If a global is just loaded and restored, realize that it is not changing

value.  This allows us to turn more globals into constants and eliminate them.
This patch implements GlobalOpt/load-store-global.llx.

Note that this patch speeds up 255.vortex from:

Output/255.vortex.out-cbe.time:program 7.640000
Output/255.vortex.out-llc.time:program 9.810000

to:

Output/255.vortex.out-cbe.time:program 7.250000
Output/255.vortex.out-llc.time:program 9.490000

Which isn't bad at all!

llvm-svn: 17746

1 files changed, 9 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
index 054c5ee4306..b8edc32f16f 100644
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -155,14 +155,20 @@ static bool AnalyzeGlobal(Value *V, GlobalStatus &GS,
         // stores.
         if (GS.StoredType != GlobalStatus::isStored)
           if (GlobalVariable *GV = dyn_cast<GlobalVariable>(SI->getOperand(1))){
-            if (SI->getOperand(0) == GV->getInitializer()) {
+            Value *StoredVal = SI->getOperand(0);
+            if (StoredVal == GV->getInitializer()) {
+              if (GS.StoredType < GlobalStatus::isInitializerStored)
+                GS.StoredType = GlobalStatus::isInitializerStored;
+            } else if (isa<LoadInst>(StoredVal) &&
+                       cast<LoadInst>(StoredVal)->getOperand(0) == GV) {
+              // G = G
               if (GS.StoredType < GlobalStatus::isInitializerStored)
                 GS.StoredType = GlobalStatus::isInitializerStored;
             } else if (GS.StoredType < GlobalStatus::isStoredOnce) {
               GS.StoredType = GlobalStatus::isStoredOnce;
-              GS.StoredOnceValue = SI->getOperand(0);
+              GS.StoredOnceValue = StoredVal;
             } else if (GS.StoredType == GlobalStatus::isStoredOnce &&
-                       GS.StoredOnceValue == SI->getOperand(0)) {
+                       GS.StoredOnceValue == StoredVal) {
               // noop.
             } else {
               GS.StoredType = GlobalStatus::isStored;