[llvm] [Attributor] Do not optimize away externally_initialized loads. (PR #128170)

[Pierre van Houtryve via llvm-commits]

https://github.com/Pierre-vh updated https://github.com/llvm/llvm-project/pull/128170

>From 06c412f40f717506e3122ed860827a8f03eb32f4 Mon Sep 17 00:00:00 2001
From: pvanhout <pierre.vanhoutryve at amd.com>
Date: Fri, 21 Feb 2025 13:55:16 +0100
Subject: [PATCH 1/3] [Attributor] Do not optimize away externally_initialized
 loads.

Fixes SWDEV-515029
---
 llvm/lib/Transforms/IPO/Attributor.cpp        |  3 ++-
 .../Transforms/Attributor/value-simplify.ll   | 19 +++++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/IPO/Attributor.cpp b/llvm/lib/Transforms/IPO/Attributor.cpp
index a1e1a51b201b0..0865e4da1a071 100644
--- a/llvm/lib/Transforms/IPO/Attributor.cpp
+++ b/llvm/lib/Transforms/IPO/Attributor.cpp
@@ -260,7 +260,8 @@ AA::getInitialValueForObj(Attributor &A, const AbstractAttribute &QueryingAA,
       return nullptr;
   } else {
     if (!GV->hasLocalLinkage() &&
-        (GV->isInterposable() || !(GV->isConstant() && GV->hasInitializer())))
+        (GV->isInterposable() || GV->isExternallyInitialized() ||
+         !(GV->isConstant() && GV->hasInitializer())))
       return nullptr;
     if (!GV->hasInitializer())
       return UndefValue::get(&Ty);
diff --git a/llvm/test/Transforms/Attributor/value-simplify.ll b/llvm/test/Transforms/Attributor/value-simplify.ll
index a90fb54fbe74a..59160c0834980 100644
--- a/llvm/test/Transforms/Attributor/value-simplify.ll
+++ b/llvm/test/Transforms/Attributor/value-simplify.ll
@@ -12,6 +12,7 @@ declare ptr @llvm.call.preallocated.arg(token, i32)
 @ConstPtr = constant i32 0, align 4
 @ConstWeakPtr = weak constant i32 0, align 4
 @ConstWeakODRPtr = weak_odr constant i32 0, align 4
+ at ExtInitZeroInit = externally_initialized constant i32 zeroinitializer, align 4
 
 ;.
 ; CHECK: @str = private unnamed_addr addrspace(4) constant [1 x i8] zeroinitializer, align 1
@@ -19,6 +20,7 @@ declare ptr @llvm.call.preallocated.arg(token, i32)
 ; CHECK: @ConstPtr = constant i32 0, align 4
 ; CHECK: @ConstWeakPtr = weak constant i32 0, align 4
 ; CHECK: @ConstWeakODRPtr = weak_odr constant i32 0, align 4
+; CHECK: @ExtInitZeroInit = externally_initialized constant i32 0, align 4
 ; CHECK: @S = external global %struct.X
 ; CHECK: @g = internal constant { [2 x ptr] } { [2 x ptr] [ptr @f1, ptr @f2] }
 ; CHECK: @x = external global i32
@@ -1651,6 +1653,23 @@ define i32 @readWeakOdrConst() {
   ret i32 %l
 }
 
+define i32 @readExtInitZeroInit() {
+; TUNIT: Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none)
+; TUNIT-LABEL: define {{[^@]+}}@readExtInitZeroInit
+; TUNIT-SAME: () #[[ATTR2]] {
+; TUNIT-NEXT:    [[L:%.*]] = load i32, ptr @ExtInitZeroInit, align 4
+; TUNIT-NEXT:    ret i32 [[L]]
+;
+; CGSCC: Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none)
+; CGSCC-LABEL: define {{[^@]+}}@readExtInitZeroInit
+; CGSCC-SAME: () #[[ATTR1]] {
+; CGSCC-NEXT:    [[L:%.*]] = load i32, ptr @ExtInitZeroInit, align 4
+; CGSCC-NEXT:    ret i32 [[L]]
+;
+  %l = load i32, ptr @ExtInitZeroInit
+  ret i32 %l
+}
+
 ;.
 ; TUNIT: attributes #[[ATTR0:[0-9]+]] = { nocallback nofree nosync nounwind willreturn }
 ; TUNIT: attributes #[[ATTR1]] = { memory(readwrite, argmem: none) }

>From 9ac5ece524825420c4778f2c4ec545015323c6a5 Mon Sep 17 00:00:00 2001
From: pvanhout <pierre.vanhoutryve at amd.com>
Date: Fri, 21 Feb 2025 14:41:05 +0100
Subject: [PATCH 2/3] Comments

---
 llvm/lib/Transforms/IPO/Attributor.cpp | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/llvm/lib/Transforms/IPO/Attributor.cpp b/llvm/lib/Transforms/IPO/Attributor.cpp
index 0865e4da1a071..15ea2c8e7330a 100644
--- a/llvm/lib/Transforms/IPO/Attributor.cpp
+++ b/llvm/lib/Transforms/IPO/Attributor.cpp
@@ -259,12 +259,16 @@ AA::getInitialValueForObj(Attributor &A, const AbstractAttribute &QueryingAA,
     if (!Initializer)
       return nullptr;
   } else {
-    if (!GV->hasLocalLinkage() &&
-        (GV->isInterposable() || GV->isExternallyInitialized() ||
-         !(GV->isConstant() && GV->hasInitializer())))
-      return nullptr;
-    if (!GV->hasInitializer())
-      return UndefValue::get(&Ty);
+    if (GV->hasLocalLinkage()) {
+      // uninitialized local variable
+      if (!GV->hasInitializer())
+        return UndefValue::get(&Ty);
+    } else {
+      // externally visible variable with uncertain initializer or
+      // uncertain values.
+      if (!GV->hasDefinitiveInitializer() || !GV->isConstant())
+        return nullptr;
+    }
 
     if (!Initializer)
       Initializer = GV->getInitializer();

>From 4c712ce26705c15f27c1b51d7a3a920a8b9ec054 Mon Sep 17 00:00:00 2001
From: pvanhout <pierre.vanhoutryve at amd.com>
Date: Mon, 24 Feb 2025 08:58:57 +0100
Subject: [PATCH 3/3] Commentss

---
 llvm/lib/Transforms/IPO/Attributor.cpp | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Transforms/IPO/Attributor.cpp b/llvm/lib/Transforms/IPO/Attributor.cpp
index 15ea2c8e7330a..823aa58fa007b 100644
--- a/llvm/lib/Transforms/IPO/Attributor.cpp
+++ b/llvm/lib/Transforms/IPO/Attributor.cpp
@@ -260,12 +260,12 @@ AA::getInitialValueForObj(Attributor &A, const AbstractAttribute &QueryingAA,
       return nullptr;
   } else {
     if (GV->hasLocalLinkage()) {
-      // uninitialized local variable
+      // Uninitialized global with local linkage.
       if (!GV->hasInitializer())
         return UndefValue::get(&Ty);
     } else {
-      // externally visible variable with uncertain initializer or
-      // uncertain values.
+      // Externally visible global that's either non-constant,
+      // or a constant with an uncertain initializer.
       if (!GV->hasDefinitiveInitializer() || !GV->isConstant())
         return nullptr;
     }