[SelectionDAGBuilder] Let asm constraint "i" accept an undef operand.

[jonpa]

Summary

Make LowerAsmOperandForConstraint() accept an undef operand for the "i" constraint.

[nickdesaulniers]

Interesting, did this come from actual C code? It looks to me like GCC and Clang both currently reject such input:

void foo(void) {
    int x;
    asm volatile (""::"i"(x));
}

Why should we allow that? Seems more indicative of a bug in the source code? Or do you have a different test case that more clearly illuminates the issue in LLVM?

llvm/test/CodeGen/SystemZ/asm-20.ll

3

Probably don't need to reorder these?

[jonpa]

Thanks for review, but as you suspected this test case did not reflect the true problem. A new patch is suggested at: https://reviews.llvm.org/D90231.

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

while (1) {
                                             SDLoc(C), MVT::i64));
         return;
       } else if ((BA = dyn_cast<BlockAddressSDNode>(Op)) &&
                  ConstraintLetter != 'n') {
         Ops.push_back(DAG.getTargetBlockAddress(
             BA->getBlockAddress(), BA->getValueType(0),
             Offset + BA->getOffset(), BA->getTargetFlags()));
         return;
+      } else if (Op.isUndef()) {
+        Ops.push_back(Op);
       } else {
         const unsigned OpCode = Op.getOpcode();
         if (OpCode == ISD::ADD || OpCode == ISD::SUB) {
           if ((C = dyn_cast<ConstantSDNode>(Op.getOperand(0))))
             Op = Op.getOperand(1);
           // Subtraction is not commutative.
           else if (OpCode == ISD::ADD &&
                    (C = dyn_cast<ConstantSDNode>(Op.getOperand(1))))

llvm/test/CodeGen/SystemZ/asm-20.ll

-; Test that asm goto can be compiled.
-;
 ; RUN: llc < %s -mtriple=s390x-linux-gnu -mcpu=z14
 
+; Test that asm goto can be compiled.
 define i32 @c() {
 entry:
   callbr void asm sideeffect "j d", "X"(i8* blockaddress(@c, %d))
           to label %asm.fallthrough [label %d]
 
 asm.fallthrough:               ; preds = %entry
   br label %d
 
 d:                             ; preds = %asm.fallthrough, %entry
   ret i32 undef
 }
+
+; Test that constraint "i" accepts an undef operand.
+define void @fun1() {
+entry:
+  tail call void asm sideeffect "", "i"(i32 undef)
+  ret void
+}