From 840257a49c26c5c7d66ee17eeaaf5a43fad0b9da Mon Sep 17 00:00:00 2001
From: Hal Finkel <hfinkel@anl.gov>
Date: Mon, 3 Nov 2014 23:19:16 +0000
Subject: Use AA in LoadCombine

LoadCombine can be smarter about aborting when a writing instruction is
encountered, instead of aborting upon encountering any writing instruction, use
an AliasSetTracker, and only abort when encountering some write that might
alias with the loads that could potentially be combined.

This was originally motivated by comments made (and a test case provided) by
David Majnemer in response to PR21448. It turned out that LoadCombine was not
responsible for that PR, but LoadCombine should also be improved so that
unrelated stores (and @llvm.assume) don't interrupt load combining.

llvm-svn: 221203
---
 llvm/lib/Transforms/Scalar/LoadCombine.cpp | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

(limited to 'llvm/lib/Transforms/Scalar/LoadCombine.cpp')

diff --git a/llvm/lib/Transforms/Scalar/LoadCombine.cpp b/llvm/lib/Transforms/Scalar/LoadCombine.cpp
index 648626a7c12..11e4d7606d9 100644
--- a/llvm/lib/Transforms/Scalar/LoadCombine.cpp
+++ b/llvm/lib/Transforms/Scalar/LoadCombine.cpp
@@ -15,6 +15,8 @@
 
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/AliasSetTracker.h"
 #include "llvm/Analysis/TargetFolder.h"
 #include "llvm/Pass.h"
 #include "llvm/IR/DataLayout.h"
@@ -51,11 +53,12 @@ struct LoadPOPPair {
 class LoadCombine : public BasicBlockPass {
   LLVMContext *C;
   const DataLayout *DL;
+  AliasAnalysis *AA;
 
 public:
   LoadCombine()
       : BasicBlockPass(ID),
-        C(nullptr), DL(nullptr) {
+        C(nullptr), DL(nullptr), AA(nullptr) {
     initializeSROAPass(*PassRegistry::getPassRegistry());
   }
   
@@ -225,19 +228,23 @@ bool LoadCombine::runOnBasicBlock(BasicBlock &BB) {
   if (skipOptnoneFunction(BB) || !DL)
     return false;
 
+  AA = &getAnalysis<AliasAnalysis>();
+
   IRBuilder<true, TargetFolder>
   TheBuilder(BB.getContext(), TargetFolder(DL));
   Builder = &TheBuilder;
 
   DenseMap<const Value *, SmallVector<LoadPOPPair, 8>> LoadMap;
+  AliasSetTracker AST(*AA);
 
   bool Combined = false;
   unsigned Index = 0;
   for (auto &I : BB) {
-    if (I.mayWriteToMemory() || I.mayThrow()) {
+    if (I.mayThrow() || (I.mayWriteToMemory() && AST.containsUnknown(&I))) {
       if (combineLoads(LoadMap))
         Combined = true;
       LoadMap.clear();
+      AST.clear();
       continue;
     }
     LoadInst *LI = dyn_cast<LoadInst>(&I);
@@ -250,6 +257,7 @@ bool LoadCombine::runOnBasicBlock(BasicBlock &BB) {
     if (!POP.Pointer)
       continue;
     LoadMap[POP.Pointer].push_back(LoadPOPPair(LI, POP, Index++));
+    AST.add(LI);
   }
   if (combineLoads(LoadMap))
     Combined = true;
@@ -258,6 +266,9 @@ bool LoadCombine::runOnBasicBlock(BasicBlock &BB) {
 
 void LoadCombine::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.setPreservesCFG();
+
+  AU.addRequired<AliasAnalysis>();
+  AU.addPreserved<AliasAnalysis>();
 }
 
 char LoadCombine::ID = 0;
@@ -266,5 +277,9 @@ BasicBlockPass *llvm::createLoadCombinePass() {
   return new LoadCombine();
 }
 
-INITIALIZE_PASS(LoadCombine, "load-combine", "Combine Adjacent Loads", false,
-                false)
+INITIALIZE_PASS_BEGIN(LoadCombine, "load-combine", "Combine Adjacent Loads",
+                      false, false)
+INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
+INITIALIZE_PASS_END(LoadCombine, "load-combine", "Combine Adjacent Loads",
+                    false, false)
+
-- 
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