[llvm] [Vectorizer] Conversion of icmp eq to icmp ult to enable vectorization (PR #202368)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-vectorizers

Author: Bala_Bhuvan_Varma (Bhuvan1527)

<details>
<summary>Changes</summary>

Resolves: https://github.com/llvm/llvm-project/issues/190505

The current slp-vectorizer donot recognize the following pattern 
```
icmp eq %x, 0
icmp ult %y, ..
icmp ult
..
```
It will not convert icmp eq to icmp ult. Hence, full vectorization is skipped. Instead it performs altshuffle vectorization. Creating two vector instructions (one for only eq, other for ult) followed by shuffleVec instructions to combine the results.

In this pr, we are supporting this pattern by checking and canonicalizing the icmp eq to icmp ult before computing Legality.getInstructionsState().

---
Full diff: https://github.com/llvm/llvm-project/pull/202368.diff


1 Files Affected:

- (modified) llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp (+32) 


``````````diff
diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index f9532a0bd0616..408ebbae1f614 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -12800,6 +12800,38 @@ void BoUpSLP::buildTreeRec(ArrayRef<Value *> VLRef, unsigned Depth,
   SmallVector<int> ReuseShuffleIndices;
   SmallVector<Value *> VL(VLRef);
 
+  if (VL.size() > 1 && isa<ICmpInst>(VL[0])) {
+  // Changes icmp eq %x, 0 -> icmp ult %x, 1 when the rest of
+  // the instructions are icmp ult .. . This prevents to move 
+  // towards altShuffle path.
+  bool HasEqZero = false;
+  bool AllUltOrEqZero = all_of(VL, [&HasEqZero](Value *V) {
+    auto *Cmp = dyn_cast<ICmpInst>(V);
+    if (!Cmp)
+      return isa<PoisonValue>(V);
+    if (Cmp->getPredicate() == ICmpInst::ICMP_ULT)
+      return true;
+    if (Cmp->getPredicate() == ICmpInst::ICMP_EQ)
+      if (auto *C = dyn_cast<ConstantInt>(Cmp->getOperand(1)))
+        if (C->isZero()) {
+          HasEqZero = true;
+          return true;
+        }
+    return false;
+  });
+
+  if (HasEqZero && AllUltOrEqZero) {
+    for (Value *V : VL) {
+      auto *Cmp = dyn_cast<ICmpInst>(V);
+      if (!Cmp || Cmp->getPredicate() != ICmpInst::ICMP_EQ)
+        continue;
+      Cmp->setPredicate(ICmpInst::ICMP_ULT);
+      Cmp->setOperand(1, ConstantInt::get(Cmp->getOperand(1)->getType(), 1));
+      LLVM_DEBUG(dbgs() << "SLP: Canonicalized icmp eq 0 to ult 1: "
+                        << *Cmp << "\n");
+    }
+  }
+}
   // Tries to build split node.
   auto TrySplitNode = [&](const InstructionsState &LocalState) {
     SmallVector<Value *> Op1, Op2;

``````````

</details>


https://github.com/llvm/llvm-project/pull/202368