fix an issue in IsPointerOffset that prevented us from recognizing that

P and P+1 are relative to the same base pointer.

llvm-svn: 123087

2 files changed, 31 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
index a4f60878e6f..c2844375c44 100644
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -71,13 +71,30 @@ static int64_t GetOffsetFromIndex(const GetElementPtrInst *GEP, unsigned Idx,
 /// be &A[42], and Ptr2 might be &A[40].  In this case offset would be -8.
 static bool IsPointerOffset(Value *Ptr1, Value *Ptr2, int64_t &Offset,
                             const TargetData &TD) {
+  Ptr1 = Ptr1->stripPointerCasts();
+  Ptr2 = Ptr2->stripPointerCasts();
+  GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(Ptr1);
+  GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(Ptr2);
+  
+  bool VariableIdxFound = false;
+
+  // If one pointer is a GEP and the other isn't, then see if the GEP is a
+  // constant offset from the base, as in "P" and "gep P, 1".
+  if (GEP1 && GEP2 == 0 && GEP1->getOperand(0)->stripPointerCasts() == Ptr2) {
+    Offset = -GetOffsetFromIndex(GEP1, 1, VariableIdxFound, TD);
+    return !VariableIdxFound;
+  }
+
+  if (GEP2 && GEP1 == 0 && GEP2->getOperand(0)->stripPointerCasts() == Ptr1) {
+    Offset = GetOffsetFromIndex(GEP2, 1, VariableIdxFound, TD);
+    return !VariableIdxFound;
+  }
+  
   // Right now we handle the case when Ptr1/Ptr2 are both GEPs with an identical
   // base.  After that base, they may have some number of common (and
   // potentially variable) indices.  After that they handle some constant
   // offset, which determines their offset from each other.  At this point, we
   // handle no other case.
-  GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(Ptr1);
-  GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(Ptr2);
   if (!GEP1 || !GEP2 || GEP1->getOperand(0) != GEP2->getOperand(0))
     return false;
   
@@ -87,7 +104,6 @@ static bool IsPointerOffset(Value *Ptr1, Value *Ptr2, int64_t &Offset,
     if (GEP1->getOperand(Idx) != GEP2->getOperand(Idx))
       break;
 
-  bool VariableIdxFound = false;
   int64_t Offset1 = GetOffsetFromIndex(GEP1, Idx, VariableIdxFound, TD);
   int64_t Offset2 = GetOffsetFromIndex(GEP2, Idx, VariableIdxFound, TD);
   if (VariableIdxFound) return false;
diff --git a/llvm/test/Transforms/MemCpyOpt/form-memset.ll b/llvm/test/Transforms/MemCpyOpt/form-memset.ll
index 04bd6742efe..523615d72d2 100644
--- a/llvm/test/Transforms/MemCpyOpt/form-memset.ll
+++ b/llvm/test/Transforms/MemCpyOpt/form-memset.ll
@@ -177,6 +177,18 @@ entry:
 ; CHECK: call void @llvm.memset.p0i8.i64(i8* %1, i8 0, i64 15, i32 4, i1 false)
 }
 
+define void @test4(i32* nocapture %P) nounwind ssp {
+entry:
+  store i32 0, i32* %P, align 4
+  %add.ptr = getelementptr inbounds i32* %P, i64 1
+  %0 = bitcast i32* %add.ptr to i8*
+  tail call void @llvm.memset.p0i8.i64(i8* %0, i8 0, i64 11, i32 1, i1 false)
+  ret void
+; CHECK: @test4
+; CHECK-NOT: store
+; CHECK: call void @llvm.memset.p0i8.i64(i8* %1, i8 0, i64 15, i32 4, i1 false)
+}
+
 declare void @llvm.memset.p0i8.i64(i8* nocapture, i8, i64, i32, i1) nounwind