[PATCH] Optimize memcmp(x, y, n)==0 for small n and suitably aligned x/y.

[Eelis]

This fixes http://llvm.org/bugs/show_bug.cgi?id=20673

http://reviews.llvm.org/D6952

Files:
  lib/Transforms/Utils/SimplifyLibCalls.cpp
  test/Transforms/InstCombine/memcmp-1.ll

Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -32,6 +32,7 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Target/TargetLibraryInfo.h"
 #include "llvm/Transforms/Utils/BuildLibCalls.h"
+#include "llvm/Transforms/Utils/Local.h"
 
 using namespace llvm;
 using namespace PatternMatch;
@@ -949,6 +950,27 @@
     return B.CreateSub(LHSV, RHSV, "chardiff");
   }
 
+  // memcmp(S1,S2,8)==0 -> (*(int64_t*)S1 != *(int64_t*)S2)==0
+  // (And same for 16/32 bit.)
+  if (Len <= 8 && !(Len & (Len-1))
+      && isOnlyUsedInZeroEqualityComparison(CI)
+      && getKnownAlignment(LHS, DL, nullptr, CI) >= Len
+      && getKnownAlignment(RHS, DL, nullptr, CI) >= Len) {
+
+    if (Type *IntType = IntegerType::get(CI->getContext(), Len << 3)) {
+
+      Type *LHSPtrTy = PointerType::get(IntType,
+        cast<PointerType>(LHS->getType())->getAddressSpace());
+      Type *RHSPtrTy = PointerType::get(IntType,
+        cast<PointerType>(RHS->getType())->getAddressSpace());
+
+      Value *LHSV = B.CreateLoad(B.CreateBitCast(LHS, LHSPtrTy, "lhsc"), "lhsv");
+      Value *RHSV = B.CreateLoad(B.CreateBitCast(RHS, RHSPtrTy, "rhsc"), "rhsv");
+
+      return B.CreateZExt(B.CreateICmpNE(LHSV, RHSV), CI->getType(), "memcmp");
+    }
+  }
+
   // Constant folding: memcmp(x, y, l) -> cnst (all arguments are constant)
   StringRef LHSStr, RHSStr;
   if (getConstantStringInfo(LHS, LHSStr) &&
Index: test/Transforms/InstCombine/memcmp-1.ll
===================================================================
--- test/Transforms/InstCombine/memcmp-1.ll
+++ test/Transforms/InstCombine/memcmp-1.ll
@@ -70,3 +70,54 @@
   ret i32 %ret
 ; CHECK: ret i32 -1
 }
+
+; Check memcmp(mem1, mem2, 8)==0 -> *(int64_t*)mem1 == *(int64_t*)mem2
+
+define i1 @test_simplify7(i64 %x, i64 %y) {
+; CHECK-label: @test_simplify7(
+  %x.addr = alloca i64, align 8
+  %y.addr = alloca i64, align 8
+  store i64 %x, i64* %x.addr, align 8
+  store i64 %y, i64* %y.addr, align 8
+  %xptr = bitcast i64* %x.addr to i8*
+  %yptr = bitcast i64* %y.addr to i8*
+  %call = call i32 @memcmp(i8* %xptr, i8* %yptr, i32 8)
+  %cmp = icmp eq i32 %call, 0
+  ret i1 %cmp
+; CHECK: %cmp = icmp eq i64 %x, %y
+; CHECK: ret i1 %cmp
+}
+
+; Check memcmp(mem1, mem2, 4)==0 -> *(int32_t*)mem1 == *(int32_t*)mem2
+
+define i1 @test_simplify8(i32 %x, i32 %y) {
+; CHECK-label: @test_simplify8(
+  %x.addr = alloca i32, align 4
+  %y.addr = alloca i32, align 4
+  store i32 %x, i32* %x.addr, align 4
+  store i32 %y, i32* %y.addr, align 4
+  %xptr = bitcast i32* %x.addr to i8*
+  %yptr = bitcast i32* %y.addr to i8*
+  %call = call i32 @memcmp(i8* %xptr, i8* %yptr, i32 4)
+  %cmp = icmp eq i32 %call, 0
+  ret i1 %cmp
+; CHECK: %cmp = icmp eq i32 %x, %y
+; CHECK: ret i1 %cmp
+}
+
+; Check memcmp(mem1, mem2, 2)==0 -> *(int16_t*)mem1 == *(int16_t*)mem2
+
+define i1 @test_simplify9(i16 %x, i16 %y) {
+; CHECK-label: @test_simplify9(
+  %x.addr = alloca i16, align 2
+  %y.addr = alloca i16, align 2
+  store i16 %x, i16* %x.addr, align 2
+  store i16 %y, i16* %y.addr, align 2
+  %xptr = bitcast i16* %x.addr to i8*
+  %yptr = bitcast i16* %y.addr to i8*
+  %call = call i32 @memcmp(i8* %xptr, i8* %yptr, i32 2)
+  %cmp = icmp eq i32 %call, 0
+  ret i1 %cmp
+; CHECK: %cmp = icmp eq i16 %x, %y
+; CHECK: ret i1 %cmp
+}

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