[ValueTracking] Add `shl nsw %val, %cnt != 0` if `%val != 0`.

[goldstein.w.n]

Summary

Alive2 Link: https://alive2.llvm.org/ce/z/mxZLJn

[goldstein.w.n]

ping, this is a pretty simple patch.

[nikic]

LGTM

[goldstein.w.n]

Rebase

llvm/lib/Analysis/ValueTracking.cpp

case Instruction::Or:
     return isKnownNonZero(I->getOperand(0), DemandedElts, Depth, Q) ||
            isKnownNonZero(I->getOperand(1), DemandedElts, Depth, Q);
   case Instruction::SExt:
   case Instruction::ZExt:
     // ext X != 0 if X != 0.
     return isKnownNonZero(I->getOperand(0), Depth, Q);
 
   case Instruction::Shl: {
-    // shl nuw can't remove any non-zero bits.
+    // shl nsw/nuw can't remove any non-zero bits.
     const OverflowingBinaryOperator *BO = cast<OverflowingBinaryOperator>(V);
-    if (Q.IIQ.hasNoUnsignedWrap(BO))
+    if (Q.IIQ.hasNoUnsignedWrap(BO) || Q.IIQ.hasNoSignedWrap(BO))
       return isKnownNonZero(I->getOperand(0), Depth, Q);
 
     // shl X, Y != 0 if X is odd.  Note that the value of the shift is undefined
     // if the lowest bit is shifted off the end.
     KnownBits Known(BitWidth);
     computeKnownBits(I->getOperand(0), DemandedElts, Known, Depth, Q);
     if (Known.One[0])
       return true;

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

;
   %r = icmp eq i32 %shl, 0
   ret i1 %r
 }
 
 define i1 @shl_non_zero_nsw(i8 %s, i8 %cnt) {
 ; CHECK-LABEL: @shl_non_zero_nsw(
 ; CHECK-NEXT:    [[NZ:%.*]] = icmp ne i8 [[S:%.*]], 0
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[NZ]])
-; CHECK-NEXT:    [[V:%.*]] = shl nsw i8 [[S]], [[CNT:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[V]], 0
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 false
 ;
   %nz = icmp ne i8 %s, 0
   call void @llvm.assume(i1 %nz)
   %v = shl nsw i8 %s, %cnt
   %r = icmp eq i8 %v, 0
   ret i1 %r
 }