[ValueTracking] isKnownNonZero, computeKnownBits for freeze

This implements support for isKnownNonZero, computeKnownBits when freeze is involved.

```
  br (x != 0), BB1, BB2
BB1:
  y = freeze x
```

In the above program, we can say that y is non-zero. The reason is as follows:

(1) If x was poison, `br (x != 0)` raised UB
(2) If x was fully undef, the branch again raised UB
(3) If x was non-zero partially undef, say `undef | 1`, `freeze x` will return a nondeterministic value which is also non-zero.
(4) If x was just a concrete value, it is trivial

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D75808

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 469257d..1a89495 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1872,6 +1872,10 @@ static void computeKnownBitsFromOperator(const Operator *I,
       }
     }
     break;
+  case Instruction::Freeze:
+    if (isGuaranteedNotToBePoison(I->getOperand(0), Q.CxtI, Q.DT, Depth + 1))
+      computeKnownBits(I->getOperand(0), Known, Depth + 1, Q);
+    break;
   }
 }
 
@@ -2577,6 +2581,13 @@ bool isKnownNonZero(const Value *V, const APInt &DemandedElts, unsigned Depth,
       return isKnownNonZero(Vec, DemandedVecElts, Depth, Q);
     }
   }
+  // Freeze
+  else if (const FreezeInst *FI = dyn_cast<FreezeInst>(V)) {
+    auto *Op = FI->getOperand(0);
+    if (isKnownNonZero(Op, Depth, Q) &&
+        isGuaranteedNotToBePoison(Op, Q.CxtI, Q.DT, Depth))
+      return true;
+  }
 
   KnownBits Known(BitWidth);
   computeKnownBits(V, DemandedElts, Known, Depth, Q);