[ValueTracking] improve isKnownNonZero precision for smax

Instead of relying on known-bits for strictly positive, use the
`isKnownPositive` API. This will use `isKnownNonZero` which is more
accurate.

Author: goldsteinn

llvm/lib/Analysis/ValueTracking.cpp

@@ -2861,21 +2861,18 @@ static bool isKnownNonZeroFromOperator(const Operator *I,
                isKnownNonZero(II->getArgOperand(0), DemandedElts, Depth, Q);
       case Intrinsic::smin:
       case Intrinsic::smax: {
-        auto KnownOpImpliesNonZero = [&](const KnownBits &K) {
-          return II->getIntrinsicID() == Intrinsic::smin
-                     ? K.isNegative()
-                     : K.isStrictlyPositive();
+        bool AllNonZero = true;
+        auto KnownOpImpliesNonZero = [&](const Value *Op) {
+          KnownBits TmpKnown(getBitWidth(Op->getType(), Q.DL));
+          bool Ret =
+              II->getIntrinsicID() == Intrinsic::smin
+                  ? ::isKnownNegative(Op, DemandedElts, TmpKnown, Q, Depth)
+                  : ::isKnownPositive(Op, DemandedElts, TmpKnown, Q, Depth);
+          AllNonZero &= TmpKnown.isNonZero();
+          return Ret;
         };
-        KnownBits XKnown =
-            computeKnownBits(II->getArgOperand(0), DemandedElts, Depth, Q);
-        if (KnownOpImpliesNonZero(XKnown))
-          return true;
-        KnownBits YKnown =
-            computeKnownBits(II->getArgOperand(1), DemandedElts, Depth, Q);
-        if (KnownOpImpliesNonZero(YKnown))
-          return true;
-
-        if (XKnown.isNonZero() && YKnown.isNonZero())
+        if (KnownOpImpliesNonZero(II->getArgOperand(0)) ||
+            KnownOpImpliesNonZero(II->getArgOperand(1)) || AllNonZero)
           return true;
       }
         [[fallthrough]];

llvm/test/Transforms/InstSimplify/known-non-zero.ll

@@ -169,11 +169,7 @@ B:
 
 define i1 @smax_non_zero(i8 %xx, i8 %y) {
 ; CHECK-LABEL: @smax_non_zero(
-; CHECK-NEXT:    [[X0:%.*]] = and i8 [[XX:%.*]], 63
-; CHECK-NEXT:    [[X:%.*]] = add i8 [[X0]], 1
-; CHECK-NEXT:    [[V:%.*]] = call i8 @llvm.smax.i8(i8 [[X]], i8 [[Y:%.*]])
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[V]], 0
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 false
 ;
   %x0 = and i8 %xx, 63
   %x = add i8 %x0, 1