[llvm] [NFC][IRCE] Don't require LoopStructure to determine IRCE profitability (PR #116384)

[Jan Ječmen via llvm-commits]

https://github.com/JanJecmen created https://github.com/llvm/llvm-project/pull/116384

This refactoring hoists the profitability check earlier in the pipeline, so that for loops that are not profitable to transform there is no iteration over the basic blocks or LoopStructure computation.

Motivated by PR #104659 that tweaks how the profitability of individual branches is evaluated.

>From 6302fab8bdbea5176b0d54eb73bb55bf323588e0 Mon Sep 17 00:00:00 2001
From: Jan Jecmen <jjecmen at azul.com>
Date: Fri, 15 Nov 2024 11:24:36 +0000
Subject: [PATCH] [NFC][IRCE] Don't require LoopStructure to determine IRCE
 profitability

This refactoring hoists the profitability check earlier in the pipeline, so that for loops that are not profitable to transform there is no iteration over the basic blocks or LoopStructure computation.

Motivated by PR #104659 that tweaks how the profitability of individual branches is evaluated.
---
 .../Scalar/InductiveRangeCheckElimination.cpp | 24 ++++++++++++-------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp b/llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
index a49dc7d30a00b0..0bc783412595e5 100644
--- a/llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
@@ -248,7 +248,7 @@ class InductiveRangeCheckElimination {
 
   // Returns true if it is profitable to do a transform basing on estimation of
   // number of iterations.
-  bool isProfitableToTransform(const Loop &L, LoopStructure &LS);
+  bool isProfitableToTransform(const Loop &L);
 
 public:
   InductiveRangeCheckElimination(ScalarEvolution &SE,
@@ -938,14 +938,12 @@ PreservedAnalyses IRCEPass::run(Function &F, FunctionAnalysisManager &AM) {
   return getLoopPassPreservedAnalyses();
 }
 
-bool
-InductiveRangeCheckElimination::isProfitableToTransform(const Loop &L,
-                                                        LoopStructure &LS) {
+bool InductiveRangeCheckElimination::isProfitableToTransform(const Loop &L) {
   if (SkipProfitabilityChecks)
     return true;
   if (GetBFI) {
     BlockFrequencyInfo &BFI = (*GetBFI)();
-    uint64_t hFreq = BFI.getBlockFreq(LS.Header).getFrequency();
+    uint64_t hFreq = BFI.getBlockFreq(L.getHeader()).getFrequency();
     uint64_t phFreq = BFI.getBlockFreq(L.getLoopPreheader()).getFrequency();
     if (phFreq != 0 && hFreq != 0 && (hFreq / phFreq < MinRuntimeIterations)) {
       LLVM_DEBUG(dbgs() << "irce: could not prove profitability: "
@@ -958,8 +956,17 @@ InductiveRangeCheckElimination::isProfitableToTransform(const Loop &L,
 
   if (!BPI)
     return true;
+
+  auto *Latch = L.getLoopLatch();
+  if (!Latch)
+    return true;
+  auto *LatchBr = dyn_cast<BranchInst>(Latch->getTerminator());
+  if (!LatchBr)
+    return true;
+  auto LatchBrExitIdx = LatchBr->getSuccessor(0) == L.getHeader() ? 1 : 0;
+
   BranchProbability ExitProbability =
-      BPI->getEdgeProbability(LS.Latch, LS.LatchBrExitIdx);
+      BPI->getEdgeProbability(Latch, LatchBrExitIdx);
   if (ExitProbability > BranchProbability(1, MinRuntimeIterations)) {
     LLVM_DEBUG(dbgs() << "irce: could not prove profitability: "
                       << "the exit probability is too big " << ExitProbability
@@ -982,6 +989,9 @@ bool InductiveRangeCheckElimination::run(
     return false;
   }
 
+  if (!isProfitableToTransform(*L))
+    return false;
+
   LLVMContext &Context = Preheader->getContext();
   SmallVector<InductiveRangeCheck, 16> RangeChecks;
   bool Changed = false;
@@ -1017,8 +1027,6 @@ bool InductiveRangeCheckElimination::run(
     return Changed;
   }
   LoopStructure LS = *MaybeLoopStructure;
-  if (!isProfitableToTransform(*L, LS))
-    return Changed;
   const SCEVAddRecExpr *IndVar =
       cast<SCEVAddRecExpr>(SE.getMinusSCEV(SE.getSCEV(LS.IndVarBase), SE.getSCEV(LS.IndVarStep)));