[SystemZ] Fix addcarry of usubo (PR42512)

[nikic]

Summary

Fixes https://bugs.llvm.org/show_bug.cgi?id=42512.

If I'm understanding things right, the current code assumes that we always see uaddo+addcarry and usubo+subcarry, but no mixtures like usubo+addcarry. If this happens we need to use different flags for the two setcc operations.

Disclaimer: I have no familiarity with SystemZ.

[uweigand]

Unfortunately, I think this gets the semantics wrong, because "carry" and "borrow" use *different* CC values on SystemZ.

In your example,

slgfi %r0, 1

will set CC 1 if an overflow ("borrow") happened, and CC 2 or 3 if no overflow happened.
But the next instruction,

alcr %r2, %r0

will interpret CC [0 and] 1 as *no* carry to be added, and CC 2 and 3 as carry.

This seems a rather unusual case anyway, can we simply always fall back to the default expansion if the source of the overflow doesn't match the current instruction? I.e. a ISD::ADDCARRY may take its overflow value from a ISD::UADDO or another ISD::ADDCARRY, and a ISD::SUBCARRY may take its overflow value from a ISD::USUBO or another ISD::SUBCARRY, but everything else gets the default expansion?

[nikic]

Fall back to default legalization on add/sub mismatch of the carry generating and consuming operation.

[uweigand]

LGTM, thanks!

llvm/trunk/lib/Target/SystemZ/SystemZISelLowering.cpp

SDValue SystemZTargetLowering::lowerADDSUBCARRY(SDValue Op,
   SDLoc DL(N);
   unsigned BaseOp = 0;
   unsigned CCValid = 0;
   unsigned CCMask = 0;
 
   switch (Op.getOpcode()) {
   default: llvm_unreachable("Unknown instruction!");
   case ISD::ADDCARRY:
+    if (Carry.getOpcode() != ISD::UADDO && Carry.getOpcode() != ISD::ADDCARRY)
+      return SDValue();
+
     BaseOp = SystemZISD::ADDCARRY;
     CCValid = SystemZ::CCMASK_LOGICAL;
     CCMask = SystemZ::CCMASK_LOGICAL_CARRY;
     break;
   case ISD::SUBCARRY:
+    if (Carry.getOpcode() != ISD::USUBO && Carry.getOpcode() != ISD::SUBCARRY)
+      return SDValue();
+
     BaseOp = SystemZISD::SUBCARRY;
     CCValid = SystemZ::CCMASK_LOGICAL;
     CCMask = SystemZ::CCMASK_LOGICAL_BORROW;
     break;
   }
 
   // Set the condition code from the carry flag.
   Carry = DAG.getNode(SystemZISD::GET_CCMASK, DL, MVT::i32, Carry,

llvm/trunk/test/CodeGen/SystemZ/pr42512.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
+
+define i8 @test(i8 %x) {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    slgfi %r0, 1
+; CHECK-NEXT:    ipm %r0
+; CHECK-NEXT:    afi %r0, -536870912
+; CHECK-NEXT:    srl %r0, 31
+; CHECK-NEXT:    ar %r2, %r0
+; CHECK-NEXT:    br %r14
+  %usubo = tail call { i64, i1 } @llvm.usub.with.overflow.i64(i64 undef, i64 1)
+  %ov = extractvalue { i64, i1 } %usubo, 1
+  %ovext = zext i1 %ov to i8
+  %ret = add i8 %x, %ovext
+  ret i8 %ret
+}
+
+; Function Attrs: nounwind readnone speculatable
+declare { i64, i1 } @llvm.usub.with.overflow.i64(i64, i64) #0
+
+attributes #0 = { nounwind readnone speculatable }