[ValueTracking] Add logic for `udiv x,y != 0` if `y u<= x`

[goldstein.w.n]

Summary

Alive2 Link:

https://alive2.llvm.org/ce/z/2DKh46

[goldstein.w.n]

Rebase

[nikic]

I don't think you need the isKnownNonZero() check? If X >= Y and X == 0 then Y == 0, which is UB. https://alive2.llvm.org/ce/z/hw8Gs9 Or maybe even simpler: https://alive2.llvm.org/ce/z/QhiNtE

llvm/lib/Analysis/ValueTracking.cpp

2778–2782

I found the swapped operand order confusing.

[goldstein.w.n]

Remove extreneous isknownonzero on X. Cleanup logic. Flip YUleX - >XUgeY

[goldstein.w.n]

In D149203#4298080, @nikic wrote:

I don't think you need the isKnownNonZero() check? If X >= Y and X == 0 then Y == 0, which is UB. https://alive2.llvm.org/ce/z/hw8Gs9 Or maybe even simpler: https://alive2.llvm.org/ce/z/QhiNtE

Bright. Updated title and summary with your alive2 link.

[goldstein.w.n]

Fix type in comment

[nikic]

LGTM

llvm/lib/Analysis/ValueTracking.cpp

KnownBits Known =
         computeKnownBits(I->getOperand(0), DemandedElts, Depth, Q);
     if (Known.isNegative())
       return true;
 
     return isNonZeroShift(I, DemandedElts, Depth, Q, Known);
   }
   case Instruction::UDiv:
   case Instruction::SDiv:
+    // X / Y
     // div exact can only produce a zero if the dividend is zero.
     if (cast<PossiblyExactOperator>(I)->isExact())
       return isKnownNonZero(I->getOperand(0), DemandedElts, Depth, Q);
+    if (I->getOpcode() == Instruction::UDiv) {
+      std::optional<bool> XUgeY;
+      KnownBits XKnown =
+          computeKnownBits(I->getOperand(0), DemandedElts, Depth, Q);
+      if (!XKnown.isUnknown()) {
+        KnownBits YKnown =
+            computeKnownBits(I->getOperand(1), DemandedElts, Depth, Q);
+        // If X u>= Y then div is non zero (0/0 is UB).
+        XUgeY = KnownBits::uge(XKnown, YKnown);
+      }
+      // If X is total unknown or X u<= Y we won't be able to prove non-zero
+      // with compute known bits so just return early.
+      return XUgeY && *XUgeY;
+    }
     break;
   case Instruction::Add: {
     // X + Y.
     KnownBits XKnown =
         computeKnownBits(I->getOperand(0), DemandedElts, Depth, Q);
     KnownBits YKnown =
         computeKnownBits(I->getOperand(1), DemandedElts, Depth, Q);
 

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

;
   call void @llvm.assume(i1 %x_nz)
   %a = add nsw i8 %x, %y
   %r = icmp eq i8 %a, 0
   ret i1 %r
 }
 
 define i1 @udiv_y_le_x(i8 %xx, i8 %yy, i8 %z) {
 ; CHECK-LABEL: @udiv_y_le_x(
-; CHECK-NEXT:    [[X:%.*]] = or i8 [[XX:%.*]], 7
-; CHECK-NEXT:    [[Y:%.*]] = and i8 [[YY:%.*]], 7
-; CHECK-NEXT:    [[D:%.*]] = udiv i8 [[X]], [[Y]]
-; CHECK-NEXT:    [[O:%.*]] = or i8 [[D]], [[Z:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[O]], 0
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 false
 ;
   %x = or i8 %xx, 7
   %y = and i8 %yy, 7
   %d = udiv i8 %x, %y
   %o = or i8 %d, %z
   %r = icmp eq i8 %o, 0
   ret i1 %r
 }