[llvm] [LV][EVL] Simplify EVL recipe transformation by using a single EVL mask. nfc (PR #152479)

[Mel Chen via llvm-commits]

https://github.com/Mel-Chen created https://github.com/llvm/llvm-project/pull/152479

The EVL mask is always defined as `icmp ult (step-vector, EVL)`, so we only need to generate it once per plan in the header. Then, we replace all uses of the header mask with the EVL mask, and recursively optimize the users of EVL mask into EVL recipes. This way, the transformation to EVL recipes can be done with just a single loop.

>From a4280e01d4d27103b2700d863ff0a439b5d1597c Mon Sep 17 00:00:00 2001
From: Mel Chen <mel.chen at sifive.com>
Date: Thu, 7 Aug 2025 03:29:28 -0700
Subject: [PATCH] nfc, unified evl mask

---
 .../Transforms/Vectorize/VPlanTransforms.cpp  | 73 +++++++++----------
 1 file changed, 36 insertions(+), 37 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
index 0cb704c85ba40..4afaa9c1ece53 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
@@ -2227,48 +2227,47 @@ static void transformRecipestoEVLRecipes(VPlan &Plan, VPValue &EVL) {
     }
   }
 
-  // Try to optimize header mask recipes away to their EVL variants.
+  // Replace header masks with a mask equivalent to predicating by EVL:
+  //
+  // icmp ule widen-canonical-iv backedge-taken-count
+  // ->
+  // icmp ult step-vector, EVL
+  VPRecipeBase *EVLR = EVL.getDefiningRecipe();
+  VPBuilder Builder(EVLR->getParent(), std::next(EVLR->getIterator()));
+  Type *EVLType = TypeInfo.inferScalarType(&EVL);
+  VPValue *EVLMask = Builder.createICmp(
+      CmpInst::ICMP_ULT,
+      Builder.createNaryOp(VPInstruction::StepVector, {}, EVLType), &EVL);
   for (VPValue *HeaderMask : collectAllHeaderMasks(Plan)) {
-    // TODO: Split optimizeMaskToEVL out and move into
-    // VPlanTransforms::optimize. transformRecipestoEVLRecipes should be run in
-    // tryToBuildVPlanWithVPRecipes beforehand.
-    for (VPUser *U : collectUsersRecursively(HeaderMask)) {
-      auto *CurRecipe = cast<VPRecipeBase>(U);
-      VPRecipeBase *EVLRecipe =
-          optimizeMaskToEVL(HeaderMask, *CurRecipe, TypeInfo, *AllOneMask, EVL);
-      if (!EVLRecipe)
-        continue;
-
-      [[maybe_unused]] unsigned NumDefVal = EVLRecipe->getNumDefinedValues();
-      assert(NumDefVal == CurRecipe->getNumDefinedValues() &&
-             "New recipe must define the same number of values as the "
-             "original.");
-      assert(
-          NumDefVal <= 1 &&
-          "Only supports recipes with a single definition or without users.");
-      EVLRecipe->insertBefore(CurRecipe);
-      if (isa<VPSingleDefRecipe, VPWidenLoadEVLRecipe>(EVLRecipe)) {
-        VPValue *CurVPV = CurRecipe->getVPSingleValue();
-        CurVPV->replaceAllUsesWith(EVLRecipe->getVPSingleValue());
-      }
-      ToErase.push_back(CurRecipe);
-    }
-
-    // Replace header masks with a mask equivalent to predicating by EVL:
-    //
-    // icmp ule widen-canonical-iv backedge-taken-count
-    // ->
-    // icmp ult step-vector, EVL
-    VPRecipeBase *EVLR = EVL.getDefiningRecipe();
-    VPBuilder Builder(EVLR->getParent(), std::next(EVLR->getIterator()));
-    Type *EVLType = TypeInfo.inferScalarType(&EVL);
-    VPValue *EVLMask = Builder.createICmp(
-        CmpInst::ICMP_ULT,
-        Builder.createNaryOp(VPInstruction::StepVector, {}, EVLType), &EVL);
     HeaderMask->replaceAllUsesWith(EVLMask);
     ToErase.push_back(HeaderMask->getDefiningRecipe());
   }
 
+  // Try to optimize header mask recipes away to their EVL variants.
+  // TODO: Split optimizeMaskToEVL out and move into
+  // VPlanTransforms::optimize. transformRecipestoEVLRecipes should be run in
+  // tryToBuildVPlanWithVPRecipes beforehand.
+  for (VPUser *U : collectUsersRecursively(EVLMask)) {
+    auto *CurRecipe = cast<VPRecipeBase>(U);
+    VPRecipeBase *EVLRecipe =
+        optimizeMaskToEVL(EVLMask, *CurRecipe, TypeInfo, *AllOneMask, EVL);
+    if (!EVLRecipe)
+      continue;
+
+    [[maybe_unused]] unsigned NumDefVal = EVLRecipe->getNumDefinedValues();
+    assert(NumDefVal == CurRecipe->getNumDefinedValues() &&
+           "New recipe must define the same number of values as the "
+           "original.");
+    assert(NumDefVal <= 1 &&
+           "Only supports recipes with a single definition or without users.");
+    EVLRecipe->insertBefore(CurRecipe);
+    if (isa<VPSingleDefRecipe, VPWidenLoadEVLRecipe>(EVLRecipe)) {
+      VPValue *CurVPV = CurRecipe->getVPSingleValue();
+      CurVPV->replaceAllUsesWith(EVLRecipe->getVPSingleValue());
+    }
+    ToErase.push_back(CurRecipe);
+  }
+
   for (VPRecipeBase *R : reverse(ToErase)) {
     SmallVector<VPValue *> PossiblyDead(R->operands());
     R->eraseFromParent();