Fix a logic bug in inline expansion of memcpy / memset with an overlapping

load / store pair. It's not legal to use a wider load than the size of
the remaining bytes if it's the first pair of load / store.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@170018 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -3469,9 +3469,6 @@ static bool FindOptimalMemOpLowering(std::vector<EVT> &MemOps,
 
   unsigned NumMemOps = 0;
   while (Size != 0) {
-    if (++NumMemOps > Limit)
-      return false;
-
     unsigned VTSize = VT.getSizeInBits() / 8;
     while (VTSize > Size) {
       // For now, only use non-vector load / store's for the left-over pieces.
@@ -3507,7 +3504,8 @@ static bool FindOptimalMemOpLowering(std::vector<EVT> &MemOps,
       // FIXME: Only does this for 64-bit or more since we don't have proper
       // cost model for unaligned load / store.
       bool Fast;
-      if (AllowOverlap && VTSize >= 8 && NewVTSize < Size &&
+      if (NumMemOps && AllowOverlap &&
+          VTSize >= 8 && NewVTSize < Size &&
           TLI.allowsUnalignedMemoryAccesses(VT, &Fast) && Fast)
         VTSize = Size;
       else {
@@ -3516,6 +3514,9 @@ static bool FindOptimalMemOpLowering(std::vector<EVT> &MemOps,
       }
     }
 
+    if (++NumMemOps > Limit)
+      return false;
+
     MemOps.push_back(VT);
     Size -= VTSize;
   }

test/CodeGen/Mips/2012-12-12-ExpandMemcpy.ll

@@ -0,0 +1,11 @@
+; RUN: llc -march=mips64el -mcpu=mips64r2 < %s
+
+@.str = private unnamed_addr constant [7 x i8] c"hello\0A\00", align 1
+
+define void @t(i8* %ptr) {
+entry:
+  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %ptr, i8* getelementptr inbounds ([7 x i8]* @.str, i64 0, i64 0), i64 7, i32 1, i1 false)
+  ret void
+}
+
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture, i64, i32, i1) nounwind