[InstCombine] Simplify select using KnownBits of condition (#95923)

Simplify the arms of a select based on the KnownBits implied by its condition.
For now this only handles the case where the select arm folds to a constant,
but this can be generalized to handle other patterns by using
SimplifyDemandedBits instead (in that case we would also have to limit to
non-undef conditions).

This is implemented by adding a new member to SimplifyQuery that can be used
to inject an additional condition. The affected values are pre-computed and
we don't call computeKnownBits() if the select arms don't contain affected
values. This reduces the cost in some pathological cases.

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 7deb7bd..c0d49ca 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -771,6 +771,10 @@ static void computeKnownBitsFromCond(const Value *V, Value *Cond,
 
 void llvm::computeKnownBitsFromContext(const Value *V, KnownBits &Known,
                                        unsigned Depth, const SimplifyQuery &Q) {
+  // Handle injected condition.
+  if (Q.CC && Q.CC->AffectedValues.contains(V))
+    computeKnownBitsFromCond(V, Q.CC->Cond, Known, Depth, Q, Q.CC->Invert);
+
   if (!Q.CxtI)
     return;