[SCCP] Skip computing intrinsics if one of its args is unknownOrUndef

[StephenFan]

Summary

For constant range supported intrinsics, we got consantrange from args
no matter if they are unknown or undef. And the constant range computed
from unknown or undef value state is full range.

I think compute with full constant range is harmful since although we
can do mergeIn after these args value state are changed, the merge
operation of two constant ranges is union.

[nikic]

LGTM

llvm/test/Transforms/SCCP/intrinsics.ll

102

Please precommit the test.

[StephenFan]

Precommit test and rebase.

llvm/lib/Transforms/Utils/SCCPSolver.cpp

if (auto *II = dyn_cast<IntrinsicInst>(&CB)) {
 
     if (ConstantRange::isIntrinsicSupported(II->getIntrinsicID())) {
       // Compute result range for intrinsics supported by ConstantRange.
       // Do this even if we don't know a range for all operands, as we may
       // still know something about the result range, e.g. of abs(x).
       SmallVector<ConstantRange, 2> OpRanges;
       for (Value *Op : II->args()) {
         const ValueLatticeElement &State = getValueState(Op);
+        if (State.isUnknownOrUndef())
+          return;
         OpRanges.push_back(getConstantRange(State, Op->getType()));
       }
 
       ConstantRange Result =
           ConstantRange::intrinsic(II->getIntrinsicID(), OpRanges);
       return (void)mergeInValue(II, ValueLatticeElement::getRange(Result));
     }
   }

llvm/test/Transforms/SCCP/intrinsics.ll

 ; CHECK-NEXT:    ret void
 ;
   %m = call i8 @llvm.umax.i8(i8 %x, i8 10)
   %cmp = icmp uge i8 %m, 10
   call void @use(i1 %cmp)
   ret void
 }
 
 define i8 @umax_including_undef(i1 %c.1, i1 %c.2) {
 ; CHECK-LABEL: @umax_including_undef(
 ; CHECK-NEXT:    br i1 [[C_1:%.*]], label [[TRUE:%.*]], label [[FALSE:%.*]]
 ; CHECK:       true:
 ; CHECK-NEXT:    br label [[EXIT:%.*]]
 ; CHECK:       false:
 ; CHECK-NEXT:    br label [[EXIT]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[P_UMAX:%.*]] = call i8 @llvm.umax.i8(i8 3, i8 1)
-; CHECK-NEXT:    ret i8 [[P_UMAX]]
+; CHECK-NEXT:    ret i8 3
 ;
   br i1 %c.1, label %true, label %false
 
 true:
   br label %exit
 
 false:
   br label %exit
 
 exit:
   %p = phi i8 [ 3, %true], [ undef, %false ]
   %p_umax = call i8 @llvm.umax.i8(i8 %p, i8 1)
   ret i8 %p_umax
 }