[ValueTracking] Add logic for `isKnownNonZero(ssub.sat X, Y)`

[goldstein.w.n]

Summary

The logic here is the same for normal (sub X, Y), so just reused
isNonZeroSub.

Alive2 Link:

https://alive2.llvm.org/ce/z/9kSkMv

[nikic]

LGTM. It looks like we're missing the corresponding InstCombine for this: https://alive2.llvm.org/ce/z/hLC6aQ

[goldstein.w.n]

In D149412#4305078, @nikic wrote:

LGTM. It looks like we're missing the corresponding InstCombine for this: https://alive2.llvm.org/ce/z/hLC6aQ

Will post patch for that tomorrow.

[goldstein.w.n]

Done

[goldstein.w.n]

In D149412#4305078, @nikic wrote:

LGTM. It looks like we're missing the corresponding InstCombine for this: https://alive2.llvm.org/ce/z/hLC6aQ

See: D149521

[goldstein.w.n]

Rebase

[goldstein.w.n]

Rebase

llvm/lib/Analysis/ValueTracking.cpp

if (auto *II = dyn_cast<IntrinsicInst>(I)) {
       case Intrinsic::ushl_sat:
       case Intrinsic::abs:
       case Intrinsic::bitreverse:
       case Intrinsic::bswap:
       case Intrinsic::ctpop:
         if (isKnownNonZero(II->getArgOperand(0), DemandedElts, Depth, Q))
           return true;
         break;
+      case Intrinsic::ssub_sat:
+        return isNonZeroSub(DemandedElts, Depth, Q, BitWidth,
+                            II->getArgOperand(0), II->getArgOperand(1));
       case Intrinsic::uadd_sat:
         if (isKnownNonZero(II->getArgOperand(0), DemandedElts, Depth, Q) ||
             isKnownNonZero(II->getArgOperand(1), DemandedElts, Depth, Q))
           return true;
         break;
       case Intrinsic::cttz:
         return computeKnownBits(II->getArgOperand(0), DemandedElts, Depth, Q)
             .Zero[0];

llvm/test/Analysis/ValueTracking/known-non-zero.ll

;
   %x = call i8 @llvm.ushl.sat.i8(i8 %xx, i8 %y)
   %z = or i8 %x, %ind
   %r = icmp eq i8 %z, 0
   ret i1 %r
 }
 
 define i1 @ssub_sat_nonzero(i8 %xx, i8 %yy, i8 %ind) {
 ; CHECK-LABEL: @ssub_sat_nonzero(
-; CHECK-NEXT:    [[XA:%.*]] = and i8 [[XX:%.*]], -65
-; CHECK-NEXT:    [[YO:%.*]] = or i8 [[YY:%.*]], 64
-; CHECK-NEXT:    [[X:%.*]] = call i8 @llvm.ssub.sat.i8(i8 [[XA]], i8 [[YO]])
-; CHECK-NEXT:    [[Z:%.*]] = or i8 [[X]], [[IND:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[Z]], 0
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 false
 ;
   %xa = and i8 %xx, 191
   %yo = or i8 %yy, 64
   %x = call i8 @llvm.ssub.sat.i8(i8 %xa, i8 %yo)
   %z = or i8 %x, %ind
   %r = icmp eq i8 %z, 0
   ret i1 %r
 }