[PATCH] D11900: [GMR] Be a bit smarter about which globals alias when doing recursive lookups

[Michael Kuperstein via llvm-commits]

mkuper updated this revision to Diff 31664.
mkuper added a comment.

Thanks, David!

Added the check mayBeOverwritten() check.

About the zero-size check, what if you have two consecutive zero-size globals, followed by a third (non-zero-size) global? (That sounds like nonsense, but I don't quite understand the semantics of zero-size globals to begin with.)


http://reviews.llvm.org/D11900

Files:
  lib/Analysis/IPA/GlobalsModRef.cpp
  test/Analysis/GlobalsModRef/nonescaping-noalias.ll

Index: test/Analysis/GlobalsModRef/nonescaping-noalias.ll
===================================================================
--- test/Analysis/GlobalsModRef/nonescaping-noalias.ll
+++ test/Analysis/GlobalsModRef/nonescaping-noalias.ll
@@ -61,7 +61,9 @@
   ret i32 %v
 }
 
-define i32 @test4(i32* %param, i32 %n, i1 %c1, i1 %c2) {
+ at g3 = internal global i32 1
+
+define i32 @test4(i32* %param, i32 %n, i1 %c1, i1 %c2, i1 %c3) {
 ; Ensure that we can fold a store to a load of a global across a store to
 ; the pointer loaded from that global even when the load is behind PHIs and
 ; selects, and there is a mixture of a load and another global or argument.
@@ -77,14 +79,15 @@
   store i32 42, i32* @g1
   %ptr1 = load i32*, i32** @g2
   %ptr2 = select i1 %c1, i32* %ptr1, i32* %param
+  %ptr3 = select i1 %c3, i32* %ptr2, i32* @g3
   br label %loop
 
 loop:
   %iv = phi i32 [ 0, %entry ], [ %inc, %loop ]
-  %ptr = phi i32* [ %ptr2, %entry ], [ %ptr4, %loop ]
+  %ptr = phi i32* [ %ptr3, %entry ], [ %ptr5, %loop ]
   store i32 7, i32* %ptr
-  %ptr3 = load i32*, i32** @g2
-  %ptr4 = select i1 %c2, i32* %ptr3, i32* %call
+  %ptr4 = load i32*, i32** @g2
+  %ptr5 = select i1 %c2, i32* %ptr4, i32* %call
   %inc = add i32 %iv, 1
   %test = icmp slt i32 %inc, %n
   br i1 %test, label %loop, label %exit
Index: lib/Analysis/IPA/GlobalsModRef.cpp
===================================================================
--- lib/Analysis/IPA/GlobalsModRef.cpp
+++ lib/Analysis/IPA/GlobalsModRef.cpp
@@ -717,12 +717,20 @@
       if (InputGV == GV)
         return false;
 
-      // FIXME: It would be good to handle other obvious no-alias cases here, but
-      // it isn't clear how to do so reasonbly without building a small version
-      // of BasicAA into this code. We could recurse into AliasAnalysis::alias
-      // here but that seems likely to go poorly as we're inside the
-      // implementation of such a query. Until then, just conservatievly retun
-      // false.
+      // Distinct GlobalVariables never alias, unless zero-sized.
+      // FIXME: The condition can be refined, but be conservative for now.
+      auto *GVar = dyn_cast<GlobalVariable>(GV);
+      auto *InputGVar = dyn_cast<GlobalVariable>(InputGV);
+      
+      if (GVar && InputGVar &&
+          !GVar->isDeclaration() && !InputGVar->isDeclaration() &&
+          !GVar->mayBeOverridden() && !InputGVar->mayBeOverridden() &&
+          DL->getTypeAllocSize(GVar->getInitializer()->getType()) &&
+          DL->getTypeAllocSize(InputGVar->getInitializer()->getType()))
+        continue;
+
+      // Conservatively return false, even though we could be smarter
+      // (e.g. look through GlobalAliases).
       return false;
     }
 
@@ -767,7 +775,12 @@
       continue;
     }
 
-    // Unknown instruction, bail.
+    // FIXME: It would be good to handle other obvious no-alias cases here, but
+    // it isn't clear how to do so reasonbly without building a small version
+    // of BasicAA into this code. We could recurse into AliasAnalysis::alias
+    // here but that seems likely to go poorly as we're inside the
+    // implementation of such a query. Until then, just conservatievly retun
+    // false.
     return false;
   } while (!Inputs.empty());
 


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