[WASM] Expand memcmp for small size

llvm · badumbatish · Jul 20, 2025 · Jul 11, 2025 · Jul 11, 2025 · Jul 13, 2025
commit d5103f62e28173fff2c720a64bbe32a7db9cdd22
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
@@ -46,6 +46,10 @@ WebAssemblyTargetLowering::WebAssemblyTargetLowering(
     : TargetLowering(TM), Subtarget(&STI) {
   auto MVTPtr = Subtarget->hasAddr64() ? MVT::i64 : MVT::i32;
 
+  // Set the load count for memcmp expand optimization
+  MaxLoadsPerMemcmp = 3;
+  MaxLoadsPerMemcmpOptSize = 2;
+
   // Booleans always contain 0 or 1.
   setBooleanContents(ZeroOrOneBooleanContent);
   // Except in SIMD vectors

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyTargetTransformInfo.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyTargetTransformInfo.cpp
@@ -141,6 +141,16 @@ InstructionCost WebAssemblyTTIImpl::getCastInstrCost(
   return BaseT::getCastInstrCost(Opcode, Dst, Src, CCH, CostKind, I);
 }
 
+WebAssemblyTTIImpl::TTI::MemCmpExpansionOptions
+WebAssemblyTTIImpl::enableMemCmpExpansion(bool OptSize, bool IsZeroCmp) const {
+  TTI::MemCmpExpansionOptions Options;
+  // INFO: I'm not sure what determines this, setting 2 conservatively
+  Options.NumLoadsPerBlock = 2;
+  Options.LoadSizes.append({8, 4, 2, 1});
+  Options.MaxNumLoads = TLI->getMaxExpandSizeMemcmp(OptSize);
+  return Options;
+}
+
 InstructionCost WebAssemblyTTIImpl::getMemoryOpCost(
     unsigned Opcode, Type *Ty, Align Alignment, unsigned AddressSpace,
     TTI::TargetCostKind CostKind, TTI::OperandValueInfo OpInfo,

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyTargetTransformInfo.h b/llvm/lib/Target/WebAssembly/WebAssemblyTargetTransformInfo.h
@@ -73,6 +73,10 @@ class WebAssemblyTTIImpl final : public BasicTTIImplBase<WebAssemblyTTIImpl> {
   getCastInstrCost(unsigned Opcode, Type *Dst, Type *Src,
                    TTI::CastContextHint CCH, TTI::TargetCostKind CostKind,
                    const Instruction *I = nullptr) const override;
+
+  TTI::MemCmpExpansionOptions
+  enableMemCmpExpansion(bool OptSize, bool IsZeroCmp) const override;
+
   InstructionCost getMemoryOpCost(
       unsigned Opcode, Type *Src, Align Alignment, unsigned AddressSpace,
       TTI::TargetCostKind CostKind,

diff --git a/llvm/test/CodeGen/WebAssembly/memcmp-expand.ll b/llvm/test/CodeGen/WebAssembly/memcmp-expand.ll
@@ -9,10 +9,21 @@ define i1 @memcmp_expand_3(ptr %a, ptr %b) {
 ; CHECK-LABEL: memcmp_expand_3:
 ; CHECK:         .functype memcmp_expand_3 (i32, i32) -> (i32)
 ; CHECK-NEXT:  # %bb.0:
-; CHECK-NEXT:    i32.const $push0=, 3
-; CHECK-NEXT:    call $push1=, memcmp, $0, $1, $pop0
-; CHECK-NEXT:    i32.eqz $push2=, $pop1
-; CHECK-NEXT:    return $pop2
+; CHECK-NEXT:    i32.load16_u $push7=, 0($0):p2align=0
+; CHECK-NEXT:    i32.load16_u $push6=, 0($1):p2align=0
+; CHECK-NEXT:    i32.xor $push8=, $pop7, $pop6
+; CHECK-NEXT:    i32.const $push0=, 2
+; CHECK-NEXT:    i32.add $push3=, $0, $pop0
+; CHECK-NEXT:    i32.load8_u $push4=, 0($pop3)
+; CHECK-NEXT:    i32.const $push13=, 2
+; CHECK-NEXT:    i32.add $push1=, $1, $pop13
+; CHECK-NEXT:    i32.load8_u $push2=, 0($pop1)
+; CHECK-NEXT:    i32.xor $push5=, $pop4, $pop2
+; CHECK-NEXT:    i32.or $push9=, $pop8, $pop5
+; CHECK-NEXT:    i32.const $push10=, 65535
+; CHECK-NEXT:    i32.and $push11=, $pop9, $pop10
+; CHECK-NEXT:    i32.eqz $push12=, $pop11
+; CHECK-NEXT:    return $pop12
   %cmp_3 = call i32 @memcmp(ptr %a, ptr %b, i32 3)
   %res = icmp eq i32 %cmp_3, 0
   ret i1 %res
@@ -22,10 +33,19 @@ define i1 @memcmp_expand_5(ptr %a, ptr %b) {
 ; CHECK-LABEL: memcmp_expand_5:
 ; CHECK:         .functype memcmp_expand_5 (i32, i32) -> (i32)
 ; CHECK-NEXT:  # %bb.0:
-; CHECK-NEXT:    i32.const $push0=, 5
-; CHECK-NEXT:    call $push1=, memcmp, $0, $1, $pop0
-; CHECK-NEXT:    i32.eqz $push2=, $pop1
-; CHECK-NEXT:    return $pop2
+; CHECK-NEXT:    i32.load $push7=, 0($0):p2align=0
+; CHECK-NEXT:    i32.load $push6=, 0($1):p2align=0
+; CHECK-NEXT:    i32.xor $push8=, $pop7, $pop6
+; CHECK-NEXT:    i32.const $push0=, 4
+; CHECK-NEXT:    i32.add $push3=, $0, $pop0
+; CHECK-NEXT:    i32.load8_u $push4=, 0($pop3)
+; CHECK-NEXT:    i32.const $push11=, 4
+; CHECK-NEXT:    i32.add $push1=, $1, $pop11
+; CHECK-NEXT:    i32.load8_u $push2=, 0($pop1)
+; CHECK-NEXT:    i32.xor $push5=, $pop4, $pop2
+; CHECK-NEXT:    i32.or $push9=, $pop8, $pop5
+; CHECK-NEXT:    i32.eqz $push10=, $pop9
+; CHECK-NEXT:    return $pop10
   %cmp_5 = call i32 @memcmp(ptr %a, ptr %b, i32 5)
   %res = icmp eq i32 %cmp_5, 0
   ret i1 %res
@@ -35,10 +55,37 @@ define i1 @memcmp_expand_7(ptr %a, ptr %b) {
 ; CHECK-LABEL: memcmp_expand_7:
 ; CHECK:         .functype memcmp_expand_7 (i32, i32) -> (i32)
 ; CHECK-NEXT:  # %bb.0:
-; CHECK-NEXT:    i32.const $push0=, 7
-; CHECK-NEXT:    call $push1=, memcmp, $0, $1, $pop0
-; CHECK-NEXT:    i32.eqz $push2=, $pop1
-; CHECK-NEXT:    return $pop2
+; CHECK-NEXT:    block
+; CHECK-NEXT:    block
+; CHECK-NEXT:    i32.load $push7=, 0($0):p2align=0
+; CHECK-NEXT:    i32.load $push6=, 0($1):p2align=0
+; CHECK-NEXT:    i32.xor $push8=, $pop7, $pop6
+; CHECK-NEXT:    i32.const $push0=, 4
+; CHECK-NEXT:    i32.add $push3=, $0, $pop0
+; CHECK-NEXT:    i32.load16_u $push4=, 0($pop3):p2align=0
+; CHECK-NEXT:    i32.const $push17=, 4
+; CHECK-NEXT:    i32.add $push1=, $1, $pop17
+; CHECK-NEXT:    i32.load16_u $push2=, 0($pop1):p2align=0
+; CHECK-NEXT:    i32.xor $push5=, $pop4, $pop2
+; CHECK-NEXT:    i32.or $push9=, $pop8, $pop5
+; CHECK-NEXT:    br_if 0, $pop9 # 0: down to label1
+; CHECK-NEXT:  # %bb.1: # %loadbb1
+; CHECK-NEXT:    i32.const $2=, 0
+; CHECK-NEXT:    i32.const $push10=, 6
+; CHECK-NEXT:    i32.add $push13=, $0, $pop10
+; CHECK-NEXT:    i32.load8_u $push14=, 0($pop13)
+; CHECK-NEXT:    i32.const $push18=, 6
+; CHECK-NEXT:    i32.add $push11=, $1, $pop18
+; CHECK-NEXT:    i32.load8_u $push12=, 0($pop11)
+; CHECK-NEXT:    i32.eq $push15=, $pop14, $pop12
+; CHECK-NEXT:    br_if 1, $pop15 # 1: down to label0
+; CHECK-NEXT:  .LBB2_2: # %res_block
+; CHECK-NEXT:    end_block # label1:
+; CHECK-NEXT:    i32.const $2=, 1
+; CHECK-NEXT:  .LBB2_3: # %endblock
+; CHECK-NEXT:    end_block # label0:
+; CHECK-NEXT:    i32.eqz $push16=, $2
+; CHECK-NEXT:    return $pop16
   %cmp_7 = call i32 @memcmp(ptr %a, ptr %b, i32 7)
   %res = icmp eq i32 %cmp_7, 0
   ret i1 %res
@@ -76,10 +123,19 @@ define i1 @memcmp_expand_16(ptr %a, ptr %b) {
 ; CHECK-LABEL: memcmp_expand_16:
 ; CHECK:         .functype memcmp_expand_16 (i32, i32) -> (i32)
 ; CHECK-NEXT:  # %bb.0:
-; CHECK-NEXT:    i32.const $push0=, 16
-; CHECK-NEXT:    call $push1=, memcmp, $0, $1, $pop0
-; CHECK-NEXT:    i32.eqz $push2=, $pop1
-; CHECK-NEXT:    return $pop2
+; CHECK-NEXT:    i64.load $push7=, 0($0):p2align=0
+; CHECK-NEXT:    i64.load $push6=, 0($1):p2align=0
+; CHECK-NEXT:    i64.xor $push8=, $pop7, $pop6
+; CHECK-NEXT:    i32.const $push0=, 8
+; CHECK-NEXT:    i32.add $push3=, $0, $pop0
+; CHECK-NEXT:    i64.load $push4=, 0($pop3):p2align=0
+; CHECK-NEXT:    i32.const $push11=, 8
+; CHECK-NEXT:    i32.add $push1=, $1, $pop11
+; CHECK-NEXT:    i64.load $push2=, 0($pop1):p2align=0
+; CHECK-NEXT:    i64.xor $push5=, $pop4, $pop2
+; CHECK-NEXT:    i64.or $push9=, $pop8, $pop5
+; CHECK-NEXT:    i64.eqz $push10=, $pop9
+; CHECK-NEXT:    return $pop10
   %cmp_16 = call i32 @memcmp(ptr %a, ptr %b, i32 16)
   %res = icmp eq i32 %cmp_16, 0
   ret i1 %res