[llvm] [BasicAA] Treat IntToPtr(Argument) similarly to Argument in relation to function-local objects. (PR #134505)

[David Green via llvm-commits]

https://github.com/davemgreen created https://github.com/llvm/llvm-project/pull/134505

Given a IntToPtr or an argument, or of an extract from an argument (that can come up from coerced call parameter), it cannot alias with an object that is function local. My understanding is that essentially from dataflow alone we can prove noalias, not requiring any provenance or whether the pointer escapes via the inttoptr. That is true even for noalias arguments if my reading of D101541 and the Pointer Aliasing Rules is correct, but I am really not an expert.

See https://reviews.llvm.org/D101541 for the last time this came up in the altered test cases.

>From 1c308aa8adf9ddb03726d2d35dfa1689a438f8d5 Mon Sep 17 00:00:00 2001
From: David Green <david.green at arm.com>
Date: Sat, 5 Apr 2025 21:39:41 +0100
Subject: [PATCH] [BasicAA] Treat IntToPtr(Argument) similarly to Argument in
 relation to function-local objects.

Given a IntToPtr or an argument, or of a extract from an argument that can come
up from coerced call parameter, it cannot alias with an Object that is function
local. My understanding is that essentially from dataflow alone we can prove
noalias, not requiring any provenance or whether the pointer escapes via the
inttoptr. That is true even for noalias arguments if my reading of D101541 and
the Pointer Aliasing Rules is correct, but I am really not an expert.

See https://reviews.llvm.org/D101541 for the last time this came up in the
altered test cases.
---
 llvm/lib/Analysis/BasicAliasAnalysis.cpp      | 21 +++++++++++++++++--
 .../test/Analysis/BasicAA/noalias-inttoptr.ll | 20 ++++++++++++++----
 2 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 4d1a95a0c4b43..66b2c1050d59c 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1533,6 +1533,23 @@ AliasResult BasicAAResult::aliasPHI(const PHINode *PN, LocationSize PNSize,
   return Alias;
 }
 
+// Return true for an Argument, IntToPtr(Argument) or
+// IntToPtr(ExtractValue(Argument)). These are all known
+// to not alias with FunctionLocal objects and can come up
+// from coerced function arguments.
+static bool isArgumentOrArgumentLike(const Value *V) {
+  if (isa<Argument>(V))
+    return true;
+  if (auto *P = dyn_cast<IntToPtrInst>(V)) {
+    const Value *POp = P->getOperand(0);
+    if (isa<Argument>(POp))
+      return true;
+    if (auto *E = dyn_cast<ExtractValueInst>(POp))
+      return isa<Argument>(E->getOperand(0));
+  }
+  return false;
+}
+
 /// Provides a bunch of ad-hoc rules to disambiguate in common cases, such as
 /// array references.
 AliasResult BasicAAResult::aliasCheck(const Value *V1, LocationSize V1Size,
@@ -1585,8 +1602,8 @@ AliasResult BasicAAResult::aliasCheck(const Value *V1, LocationSize V1Size,
 
     // Function arguments can't alias with things that are known to be
     // unambigously identified at the function level.
-    if ((isa<Argument>(O1) && isIdentifiedFunctionLocal(O2)) ||
-        (isa<Argument>(O2) && isIdentifiedFunctionLocal(O1)))
+    if ((isArgumentOrArgumentLike(O1) && isIdentifiedFunctionLocal(O2)) ||
+        (isArgumentOrArgumentLike(O2) && isIdentifiedFunctionLocal(O1)))
       return AliasResult::NoAlias;
 
     // If one pointer is the result of a call/invoke or load and the other is a
diff --git a/llvm/test/Analysis/BasicAA/noalias-inttoptr.ll b/llvm/test/Analysis/BasicAA/noalias-inttoptr.ll
index 24bbcc55b3202..4e48dd048cf6b 100644
--- a/llvm/test/Analysis/BasicAA/noalias-inttoptr.ll
+++ b/llvm/test/Analysis/BasicAA/noalias-inttoptr.ll
@@ -24,10 +24,10 @@ define void @test2(i64 %Q_as_int) {
   ret void
 }
 
-; Verify that escaped noalias parameter may alias inttoptr
+; Verify that escaped noalias parameter are no alias inttoptr
 define void @test3(ptr noalias %P, i64 %Q_as_int) {
   ; CHECK-LABEL: Function: test3:
-  ; CHECK: MayAlias:	i8* %P, i8* %Q
+  ; CHECK: NoAlias:	i8* %P, i8* %Q
   call void @escape(ptr %P)
   %Q = inttoptr i64 %Q_as_int to ptr
   store i8 0, ptr %P
@@ -35,10 +35,10 @@ define void @test3(ptr noalias %P, i64 %Q_as_int) {
   ret void
 }
 
-; Verify that escaped alloca may alias inttoptr
+; Verify that escaped alloca are noalias inttoptr
 define void @test4(i64 %Q_as_int) {
   ; CHECK-LABEL: Function: test4:
-  ; CHECK: MayAlias:	i8* %P, i8* %Q
+  ; CHECK: NoAlias:	i8* %P, i8* %Q
   %P = alloca i8
   call void @escape(ptr %P)
   %Q = inttoptr i64 %Q_as_int to ptr
@@ -58,3 +58,15 @@ define void @test5(i64 %Q_as_int) {
   store i8 1, ptr %Q
   ret void
 }
+
+; Verify that extractvalue of a coerced argument are noalias a function local object
+define void @test6([2 x i64] %Q.coerce) {
+  ; CHECK-LABEL: Function: test6:
+  ; CHECK: NoAlias:	i8* %P, i8* %Q
+  %P = alloca i8
+  %Q_as_int = extractvalue [2 x i64] %Q.coerce, 1
+  %Q = inttoptr i64 %Q_as_int to ptr
+  store i8 0, ptr %P
+  store i8 1, ptr %Q
+  ret void
+}