[ValueTracking] recognize a 'not' of an assumed condition as false

[spatel]

Summary

I think this is a logical extension of the current checks in computeKnownBitsFromAssume(). Ie, it has negligible cost for the added functionality. But let me know if we need to re-think the whole approach.

This is the reason I didn't have the canonical xor-form of 'not' in the test case for D28337 - it doesn't work without this change.

[majnemer]

lib/Analysis/ValueTracking.cpp

550–551

I don't think you want to use the uint64_t constructor here., you probably want to use clearAllBits and setAllBits on the two APInts as needed.

[spatel]

Patch updated:
Use clearAllBits/setAllBits as suggested by David.

[hfinkel]

LGTM

[hfinkel]

You'll also need a corresponding update to AssumptionCache::updateAffectedValues.

[spatel]

In D28485#648132, @hfinkel wrote:

You'll also need a corresponding update to AssumptionCache::updateAffectedValues.

Thanks! I'll update that part, add the select test diff from D28337, and check it in.

lib/Analysis/ValueTracking.cpp

for (auto &AssumeVH : Q.AC->assumptions()) {
     // We're running this loop for once for each value queried resulting in a
     // runtime of ~O(#assumes * #values).
 
     assert(I->getCalledFunction()->getIntrinsicID() == Intrinsic::assume &&
            "must be an assume intrinsic");
 
     Value *Arg = I->getArgOperand(0);
 
-    if (Arg == V && isValidAssumeForContext(I, Q.CxtI, Q.DT)) {
+    if ((Arg == V || match(Arg, m_Not(m_Specific(V)))) &&
+        isValidAssumeForContext(I, Q.CxtI, Q.DT)) {
       assert(BitWidth == 1 && "assume operand is not i1?");
-      KnownZero.clearAllBits();
-      KnownOne.setAllBits();
+      KnownZero = Arg == V ? 0 : -1;
+      KnownOne = Arg == V ? -1 : 0;
       return;
     }
 
     // The remaining tests are all recursive, so bail out if we hit the limit.
     if (Depth == MaxDepth)
       continue;
 
     Value *A, *B;

test/Transforms/InstCombine/assume.ll

 ;
   %cmp = icmp sgt i32 %a, 5
   tail call void @llvm.assume(i1 %cmp)
   %t0 = zext i1 %cmp to i32
   %lnot.ext = xor i32 %t0, 1
   ret i32 %lnot.ext
 }
 
-; FIXME: If the 'not' of a condition is known true, then the condition must be false. 
+; If the 'not' of a condition is known true, then the condition must be false.
 
 define i1 @assume_not(i1 %cond) {
 ; CHECK-LABEL: @assume_not(
 ; CHECK-NEXT:    [[NOTCOND:%.*]] = xor i1 [[COND:%.*]], true
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[NOTCOND]])
-; CHECK-NEXT:    ret i1 [[COND]]
+; CHECK-NEXT:    ret i1 false
 ;
   %notcond = xor i1 %cond, true
   call void @llvm.assume(i1 %notcond)
   ret i1 %cond
 }
 
 declare void @escape(i32* %a)