[SystemZ] Fix check for zero size when lowering memcmp.

During lowering of memcmp/bcmp, the check for a size of 0 is done
in 2 different ways. In rare cases this can lead to a crash in
SystemZSelectionDAGInfo::EmitTargetCodeForMemcmp(). The root cause
is that SelectionDAGBuilder::visitMemCmpBCmpCall() checks for a
constant int value which is not yet evaluated. When the value is
turned into a SDValue, then the evaluation is done and results in
a ConstantSDNode. But EmitTargetCodeForMemcmp() expects the special
case of 0 length to be handled, which results in an assertion.

The fix is to turn the value into a SDValue, so that both functions
use the same check.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D126900

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -7902,7 +7902,7 @@ void SelectionDAGBuilder::processIntegerCallValue(const Instruction &I,
 bool SelectionDAGBuilder::visitMemCmpBCmpCall(const CallInst &I) {
   const Value *LHS = I.getArgOperand(0), *RHS = I.getArgOperand(1);
   const Value *Size = I.getArgOperand(2);
-  const ConstantInt *CSize = dyn_cast<ConstantInt>(Size);
+  const ConstantSDNode *CSize = dyn_cast<ConstantSDNode>(getValue(Size));
   if (CSize && CSize->getZExtValue() == 0) {
     EVT CallVT = DAG.getTargetLoweringInfo().getValueType(DAG.getDataLayout(),
                                                           I.getType(), true);

llvm/test/CodeGen/SystemZ/memcmp-03.ll

@@ -0,0 +1,20 @@
+; Test memcmp with 0 size.
+
+; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
+; REQUIRES: asserts
+
+declare i32 @memcmp(i8* nocapture, i8* nocapture, i64)
+
+define hidden void @fun() {
+; CHECK-LABEL: fun
+entry:
+  %len = extractvalue [2 x i64] zeroinitializer, 1
+  br i1 undef, label %end, label %call
+
+call:
+  %res = tail call signext i32 @memcmp(i8* noundef undef, i8* noundef undef, i64 noundef %len)
+  unreachable
+
+end:
+  unreachable
+}