From 17c20a6dfb7c89ac4eb13990308b731263345918 Mon Sep 17 00:00:00 2001
From: Nikita Popov <nikita.ppv@gmail.com>
Date: Tue, 5 Oct 2021 23:21:25 +0200
Subject: [PATCH] [SCEV] Avoid unnecessary domination checks (NFC)

When determining the defining scope, avoid repeatedly querying
dominationg against the function entry instruction. This ends up
begin a very common case that we can handle more efficiently.
---
 llvm/include/llvm/Analysis/ScalarEvolution.h |  5 +++--
 llvm/lib/Analysis/ScalarEvolution.cpp        | 19 +++++++++----------
 2 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/llvm/include/llvm/Analysis/ScalarEvolution.h b/llvm/include/llvm/Analysis/ScalarEvolution.h
index df1b7c2ad157..a38b1a299163 100644
--- a/llvm/include/llvm/Analysis/ScalarEvolution.h
+++ b/llvm/include/llvm/Analysis/ScalarEvolution.h
@@ -1921,9 +1921,10 @@ private:
   SCEV::NoWrapFlags getNoWrapFlagsFromUB(const Value *V);
 
   /// Return a scope which provides an upper bound on the defining scope of
-  /// 'S'.  Specifically, return the first instruction in said bounding scope.
+  /// 'S'. Specifically, return the first instruction in said bounding scope.
+  /// Return nullptr if the scope is trivial (function entry).
   /// (See scope definition rules associated with flag discussion above)
-  const Instruction *getDefiningScopeBound(const SCEV *S);
+  const Instruction *getNonTrivialDefiningScopeBound(const SCEV *S);
 
   /// Return a scope which provides an upper bound on the defining scope for
   /// a SCEV with the operands in Ops.
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index b767adac6a59..14f5dd51c406 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -6585,25 +6585,24 @@ SCEV::NoWrapFlags ScalarEvolution::getNoWrapFlagsFromUB(const Value *V) {
   return isSCEVExprNeverPoison(BinOp) ? Flags : SCEV::FlagAnyWrap;
 }
 
-const Instruction *ScalarEvolution::getDefiningScopeBound(const SCEV *S) {
+const Instruction *
+ScalarEvolution::getNonTrivialDefiningScopeBound(const SCEV *S) {
   if (auto *AddRec = dyn_cast<SCEVAddRecExpr>(S))
     return &*AddRec->getLoop()->getHeader()->begin();
   if (auto *U = dyn_cast<SCEVUnknown>(S))
     if (auto *I = dyn_cast<Instruction>(U->getValue()))
       return I;
-  // All SCEVs are bound by the entry to F
-  return &*F.getEntryBlock().begin();
+  return nullptr;
 }
 
 const Instruction *
 ScalarEvolution::getDefiningScopeBound(ArrayRef<const SCEV *> Ops) {
-  const Instruction *Bound = &*F.getEntryBlock().begin();
-  for (auto *S : Ops) {
-    auto *DefI = getDefiningScopeBound(S);
-    if (DT.dominates(Bound, DefI))
-      Bound = DefI;
-  }
-  return Bound;
+  const Instruction *Bound = nullptr;
+  for (auto *S : Ops)
+    if (auto *DefI = getNonTrivialDefiningScopeBound(S))
+      if (!Bound || DT.dominates(Bound, DefI))
+        Bound = DefI;
+  return Bound ? Bound : &*F.getEntryBlock().begin();
 }