From: Dan Gohman <gohman@apple.com>
Date: Thu, 5 Aug 2010 22:09:15 +0000 (+0000)
Subject: Fix memdep's code for reasoning about dependences between two calls. A Ref
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=5fa417c7904f7394d4e6dcb86e366c86867bcb5a;p=oota-llvm.git

Fix memdep's code for reasoning about dependences between two calls. A Ref
response from getModRefInfo is not useful here. Instead, check for identical
calls only in the NoModRef case.

Reapply r110270, and strengthen it to compensate for the memdep changes.
When both calls are readonly, there is no dependence between them.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110382 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index 6bb84d4e458..11cba991a95 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -425,8 +425,13 @@ BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS1,
   ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
   if (CS2B == DoesNotAccessMemory) return NoModRef;
   
-  // If they both only read from memory, just return ref.
+  // If they both only read from memory, there is no dependence.
   if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
+    return NoModRef;
+
+  // If CS1 only reads memory, the only dependence on CS2 can be
+  // from CS1 reading memory written by CS2.
+  if (CS1B == OnlyReadsMemory)
     return Ref;
   
   // Otherwise, fall back to NoAA (mod+ref).
diff --git a/lib/Analysis/MemoryDependenceAnalysis.cpp b/lib/Analysis/MemoryDependenceAnalysis.cpp
index e97bc3b6727..e003d643691 100644
--- a/lib/Analysis/MemoryDependenceAnalysis.cpp
+++ b/lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -126,26 +126,15 @@ getCallSiteDependencyFrom(CallSite CS, bool isReadOnlyCall,
       // If these two calls do not interfere, look past it.
       switch (AA->getModRefInfo(CS, InstCS)) {
       case AliasAnalysis::NoModRef:
-        // If the two calls don't interact (e.g. InstCS is readnone) keep
-        // scanning.
+        // If the two calls are the same, return InstCS as a Def, so that
+        // CS can be found redundant and eliminated.
+        if (isReadOnlyCall && InstCS.onlyReadsMemory() &&
+            CS.getInstruction()->isIdenticalToWhenDefined(Inst))
+          return MemDepResult::getDef(Inst);
+
+        // Otherwise if the two calls don't interact (e.g. InstCS is readnone)
+        // keep scanning.
         continue;
-      case AliasAnalysis::Ref:
-        // If the two calls read the same memory locations and CS is a readonly
-        // function, then we have two cases: 1) the calls may not interfere with
-        // each other at all.  2) the calls may produce the same value.  In case
-        // #1 we want to ignore the values, in case #2, we want to return Inst
-        // as a Def dependence.  This allows us to CSE in cases like:
-        //   X = strlen(P);
-        //    memchr(...);
-        //   Y = strlen(P);  // Y = X
-        if (isReadOnlyCall) {
-          if (CS.getCalledFunction() != 0 &&
-              CS.getCalledFunction() == InstCS.getCalledFunction())
-            return MemDepResult::getDef(Inst);
-          // Ignore unrelated read/read call dependences.
-          continue;
-        }
-        // FALL THROUGH
       default:
         return MemDepResult::getClobber(Inst);
       }
diff --git a/test/Analysis/BasicAA/getmodrefinfo-cs-cs.ll b/test/Analysis/BasicAA/getmodrefinfo-cs-cs.ll
index 953eaa847d1..f655913f3d1 100644
--- a/test/Analysis/BasicAA/getmodrefinfo-cs-cs.ll
+++ b/test/Analysis/BasicAA/getmodrefinfo-cs-cs.ll
@@ -1,5 +1,4 @@
 ; RUN: opt < %s -aa-eval -print-all-alias-modref-info -disable-output |& FileCheck %s
-; XFAIL: *
 
 ; CHECK: Just Ref: call void @ro() <-> call void @f0()