[PATCH] D13512: [GlobalsAA] Loosen an overly conservative bailout

[James Molloy via llvm-commits]

jmolloy updated this revision to Diff 37225.
jmolloy added a comment.

Hi Chandler,

You're right, the previous patch was utter utter nonsense.

This updated patch covers all the cases I cared about and seems far more sound. Simply: instead of bailing out, recurse. If a captured value is found while following a chain of loads, all those loads are also captured.

This lets us hit the case I care about which is something like:

  static int a;
  static int **b;
  
  alias? (a, b[1][2]);

Cheers,

James


Repository:
  rL LLVM

http://reviews.llvm.org/D13512

Files:
  lib/Analysis/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
@@ -97,3 +97,20 @@
   %v = load i32, i32* @g1
   ret i32 %v
 }
+
+define i32 @test5(i32** %param) {
+; Ensure that we can fold a store to a load of a global across a store to
+; a parameter that has been dereferenced when the global is non-escaping.
+;
+; CHECK-LABEL: @test5(
+; CHECK: %p = load i32*
+; CHECK: store i32 42, i32* @g1
+; CHECK-NOT: load i32
+; CHECK: ret i32 42
+entry:
+  %p = load i32*, i32** %param
+  store i32 42, i32* @g1
+  store i32 7, i32* %p
+  %v = load i32, i32* @g1
+  ret i32 %v
+}
Index: lib/Analysis/GlobalsModRef.cpp
===================================================================
--- lib/Analysis/GlobalsModRef.cpp
+++ lib/Analysis/GlobalsModRef.cpp
@@ -666,22 +666,30 @@
       // non-addr-taken globals.
       continue;
     }
+    // Recurse through a limited number of selects, loads and PHIs. This is an
+    // arbitrary depth of 4, lower numbers could be used to fix compile time
+    // issues if needed, but this is generally expected to be only be important
+    // for small depths.
+    if (++Depth > 4)
+      return false;
+
     if (auto *LI = dyn_cast<LoadInst>(Input)) {
       // A pointer loaded from a global would have been captured, and we know
       // that the global is non-escaping, so no alias.
       if (isa<GlobalValue>(GetUnderlyingObject(LI->getPointerOperand(), DL)))
         continue;
 
-      // Otherwise, a load could come from anywhere, so bail.
-      return false;
+      auto *Ptr = GetUnderlyingObject(LI->getPointerOperand(), DL);
+      if (Ptr) {
+        // If the object this is loaded from is itself captured, then this
+        // pointer must have been captured too.
+        Inputs.push_back(Ptr);
+        continue;
+      } else {
+        // Otherwise, a load could come from anywhere, so bail.
+        return false;
+      }
     }
-
-    // Recurse through a limited number of selects and PHIs. This is an
-    // arbitrary depth of 4, lower numbers could be used to fix compile time
-    // issues if needed, but this is generally expected to be only be important
-    // for small depths.
-    if (++Depth > 4)
-      return false;
     if (auto *SI = dyn_cast<SelectInst>(Input)) {
       const Value *LHS = GetUnderlyingObject(SI->getTrueValue(), DL);
       const Value *RHS = GetUnderlyingObject(SI->getFalseValue(), DL);


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