[SelectionDAG] Teach computeKnownBits about ATOMIC_CMP_SWAP_WITH_SUCCESS boolean return value

[uweigand]

Summary

The second return value of ATOMIC_CMP_SWAP_WITH_SUCCESS is known to be a boolean, and should therefore be treated by computeKnownBits just like the second return values of SMULO / UMULO.

(I don't have a stand-alone test case for this, but this patch is required to make a test case pass that I'll be adding with a follow-on SystemZ back-end patch.)

[RKSimon]

(I don't have a stand-alone test case for this, but this patch is required to make a test case pass that I'll be adding with a follow-on SystemZ back-end patch.)

Do have you the follow up patch available that you could make dependent on this one to show the diff?

[uweigand]

In D42067#977202, @RKSimon wrote:

(I don't have a stand-alone test case for this, but this patch is required to make a test case pass that I'll be adding with a follow-on SystemZ back-end patch.)

Do have you the follow up patch available that you could make dependent on this one to show the diff?

I somehow wasn't able to create a dependent diff, but I uploaded the back-end patch (set) here:
https://reviews.llvm.org/D42115

Most of the new tests added to tests/SystemZ/cmpxchg-* will fail without this patch. In the end, this is because I want to fold tests of the success result of compare-and-swap operations into a conditional branch using the condition code set by the CSG instruction directly. But standard expansion in SelectionDAG for some reason always creates an AND with 1 when using the success result, and the presence of this AND messes up my pattern matchers. But with this patch, common code knows that this AND is redundant and we can get rid of it.

[RKSimon]

LGTM - thanks.

llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

case ISD::SELECT_CC:
     computeKnownBits(Op.getOperand(2), Known2, DemandedElts, Depth+1);
 
     // Only known if known in both the LHS and RHS.
     Known.One &= Known2.One;
     Known.Zero &= Known2.Zero;
     break;
   case ISD::SMULO:
   case ISD::UMULO:
+  case ISD::ATOMIC_CMP_SWAP_WITH_SUCCESS:
     if (Op.getResNo() != 1)
       break;
     // The boolean result conforms to getBooleanContents.
     // If we know the result of a setcc has the top bits zero, use this info.
     // We know that we have an integer-based boolean since these operations
     // are only available for integer.
     if (TLI->getBooleanContents(Op.getValueType().isVector(), false) ==
             TargetLowering::ZeroOrOneBooleanContent &&