[SystemZ] Make sure that combineGET_CCMASK() can handle a truncated SELECT_CCMASK.

[jonpa]

Summary

In cases where the SELECT_CCMASK has an additional user of the the carry a truncated SELECT_CCMASK may result as the input to a GET_CCMASK. This patch adds capability to combineGET_CCMASK() to recognize and handle these cases.

Fixes https://github.com/llvm/llvm-project/issues/59054.

[uweigand]

Looking into this a bit more, the argument to GET_CCMASK is known to be a boolean value (either 0 or 1), so we don't need to verify that after all, and can just accept truncates in general.

[jonpa]

Updated per review.

[uweigand]

LGTM, thanks!

llvm/lib/Target/SystemZ/SystemZISelLowering.cpp

SDValue SystemZTargetLowering::combineGET_CCMASK(
   auto *CCValid = dyn_cast<ConstantSDNode>(N->getOperand(1));
   auto *CCMask = dyn_cast<ConstantSDNode>(N->getOperand(2));
   if (!CCValid || !CCMask)
     return SDValue();
   int CCValidVal = CCValid->getZExtValue();
   int CCMaskVal = CCMask->getZExtValue();
 
   SDValue Select = N->getOperand(0);
+  if (Select->getOpcode() == ISD::TRUNCATE)
+    Select = Select->getOperand(0);
   if (Select->getOpcode() != SystemZISD::SELECT_CCMASK)
     return SDValue();
 
   auto *SelectCCValid = dyn_cast<ConstantSDNode>(Select->getOperand(2));
   auto *SelectCCMask = dyn_cast<ConstantSDNode>(Select->getOperand(3));
   if (!SelectCCValid || !SelectCCMask)
     return SDValue();
   int SelectCCValidVal = SelectCCValid->getZExtValue();
   int SelectCCMaskVal = SelectCCMask->getZExtValue();
 
   auto *TrueVal = dyn_cast<ConstantSDNode>(Select->getOperand(0));
   auto *FalseVal = dyn_cast<ConstantSDNode>(Select->getOperand(1));
   if (!TrueVal || !FalseVal)
     return SDValue();
-  if (TrueVal->getZExtValue() != 0 && FalseVal->getZExtValue() == 0)
+  if (TrueVal->getZExtValue() == 1 && FalseVal->getZExtValue() == 0)
     ;
-  else if (TrueVal->getZExtValue() == 0 && FalseVal->getZExtValue() != 0)
+  else if (TrueVal->getZExtValue() == 0 && FalseVal->getZExtValue() == 1)
     SelectCCMaskVal ^= SelectCCValidVal;
   else
     return SDValue();
 
   if (SelectCCValidVal & ~CCValidVal)
     return SDValue();
   if (SelectCCMaskVal != (CCMaskVal & SelectCCValidVal))
     return SDValue();

llvm/test/CodeGen/SystemZ/int-uadd-multiple-carry-users.ll

+; RUN: llc < %s -mtriple=s390x-linux-gnu -start-after=codegenprepare | FileCheck %s
+;
+; Test that compilation succeeds with multiple users of the carry resulting
+; in a truncation of the SELECT_CCMASK used by GET_CCMASK.
+
+define void @fun() {
+; CHECK-LABEL: fun:
+bb:
+  %0 = call { i64, i1 } @llvm.uadd.with.overflow.i64(i64 poison, i64 poison)
+  %ov = extractvalue { i64, i1 } %0, 1
+  %i2 = sext i1 %ov to i64
+  %i3 = select i1 %ov, i64 9223372036854775807, i64 -1
+  %i4 = sub nsw i64 %i3, %i2
+  %i5 = and i64 %i4, %i2
+  %i6 = icmp slt i64 %i5, 0
+  %i7 = xor i1 %ov, true
+  %i8 = select i1 %i6, i1 %i7, i1 false
+  %i9 = sext i1 %i8 to i16
+  store i16 %i9, ptr poison, align 2
+  unreachable
+}
+
+declare { i64, i1 } @llvm.uadd.with.overflow.i64(i64, i64)