From 516ec62bfef9dcf7390811083d07ee9fe16fe091 Mon Sep 17 00:00:00 2001
From: Duncan Sands <baldrick@free.fr>
Date: Mon, 29 Dec 2008 11:34:09 +0000
Subject: [PATCH] Allow readnone functions to read (and write!) global
 constants, since doing so is irrelevant for aliasing purposes.  While this
 doesn't increase the total number of functions marked readonly or readnone in
 MultiSource/ Applications (3089), it does result in 12 functions being marked
 readnone rather than readonly. Before:   readnone: 820   readonly: 2269
 After:   readnone: 832   readonly: 2257

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61469 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/IPO/AddReadAttrs.cpp           | 23 +++++++++++++++----
 .../AddReadAttrs/2008-12-29-Constant.ll       |  8 +++++++
 2 files changed, 27 insertions(+), 4 deletions(-)
 create mode 100644 test/Transforms/AddReadAttrs/2008-12-29-Constant.ll

diff --git a/lib/Transforms/IPO/AddReadAttrs.cpp b/lib/Transforms/IPO/AddReadAttrs.cpp
index 5460a5bde7b..2835d5e9f36 100644
--- a/lib/Transforms/IPO/AddReadAttrs.cpp
+++ b/lib/Transforms/IPO/AddReadAttrs.cpp
@@ -17,6 +17,7 @@
 #define DEBUG_TYPE "addreadattrs"
 #include "llvm/Transforms/IPO.h"
 #include "llvm/CallGraphSCCPass.h"
+#include "llvm/GlobalVariable.h"
 #include "llvm/Instructions.h"
 #include "llvm/Analysis/CallGraph.h"
 #include "llvm/ADT/SmallPtrSet.h"
@@ -40,6 +41,8 @@ namespace {
       AU.setPreservesCFG();
       CallGraphSCCPass::getAnalysisUsage(AU);
     }
+
+    bool PointsToLocalMemory(Value *V);
   };
 }
 
@@ -50,6 +53,20 @@ X("addreadattrs", "Mark functions readnone/readonly");
 Pass *llvm::createAddReadAttrsPass() { return new AddReadAttrs(); }
 
 
+/// PointsToLocalMemory - Returns whether the given pointer value points to
+/// memory that is local to the function.  Global constants are considered
+/// local to all functions.
+bool AddReadAttrs::PointsToLocalMemory(Value *V) {
+  V = V->getUnderlyingObject();
+  // An alloca instruction defines local memory.
+  if (isa<AllocaInst>(V))
+    return true;
+  // A global constant counts as local memory for our purposes.
+  if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
+    return GV->isConstant();
+  return false;
+}
+
 bool AddReadAttrs::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
   SmallPtrSet<CallGraphNode *, 8> SCCNodes;
   CallGraph &CG = getAnalysis<CallGraph>();
@@ -96,14 +113,12 @@ bool AddReadAttrs::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
         if (SCCNodes.count(CG[CS.getCalledFunction()]))
           continue;
       } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
-        Value *Target = LI->getPointerOperand()->getUnderlyingObject();
         // Ignore loads from local memory.
-        if (isa<AllocaInst>(Target))
+        if (PointsToLocalMemory(LI->getPointerOperand()))
           continue;
       } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
-        Value *Target = SI->getPointerOperand()->getUnderlyingObject();
         // Ignore stores to local memory.
-        if (isa<AllocaInst>(Target))
+        if (PointsToLocalMemory(SI->getPointerOperand()))
           continue;
       }
 
diff --git a/test/Transforms/AddReadAttrs/2008-12-29-Constant.ll b/test/Transforms/AddReadAttrs/2008-12-29-Constant.ll
new file mode 100644
index 00000000000..fe038c1be9a
--- /dev/null
+++ b/test/Transforms/AddReadAttrs/2008-12-29-Constant.ll
@@ -0,0 +1,8 @@
+; RUN: llvm-as < %s | opt -addreadattrs | llvm-dis | grep readnone
+
+@s = external constant i8		; <i8*> [#uses=1]
+
+define i8 @f() {
+	%tmp = load i8* @s		; <i8> [#uses=1]
+	ret i8 %tmp
+}