[llvm] [SLP] NFC: Simplify CandidateVFs initialization (PR #144882)

[Sander de Smalen via llvm-commits]

https://github.com/sdesmalen-arm created https://github.com/llvm/llvm-project/pull/144882

Also adds a comment to clarify the meaning of MaxRegVF.

>From ad6f2775cb2cb97dd822c229a89103f37761d35c Mon Sep 17 00:00:00 2001
From: Sander de Smalen <sander.desmalen at arm.com>
Date: Thu, 19 Jun 2025 11:02:59 +0000
Subject: [PATCH] [SLP] NFC: Simplify CandidateVFs initialization

Also adds a comment to clarify the meaning of MaxRegVF.
---
 llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index 4551a365a6967..750c93a439c16 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -21198,7 +21198,11 @@ bool SLPVectorizerPass::vectorizeStores(
         }
       }
 
+      // MaxRegVF represents the number of instructions (scalar, or vector in
+      // case of revec) that can be vectorized to naturally fit in a vector
+      // register.
       unsigned MaxRegVF = MaxVF;
+
       MaxVF = std::min<unsigned>(MaxVF, bit_floor(Operands.size()));
       if (MaxVF < MinVF) {
         LLVM_DEBUG(dbgs() << "SLP: Vectorization infeasible as MaxVF (" << MaxVF
@@ -21207,13 +21211,11 @@ bool SLPVectorizerPass::vectorizeStores(
         continue;
       }
 
-      unsigned Sz = 1 + Log2_32(MaxVF) - Log2_32(MinVF);
-      SmallVector<unsigned> CandidateVFs(Sz + (NonPowerOf2VF > 0 ? 1 : 0));
-      unsigned Size = MinVF;
-      for (unsigned &VF : reverse(CandidateVFs)) {
-        VF = Size > MaxVF ? NonPowerOf2VF : Size;
-        Size *= 2;
-      }
+      SmallVector<unsigned> CandidateVFs;
+      for (unsigned VF = std::max(MaxVF, NonPowerOf2VF); VF >= MinVF;
+           VF = divideCeil(VF, 2))
+        CandidateVFs.push_back(VF);
+
       unsigned End = Operands.size();
       unsigned Repeat = 0;
       constexpr unsigned MaxAttempts = 4;