[BasicAA] Remove incorrect rule about constant pointers

[nikic]

BasicAA currently says that any Constant cannot alias an identified local object. This is not correct if the local object escaped, as it's possible to create a pointer to the escaped object using an inttoptr constant expression base.

To compensate for this, make sure that inttoptr constant expressions are treated as escape sources, just like inttoptr instructions. This ensures that the optimization can still be applied if the local object is non-escaping. This is sufficient to still optimize the original motivation case from c53e2ec.

Fixes #76789.

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

Author: Nikita Popov (nikic)

Changes

BasicAA currently says that any Constant cannot alias an identified local object. This is not correct if the local object escaped, as it's possible to create a pointer to the escaped object using an inttoptr constant expression base.

To compensate for this, make sure that inttoptr constant expressions are treated as escape sources, just like inttoptr instructions. This ensures that the optimization can still be applied if the local object is non-escaping. This is sufficient to still optimize the original motivation case from c53e2ec.

Fixes #76789.

---

Full diff: https://github.com/llvm/llvm-project/pull/76815.diff

4 Files Affected:

• (modified) llvm/include/llvm/Analysis/AliasAnalysis.h (+2)
• (modified) llvm/lib/Analysis/AliasAnalysis.cpp (+5)
• (modified) llvm/lib/Analysis/BasicAliasAnalysis.cpp (+9-10)
• (modified) llvm/test/Analysis/BasicAA/inttoptr_constexpr.ll (+3-3)

diff --git a/llvm/include/llvm/Analysis/AliasAnalysis.h b/llvm/include/llvm/Analysis/AliasAnalysis.h
index e1cfb025fb6580..d6f732d35fd4cd 100644
--- a/llvm/include/llvm/Analysis/AliasAnalysis.h
+++ b/llvm/include/llvm/Analysis/AliasAnalysis.h
@@ -154,6 +154,8 @@ struct CaptureInfo {
 
   /// Check whether Object is not captured before instruction I. If OrAt is
   /// true, captures by instruction I itself are also considered.
+  ///
+  /// If I is nullptr, then captures at any point will be considered.
   virtual bool isNotCapturedBefore(const Value *Object, const Instruction *I,
                                    bool OrAt) = 0;
 };
diff --git a/llvm/lib/Analysis/AliasAnalysis.cpp b/llvm/lib/Analysis/AliasAnalysis.cpp
index da18279ae9b93d..6eaaad5f332eb9 100644
--- a/llvm/lib/Analysis/AliasAnalysis.cpp
+++ b/llvm/lib/Analysis/AliasAnalysis.cpp
@@ -883,6 +883,11 @@ bool llvm::isEscapeSource(const Value *V) {
   if (isa<IntToPtrInst>(V))
     return true;
 
+  // Same for inttoptr constant expressions.
+  if (auto *CE = dyn_cast<ConstantExpr>(V))
+    if (CE->getOpcode() == Instruction::IntToPtr)
+      return true;
+
   return false;
 }
 
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 3de147368f2346..410e17d5564269 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -215,7 +215,7 @@ bool EarliestEscapeInfo::isNotCapturedBefore(const Value *Object,
   auto Iter = EarliestEscapes.insert({Object, nullptr});
   if (Iter.second) {
     Instruction *EarliestCapture = FindEarliestCapture(
-        Object, *const_cast<Function *>(I->getFunction()),
+        Object, *const_cast<Function *>(DT.getRoot()->getParent()),
         /*ReturnCaptures=*/false, /*StoreCaptures=*/true, DT);
     if (EarliestCapture) {
       auto Ins = Inst2Obj.insert({EarliestCapture, {}});
@@ -228,6 +228,10 @@ bool EarliestEscapeInfo::isNotCapturedBefore(const Value *Object,
   if (!Iter.first->second)
     return true;
 
+  // No context instruction means any use is capturing.
+  if (!I)
+    return false;
+
   if (I == Iter.first->second) {
     if (OrAt)
       return false;
@@ -1504,11 +1508,6 @@ AliasResult BasicAAResult::aliasCheck(const Value *V1, LocationSize V1Size,
     if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
       return AliasResult::NoAlias;
 
-    // Constant pointers can't alias with non-const isIdentifiedObject objects.
-    if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
-        (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
-      return AliasResult::NoAlias;
-
     // Function arguments can't alias with things that are known to be
     // unambigously identified at the function level.
     if ((isa<Argument>(O1) && isIdentifiedFunctionLocal(O2)) ||
@@ -1524,11 +1523,11 @@ AliasResult BasicAAResult::aliasCheck(const Value *V1, LocationSize V1Size,
     // temporary store the nocapture argument's value in a temporary memory
     // location if that memory location doesn't escape. Or it may pass a
     // nocapture value to other functions as long as they don't capture it.
-    if (isEscapeSource(O1) &&
-        AAQI.CI->isNotCapturedBefore(O2, cast<Instruction>(O1), /*OrAt*/ true))
+    if (isEscapeSource(O1) && AAQI.CI->isNotCapturedBefore(
+                                  O2, dyn_cast<Instruction>(O1), /*OrAt*/ true))
       return AliasResult::NoAlias;
-    if (isEscapeSource(O2) &&
-        AAQI.CI->isNotCapturedBefore(O1, cast<Instruction>(O2), /*OrAt*/ true))
+    if (isEscapeSource(O2) && AAQI.CI->isNotCapturedBefore(
+                                  O1, dyn_cast<Instruction>(O2), /*OrAt*/ true))
       return AliasResult::NoAlias;
   }
 
diff --git a/llvm/test/Analysis/BasicAA/inttoptr_constexpr.ll b/llvm/test/Analysis/BasicAA/inttoptr_constexpr.ll
index afe3602d1c7e5e..6d4ad08d1621b5 100644
--- a/llvm/test/Analysis/BasicAA/inttoptr_constexpr.ll
+++ b/llvm/test/Analysis/BasicAA/inttoptr_constexpr.ll
@@ -1,6 +1,6 @@
 ; RUN: opt -passes=aa-eval -print-all-alias-modref-info -disable-output < %s 2>&1 | FileCheck %s
 
-; CHECK: NoAlias: i8* %a, i8* %gep
+; CHECK: MayAlias: i8* %a, i8* %gep
 define void @inttoptr_alloca() {
   %a = alloca i8
   %a.int = ptrtoint ptr %a to i64
@@ -11,7 +11,7 @@ define void @inttoptr_alloca() {
   ret void
 }
 
-; CHECK: NoAlias: i8* %a, i8* %gep
+; CHECK: MayAlias: i8* %a, i8* %gep
 define void @inttoptr_alloca_unknown_relation(i64 %offset) {
   %a = alloca i8
   %a.int = ptrtoint ptr %a to i64
@@ -30,7 +30,7 @@ define void @inttoptr_alloca_noescape(i64 %offset) {
   ret void
 }
 
-; CHECK: NoAlias: i8* %a, i8* %gep
+; CHECK: MayAlias: i8* %a, i8* %gep
 define void @inttoptr_noalias(ptr noalias %a) {
   %a.int = ptrtoint ptr %a to i64
   %a.int.1 = add i64 %a.int, 1

[nikic]

ping

[fhahn]

LGTM, thanks!