[ValueTracking] Add support for trunc nuw/nsw in isKnowNonZero

With `nsw`/`nuw`, the `trunc` is non-zero if its operand is non-zero.

Proofs: https://alive2.llvm.org/ce/z/iujmk6

Author: goldsteinn

llvm/lib/Analysis/ValueTracking.cpp

@@ -2632,6 +2632,13 @@ static bool isKnownNonZeroFromOperator(const Operator *I,
             Q.DL.getTypeSizeInBits(I->getType()).getFixedValue())
       return isKnownNonZero(I->getOperand(0), Q, Depth);
     break;
+  case Instruction::Trunc: {
+    auto *TI = cast<TruncInst>(I);
+    // nuw/nsw trunc preserves zero/non-zero status of input.
+    if (TI->hasNoSignedWrap() || TI->hasNoUnsignedWrap())
+      return isKnownNonZero(TI->getOperand(0), Q, Depth + 1);
+    break;
+  }
   case Instruction::Sub:
     return isNonZeroSub(DemandedElts, Depth, Q, BitWidth, I->getOperand(0),
                         I->getOperand(1));
@@ -3785,7 +3792,8 @@ static unsigned ComputeNumSignBitsImpl(const Value *V,
       // truncation, then we can make use of that. Otherwise we don't know
       // anything.
       Tmp = ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
-      unsigned OperandTyBits = U->getOperand(0)->getType()->getScalarSizeInBits();
+      unsigned OperandTyBits =
+          U->getOperand(0)->getType()->getScalarSizeInBits();
       if (Tmp > (OperandTyBits - TyBits))
         return Tmp - (OperandTyBits - TyBits);
 

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

@@ -1452,9 +1452,7 @@ define i1 @trunc_nsw_non_zero(i8 %x) {
 ; CHECK-LABEL: @trunc_nsw_non_zero(
 ; CHECK-NEXT:    [[X_NE_Z:%.*]] = icmp ne i8 [[X:%.*]], 0
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[X_NE_Z]])
-; CHECK-NEXT:    [[V:%.*]] = trunc nsw i8 [[X]] to i4
-; CHECK-NEXT:    [[R:%.*]] = icmp ne i4 [[V]], 0
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 true
 ;
   %x_ne_z = icmp ne i8 %x, 0
   call void @llvm.assume(i1 %x_ne_z)
@@ -1465,10 +1463,7 @@ define i1 @trunc_nsw_non_zero(i8 %x) {
 
 define i1 @trunc_nuw_non_zero(i8 %xx) {
 ; CHECK-LABEL: @trunc_nuw_non_zero(
-; CHECK-NEXT:    [[X:%.*]] = add nuw i8 [[XX:%.*]], 1
-; CHECK-NEXT:    [[V:%.*]] = trunc nuw i8 [[X]] to i4
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i4 [[V]], 0
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 false
 ;
   %x = add nuw i8 %xx, 1
   %v = trunc nuw i8 %x to i4