Normalize memcmp constant folding results.

The library call simplifier folds memcmp calls with all constant arguments
to a constant.  For example:

  memcmp("foo", "foo", 3) ->  0
  memcmp("hel", "foo", 3) ->  1
  memcmp("foo", "hel", 3) -> -1

The folding is implemented in terms of the system memcmp that LLVM gets
linked with.  It currently just blindly uses the value returned from
the system memcmp as the folded constant.

This patch normalizes the values returned from the system memcmp to
(-1, 0, 1) so that we get consistent results across multiple platforms.
The test cases were adjusted accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167726 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/SimplifyLibCalls.cpp

@@ -951,7 +951,14 @@ struct MemCmpOpt : public LibCallOptimization {
       // Make sure we're not reading out-of-bounds memory.
       if (Len > LHSStr.size() || Len > RHSStr.size())
         return 0;
-      uint64_t Ret = memcmp(LHSStr.data(), RHSStr.data(), Len);
+      // Fold the memcmp and normalize the result.  This way we get consistent
+      // results across multiple platforms.
+      uint64_t Ret = 0;
+      int Cmp = memcmp(LHSStr.data(), RHSStr.data(), Len);
+      if (Cmp < 0)
+        Ret = -1;
+      else if (Cmp > 0)
+        Ret = 1;
       return ConstantInt::get(CI->getType(), Ret);
     }
 

test/Transforms/InstCombine/memcmp-1.ll

@@ -59,7 +59,7 @@ define i32 @test_simplify5() {
   %mem2 = getelementptr [4 x i8]* @foo, i32 0, i32 0
   %ret = call i32 @memcmp(i8* %mem1, i8* %mem2, i32 3)
   ret i32 %ret
-; CHECK: ret i32 {{[0-9]+}}
+; CHECK: ret i32 1
 }
 
 define i32 @test_simplify6() {
@@ -68,5 +68,5 @@ define i32 @test_simplify6() {
   %mem2 = getelementptr [4 x i8]* @hel, i32 0, i32 0
   %ret = call i32 @memcmp(i8* %mem1, i8* %mem2, i32 3)
   ret i32 %ret
-; CHECK: ret i32 {{-[0-9]+}}
+; CHECK: ret i32 -1
 }