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

2 files changed, 27 insertions, 4 deletions

diff --git a/lib/Transforms/IPO/AddReadAttrs.cpp b/lib/Transforms/IPO/AddReadAttrs.cpp
index 5460a5bde7..2835d5e9f3 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 0000000000..fe038c1be9
--- /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
+}