[PATCH] D20652: [BasicAA] Extend negative offset logic from Allocas to GlobalVariables

[Michael Kuperstein via llvm-commits]

mkuper created this revision.
mkuper added reviewers: nlewycky, qcolombet, davidxl.
mkuper added subscribers: llvm-commits, dberlin, wmi.

This extends the logic of http://reviews.llvm.org/rL270777 to also apply to GlobalVariables in addition to Allocas.
I don't think there's any other kind of base object this can cover - it doesn't seem safe for arguments, noalias calls, or global aliases.

http://reviews.llvm.org/D20652

Files:
  include/llvm/Analysis/BasicAliasAnalysis.h
  lib/Analysis/BasicAliasAnalysis.cpp
  test/Analysis/BasicAA/negoffset.ll

Index: test/Analysis/BasicAA/negoffset.ll
===================================================================
--- test/Analysis/BasicAA/negoffset.ll
+++ test/Analysis/BasicAA/negoffset.ll
@@ -26,6 +26,17 @@
   ret void
 }
 
+ at gv = global i32 1
+; CHECK-LABEL: Function: global:
+; CHECK-DAG: MayAlias: i32* %p0, i32* @gv
+; CHECK-DAG: NoAlias:  i32* %p1, i32* @gv
+define void @global() {
+  %random = call i32* @random.i32(i32* @gv)
+  %p0 = getelementptr inbounds i32, i32* %random, i32 0
+  %p1 = getelementptr inbounds i32, i32* %random, i32 1
+  ret void
+}
+
 ; CHECK-LABEL: Function: struct:
 ; CHECK-DAG:  MayAlias: i32* %f0, i32* %p0
 ; CHECK-DAG:  MayAlias: i32* %f1, i32* %p0
Index: lib/Analysis/BasicAliasAnalysis.cpp
===================================================================
--- lib/Analysis/BasicAliasAnalysis.cpp
+++ lib/Analysis/BasicAliasAnalysis.cpp
@@ -964,6 +964,11 @@
 // repsect to the alloca, that means the GEP can not alias pointer (b).
 // Note that the pointer based on the alloca may not be a GEP. For
 // example, it may be the alloca itself.
+// The same applies if (b) is based on a GlobalVariable. Note that just being
+// based on isIdentifiedObject() is not enough - we need an identified object
+// that does not permit access to negative offsets. For example, a negative
+// offset from a noalias argument or call can be inbounds w.r.t the actual
+// underlying object.
 //
 // For example, consider:
 //
@@ -986,19 +991,22 @@
 // the highest %f1 can be is (%alloca + 3). This means %random can not be higher
 // than (%alloca - 1), and so is not inbounds, a contradiction.
 bool BasicAAResult::isGEPBaseAtNegativeOffset(const GEPOperator *GEPOp,
-      const DecomposedGEP &DecompGEP, const DecomposedGEP &DecompAlloca, 
-      uint64_t AllocaAccessSize) {
-  // If the alloca access size is unknown, or the GEP isn't inbounds, bail.
-  if (AllocaAccessSize == MemoryLocation::UnknownSize || !GEPOp->isInBounds())
+      const DecomposedGEP &DecompGEP, const DecomposedGEP &DecompObject, 
+      uint64_t ObjectAccessSize) {
+  // If the object access size is unknown, or the GEP isn't inbounds, bail.
+  if (ObjectAccessSize == MemoryLocation::UnknownSize || !GEPOp->isInBounds())
     return false;
 
-  // We need an alloca, and want to know the offset of the pointer
-  // from the alloca precisely, so no variable indices are allowed.
-  if (!isa<AllocaInst>(DecompAlloca.Base) || !DecompAlloca.VarIndices.empty())
+  // We need the object to be an alloca or a globalvariable, and want to know
+  // the offset of the pointer from the object precisely, so no variable
+  // indices are allowed.
+  if (!(isa<AllocaInst>(DecompObject.Base) ||
+        isa<GlobalVariable>(DecompObject.Base)) ||
+      !DecompObject.VarIndices.empty())
     return false;
 
-  int64_t AllocaBaseOffset = DecompAlloca.StructOffset +
-                             DecompAlloca.OtherOffset;
+  int64_t ObjectBaseOffset = DecompObject.StructOffset +
+                             DecompObject.OtherOffset;
 
   // If the GEP has no variable indices, we know the precise offset
   // from the base, then use it. If the GEP has variable indices, we're in
@@ -1009,7 +1017,7 @@
   if (DecompGEP.VarIndices.empty())
     GEPBaseOffset += DecompGEP.OtherOffset;
 
-  return (GEPBaseOffset >= AllocaBaseOffset + (int64_t)AllocaAccessSize);
+  return (GEPBaseOffset >= ObjectBaseOffset + (int64_t)ObjectAccessSize);
 }
 
 /// Provides a bunch of ad-hoc rules to disambiguate a GEP instruction against
Index: include/llvm/Analysis/BasicAliasAnalysis.h
===================================================================
--- include/llvm/Analysis/BasicAliasAnalysis.h
+++ include/llvm/Analysis/BasicAliasAnalysis.h
@@ -157,8 +157,8 @@
       const DataLayout &DL, AssumptionCache *AC, DominatorTree *DT);
 
   static bool isGEPBaseAtNegativeOffset(const GEPOperator *GEPOp,
-      const DecomposedGEP &DecompGEP, const DecomposedGEP &DecompAlloca,
-      uint64_t AllocaAccessSize);
+      const DecomposedGEP &DecompGEP, const DecomposedGEP &DecompObject,
+      uint64_t ObjectAccessSize);
 
   /// \brief A Heuristic for aliasGEP that searches for a constant offset
   /// between the variables.


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