {
 ; X86-NOSSE-LABEL: length32_eq_const:
 ; X86-NOSSE:       # %bb.0:
 ; X86-NOSSE-NEXT:    pushl $0
 ; X86-NOSSE-NEXT:    pushl $32
 ; X86-NOSSE-NEXT:    pushl $.L.str
 ; X86-NOSSE-NEXT:    pushl {{[0-9]+}}(%esp)
 ; X86-NOSSE-NEXT:    calll memcmp