[llvm] [ValueTracking] Handle `icmp pred (trunc X), C` in `computeKnownBitsFromCmp` (PR #82803)

[via llvm-commits]

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@@ -717,10 +717,22 @@ static void computeKnownBitsFromCond(const Value *V, Value *Cond,
     computeKnownBitsFromCond(V, B, Known, Depth + 1, SQ, Invert);
   }
 
-  if (auto *Cmp = dyn_cast<ICmpInst>(Cond))
-    computeKnownBitsFromCmp(
-        V, Invert ? Cmp->getInversePredicate() : Cmp->getPredicate(),
-        Cmp->getOperand(0), Cmp->getOperand(1), Known, SQ);
+  if (auto *Cmp = dyn_cast<ICmpInst>(Cond)) {
+    ICmpInst::Predicate Pred =
+        Invert ? Cmp->getInversePredicate() : Cmp->getPredicate();
+    Value *LHS = Cmp->getOperand(0);
+    Value *RHS = Cmp->getOperand(1);
+
+    // Handle icmp pred (trunc V), C
+    if (match(LHS, m_Trunc(m_Specific(V)))) {
+      KnownBits DstKnown(LHS->getType()->getScalarSizeInBits());
+      computeKnownBitsFromCmp(LHS, Pred, LHS, RHS, DstKnown, SQ);
+      Known = Known.unionWith(DstKnown.anyext(Known.getBitWidth()));
+      return;
+    }
+
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goldsteinn wrote:

Code all looks correct, i'm wondering, however, if this may become bugprone given the code "distance" between adding affected ops and actually supporting them.

Think there is any convenient way to fix that?

https://github.com/llvm/llvm-project/pull/82803