This just in, it is a *bad idea* to use 'udiv' on an offset of

a pointer. A very bad idea. Let's not do that. Fixes PR14105.

Note that this wasn't *that* glaring of an oversight. Originally, these
routines were only called on offsets within an alloca, which are
intrinsically positive. But over the evolution of the pass, they ended
up being called for arbitrary offsets, and things went downhill...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166095 91177308-0d34-0410-b5e6-96231b3b80d8

Author: chandlerc

lib/Transforms/Scalar/SROA.cpp

@@ -1828,7 +1828,7 @@ static Value *getNaturalGEPRecursively(IRBuilder<> &IRB, const DataLayout &TD,
     if (ElementSizeInBits % 8)
       return 0; // GEPs over non-multiple of 8 size vector elements are invalid.
     APInt ElementSize(Offset.getBitWidth(), ElementSizeInBits / 8);
-    APInt NumSkippedElements = Offset.udiv(ElementSize);
+    APInt NumSkippedElements = Offset.sdiv(ElementSize);
     if (NumSkippedElements.ugt(VecTy->getNumElements()))
       return 0;
     Offset -= NumSkippedElements * ElementSize;
@@ -1840,7 +1840,7 @@ static Value *getNaturalGEPRecursively(IRBuilder<> &IRB, const DataLayout &TD,
   if (ArrayType *ArrTy = dyn_cast<ArrayType>(Ty)) {
     Type *ElementTy = ArrTy->getElementType();
     APInt ElementSize(Offset.getBitWidth(), TD.getTypeAllocSize(ElementTy));
-    APInt NumSkippedElements = Offset.udiv(ElementSize);
+    APInt NumSkippedElements = Offset.sdiv(ElementSize);
     if (NumSkippedElements.ugt(ArrTy->getNumElements()))
       return 0;
 
@@ -1896,7 +1896,7 @@ static Value *getNaturalGEPWithOffset(IRBuilder<> &IRB, const DataLayout &TD,
   APInt ElementSize(Offset.getBitWidth(), TD.getTypeAllocSize(ElementTy));
   if (ElementSize == 0)
     return 0; // Zero-length arrays can't help us build a natural GEP.
-  APInt NumSkippedElements = Offset.udiv(ElementSize);
+  APInt NumSkippedElements = Offset.sdiv(ElementSize);
 
   Offset -= NumSkippedElements * ElementSize;
   Indices.push_back(IRB.getInt(NumSkippedElements));

test/Transforms/SROA/basictest.ll

@@ -1063,3 +1063,23 @@ entry:
   call void @llvm.lifetime.end(i64 -1, i8* %0)
   ret void
 }
+
+define void @PR14105({ [16 x i8] }* %ptr) {
+; Ensure that when rewriting the GEP index '-1' for this alloca we preserve is
+; sign as negative. We use a volatile memcpy to ensure promotion never actually
+; occurs.
+; CHECK: @PR14105
+
+entry:
+  %a = alloca { [16 x i8] }, align 8
+; CHECK: alloca [16 x i8], align 8
+
+  %gep = getelementptr inbounds { [16 x i8] }* %ptr, i64 -1
+; CHECK-NEXT: getelementptr inbounds { [16 x i8] }* %ptr, i64 -1, i32 0, i64 0
+
+  %cast1 = bitcast { [16 x i8 ] }* %gep to i8*
+  %cast2 = bitcast { [16 x i8 ] }* %a to i8*
+  call void @llvm.memcpy.p0i8.p0i8.i32(i8* %cast1, i8* %cast2, i32 16, i32 8, i1 true)
+  ret void
+; CHECK: ret
+}