Index: llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -18,6 +18,7 @@
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/Triple.h"
+#include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/OptimizationDiagnosticInfo.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/Analysis/ValueTracking.h"
@@ -751,29 +752,44 @@
   }
 
   // memcmp(S1,S2,N/8)==0 -> (*(intN_t*)S1 != *(intN_t*)S2)==0
+  // TODO: The case where both inputs are constants does not need to be limited
+  // to legal integers or equality comparison. See block below this.
   if (DL.isLegalInteger(Len * 8) && isOnlyUsedInZeroEqualityComparison(CI)) {
-
     IntegerType *IntType = IntegerType::get(CI->getContext(), Len * 8);
     unsigned PrefAlignment = DL.getPrefTypeAlignment(IntType);
 
-    if (getKnownAlignment(LHS, DL, CI) >= PrefAlignment &&
-        getKnownAlignment(RHS, DL, CI) >= PrefAlignment) {
+    // First, see if we can fold either argument to a constant.
+    Value *LHSV = nullptr;
+    if (auto *LHSC = dyn_cast<Constant>(LHS)) {
+      LHSC = ConstantExpr::getBitCast(LHSC, IntType->getPointerTo());
+      LHSV = ConstantFoldLoadFromConstPtr(LHSC, IntType, DL);
+    }
+    Value *RHSV = nullptr;
+    if (auto *RHSC = dyn_cast<Constant>(RHS)) {
+      RHSC = ConstantExpr::getBitCast(RHSC, IntType->getPointerTo());
+      RHSV = ConstantFoldLoadFromConstPtr(RHSC, IntType, DL);
+    }
 
+    // Don't generate unaligned loads. If either source is constant data,
+    // alignment doesn't matter for that source because there is no load.
+    if (!LHSV && getKnownAlignment(LHS, DL, CI) >= PrefAlignment) {
       Type *LHSPtrTy =
           IntType->getPointerTo(LHS->getType()->getPointerAddressSpace());
+      LHSV = B.CreateLoad(B.CreateBitCast(LHS, LHSPtrTy), "lhsv");
+    }
+
+    if (!RHSV && getKnownAlignment(RHS, DL, CI) >= PrefAlignment) {
       Type *RHSPtrTy =
           IntType->getPointerTo(RHS->getType()->getPointerAddressSpace());
+      RHSV = B.CreateLoad(B.CreateBitCast(RHS, RHSPtrTy), "rhsv");
+    }
 
-      Value *LHSV =
-          B.CreateLoad(B.CreateBitCast(LHS, LHSPtrTy, "lhsc"), "lhsv");
-      Value *RHSV =
-          B.CreateLoad(B.CreateBitCast(RHS, RHSPtrTy, "rhsc"), "rhsv");
-
+    if (LHSV && RHSV)
       return B.CreateZExt(B.CreateICmpNE(LHSV, RHSV), CI->getType(), "memcmp");
-    }
   }
 
-  // Constant folding: memcmp(x, y, l) -> cnst (all arguments are constant)
+  // Constant folding: memcmp(x, y, Len) -> constant (all arguments are const).
+  // TODO: This is limited to i8 arrays.
   StringRef LHSStr, RHSStr;
   if (getConstantStringInfo(LHS, LHSStr) &&
       getConstantStringInfo(RHS, RHSStr)) {
Index: llvm/trunk/test/Transforms/InstCombine/memcmp-constant-fold.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/memcmp-constant-fold.ll
+++ llvm/trunk/test/Transforms/InstCombine/memcmp-constant-fold.ll
@@ -0,0 +1,65 @@
+; RUN: opt < %s -instcombine -S -data-layout=e-n32 | FileCheck %s --check-prefix=ALL --check-prefix=LE
+; RUN: opt < %s -instcombine -S -data-layout=E-n32 | FileCheck %s --check-prefix=ALL --check-prefix=BE
+
+declare i32 @memcmp(i8*, i8*, i64)
+
+; The alignment of this constant does not matter. We constant fold the load.
+
+@charbuf = private unnamed_addr constant [4 x i8] [i8 0, i8 0, i8 0, i8 1], align 1
+
+define i1 @memcmp_4bytes_unaligned_constant_i8(i8* align 4 %x) {
+; LE-LABEL: @memcmp_4bytes_unaligned_constant_i8(
+; LE-NEXT:    [[TMP1:%.*]] = bitcast i8* %x to i32*
+; LE-NEXT:    [[LHSV:%.*]] = load i32, i32* [[TMP1]], align 4
+; LE-NEXT:    [[TMP2:%.*]] = icmp eq i32 [[LHSV]], 16777216
+; LE-NEXT:    ret i1 [[TMP2]]
+;
+; BE-LABEL: @memcmp_4bytes_unaligned_constant_i8(
+; BE-NEXT:    [[TMP1:%.*]] = bitcast i8* %x to i32*
+; BE-NEXT:    [[LHSV:%.*]] = load i32, i32* [[TMP1]], align 4
+; BE-NEXT:    [[TMP2:%.*]] = icmp eq i32 [[LHSV]], 1
+; BE-NEXT:    ret i1 [[TMP2]]
+;
+  %call = tail call i32 @memcmp(i8* %x, i8* getelementptr inbounds ([4 x i8], [4 x i8]* @charbuf, i64 0, i64 0), i64 4)
+  %cmpeq0 = icmp eq i32 %call, 0
+  ret i1 %cmpeq0
+}
+
+; We still don't care about alignment of the constant. We are not limited to constant folding only i8 arrays.
+; It doesn't matter if the constant operand is the first operand to the memcmp.
+
+@intbuf_unaligned = private unnamed_addr constant [4 x i16] [i16 1, i16 2, i16 3, i16 4], align 1
+
+define i1 @memcmp_4bytes_unaligned_constant_i16(i8* align 4 %x) {
+; LE-LABEL: @memcmp_4bytes_unaligned_constant_i16(
+; LE-NEXT:    [[TMP1:%.*]] = bitcast i8* %x to i32*
+; LE-NEXT:    [[RHSV:%.*]] = load i32, i32* [[TMP1]], align 4
+; LE-NEXT:    [[TMP2:%.*]] = icmp eq i32 [[RHSV]], 131073
+; LE-NEXT:    ret i1 [[TMP2]]
+;
+; BE-LABEL: @memcmp_4bytes_unaligned_constant_i16(
+; BE-NEXT:    [[TMP1:%.*]] = bitcast i8* %x to i32*
+; BE-NEXT:    [[RHSV:%.*]] = load i32, i32* [[TMP1]], align 4
+; BE-NEXT:    [[TMP2:%.*]] = icmp eq i32 [[RHSV]], 65538
+; BE-NEXT:    ret i1 [[TMP2]]
+;
+  %call = tail call i32 @memcmp(i8* bitcast (i16* getelementptr inbounds ([4 x i16], [4 x i16]* @intbuf_unaligned, i64 0, i64 0) to i8*), i8* %x, i64 4)
+  %cmpeq0 = icmp eq i32 %call, 0
+  ret i1 %cmpeq0
+}
+
+; TODO: Any memcmp where all arguments are constants should be constant folded. Currently, we only handle i8 array constants.
+
+@intbuf = private unnamed_addr constant [2 x i32] [i32 0, i32 1], align 4
+
+define i1 @memcmp_3bytes_aligned_constant_i32(i8* align 4 %x) {
+; ALL-LABEL: @memcmp_3bytes_aligned_constant_i32(
+; ALL-NEXT:    [[CALL:%.*]] = tail call i32 @memcmp(i8* bitcast (i32* getelementptr inbounds ([2 x i32], [2 x i32]* @intbuf, i64 0, i64 1) to i8*), i8* bitcast ([2 x i32]* @intbuf to i8*), i64 3)
+; ALL-NEXT:    [[CMPEQ0:%.*]] = icmp eq i32 [[CALL]], 0
+; ALL-NEXT:    ret i1 [[CMPEQ0]]
+;
+  %call = tail call i32 @memcmp(i8* bitcast (i32* getelementptr inbounds ([2 x i32], [2 x i32]* @intbuf, i64 0, i64 1) to i8*), i8* bitcast (i32* getelementptr inbounds ([2 x i32], [2 x i32]* @intbuf, i64 0, i64 0) to i8*), i64 3)
+  %cmpeq0 = icmp eq i32 %call, 0
+  ret i1 %cmpeq0
+}
+