[PATCH] D149413: [ValueTracking] Add logic for `isKnownNonZero(usub.sat X, Y)`

[Noah Goldstein via Phabricator via llvm-commits]

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`(usub X, Y) != 0` -> `X u> Y`

Alive2 Link:

  https://alive2.llvm.org/ce/z/o29DK4


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D149413

Files:
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/test/Analysis/ValueTracking/known-non-zero.ll


Index: llvm/test/Analysis/ValueTracking/known-non-zero.ll
===================================================================
--- llvm/test/Analysis/ValueTracking/known-non-zero.ll
+++ llvm/test/Analysis/ValueTracking/known-non-zero.ll
@@ -815,11 +815,7 @@
 ; CHECK-LABEL: @usub_sat_nonzero(
 ; CHECK-NEXT:    [[Y_ULT_31:%.*]] = icmp ult i8 [[YY:%.*]], 31
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[Y_ULT_31]])
-; CHECK-NEXT:    [[XO:%.*]] = or i8 [[XX:%.*]], 34
-; CHECK-NEXT:    [[X:%.*]] = call i8 @llvm.usub.sat.i8(i8 [[XO]], i8 [[YY]])
-; 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
 ;
   %y_ult_31 = icmp ult i8 %yy, 31
   call void @llvm.assume(i1 %y_ult_31)
Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -2903,9 +2903,18 @@
           return true;
         break;
       case Intrinsic::ssub_sat:
-        return isNonZeroSub(DemandedElts, Depth, Q, BitWidth, II->getArgOperand(0),
-                            II->getArgOperand(1));
-
+        return isNonZeroSub(DemandedElts, Depth, Q, BitWidth,
+                            II->getArgOperand(0), II->getArgOperand(1));
+      case Intrinsic::usub_sat: {
+        KnownBits XKnown =
+            computeKnownBits(II->getArgOperand(0), DemandedElts, Depth, Q);
+        if (XKnown.isUnknown())
+          return false;
+        KnownBits YKnown =
+            computeKnownBits(II->getArgOperand(1), DemandedElts, Depth, Q);
+        std::optional<bool> ugt = KnownBits::ugt(XKnown, YKnown);
+        return ugt && *ugt;
+      }
       case Intrinsic::uadd_sat:
         if (isKnownNonZero(II->getArgOperand(0), DemandedElts, Depth, Q) ||
             isKnownNonZero(II->getArgOperand(1), DemandedElts, Depth, Q))


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