InstSimplify: Don't simplify gep X, (Y-X) to Y if types differ

It's incorrect to perform this simplification if the types differ.
A bitcast would need to be inserted for this to work.

This fixes PR20771.

llvm-svn: 216597

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -2837,7 +2837,8 @@ static Value *SimplifyGEPInst(ArrayRef<Value *> Ops, const Query &Q, unsigned) {
             return Constant::getNullValue(GEPTy);
           Value *Temp;
           if (match(P, m_PtrToInt(m_Value(Temp))))
-            return Temp;
+            if (Temp->getType() == GEPTy)
+              return Temp;
           return nullptr;
         };
 

llvm/test/Transforms/InstSimplify/gep.ll

@@ -64,3 +64,17 @@ define i64* @test6(i64* %b) {
 ; CHECK-LABEL: @test6
 ; CHECK-NEXT: ret i64* null
 }
+
+define i8* @test7(i8* %b, i8** %e) {
+  %e_ptr = ptrtoint i8** %e to i64
+  %b_ptr = ptrtoint i8* %b to i64
+  %sub = sub i64 %e_ptr, %b_ptr
+  %gep = getelementptr inbounds i8* %b, i64 %sub
+  ret i8* %gep
+; CHECK-LABEL: @test7
+; CHECK-NEXT: ptrtoint
+; CHECK-NEXT: ptrtoint
+; CHECK-NEXT: sub
+; CHECK-NEXT: getelementptr
+; CHECK-NEXT: ret
+}