[PATCH] D20495: [BasicAA] An inbounds GEP and an alloca can't alias if the base of the GEP would point "below" the alloca

[David Li via llvm-commits]

davidxl added inline comments.

================
Comment at: lib/Analysis/BasicAliasAnalysis.cpp:355
@@ -356,2 +354,3 @@
+  Decomposed.OtherOffset = 0;
   do {
     // See if this is a bitcast or GEP.
----------------
Also clear the var indices?

================
Comment at: lib/Analysis/BasicAliasAnalysis.cpp:996
@@ +995,3 @@
+  // from the alloca precisely, so no variable indices are allowed.
+  if (!isa<AllocaInst>(DecompAlloca.Base) || !DecompAlloca.VarIndices.empty())
+    return false;
----------------
Is the VarIndices.empty() check too restrictive? 

struct A {
  int a[2];
  ...
};

struct B {
    int b1;
    int b2;
    int b3;
};

struct A a;
B *bp;

bp->b3 vs a.a[i] should produce no aliasing

================
Comment at: lib/Analysis/BasicAliasAnalysis.cpp:998
@@ -997,3 @@
-        // assert.
-        if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
-          return MayAlias;
----------------
Extract this unrelated cleanup into  separate patch.

================
Comment at: lib/Analysis/BasicAliasAnalysis.cpp:1035
@@ -1034,3 @@
-    // FIXME: They always have a DataLayout, so this should become an assert.
-    if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
-      return MayAlias;
----------------
separate out clean up code into different patch.

================
Comment at: lib/Analysis/BasicAliasAnalysis.cpp:1085
@@ -1084,3 @@
-    // FIXME: They always have a DataLayout, so this should become an assert.
-    if (GEP1BasePtr != UnderlyingV1) {
-      return MayAlias;
----------------
Ditto -- cleanup patch.


http://reviews.llvm.org/D20495