[llvm] Prevent GlobalOpt from folding away addrspacecasts which may be runtime operations. (PR #153753)

[Owen Anderson via llvm-commits]

https://github.com/resistor created https://github.com/llvm/llvm-project/pull/153753

Specifically in the context of the once-stored transformation, GlobalOpt would strip
all pointer casts unconditionally, even though addrspacecasts might be runtime operations.
This manifested particularly on CHERI targets.

This patch was inspired by an existing change in CHERI LLVM (https://github.com/CHERIoT-Platform/llvm-project/commit/91afa60f17ea1b91e5cdba21a5c90400024b2b6a), but has been reimplemented with updated conventions, and a testcase constructed from scratch.


>From 7db799a857978e7aca11e03d62e28eb05d402eeb Mon Sep 17 00:00:00 2001
From: Owen Anderson <resistor at mac.com>
Date: Fri, 15 Aug 2025 14:36:49 +0800
Subject: [PATCH] Prevent GlobalOpt from folding away addrspacecasts which may
 be runtime operations.

Specifically in the context of the once-stored transformation, GlobalOpt would strip
all pointer casts unconditionally, even though addrspacecasts might be runtime operations.
This manifested particularly on CHERI targets.

This patch was inspired by an existing change in CHERI LLVM (https://github.com/CHERIoT-Platform/llvm-project/commit/91afa60f17ea1b91e5cdba21a5c90400024b2b6a), but has been reimplemented with updated conventions, and a testcase constructed from scratch.
---
 llvm/lib/Transforms/IPO/GlobalOpt.cpp         |  5 +--
 .../GlobalOpt/stored-once-addrspacecast.ll    | 31 +++++++++++++++++++
 2 files changed, 34 insertions(+), 2 deletions(-)
 create mode 100644 llvm/test/Transforms/GlobalOpt/stored-once-addrspacecast.ll

diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
index bdda4980c1005..89a811e3f74d5 100644
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -1133,8 +1133,9 @@ static bool
 optimizeOnceStoredGlobal(GlobalVariable *GV, Value *StoredOnceVal,
                          const DataLayout &DL,
                          function_ref<TargetLibraryInfo &(Function &)> GetTLI) {
-  // Ignore no-op GEPs and bitcasts.
-  StoredOnceVal = StoredOnceVal->stripPointerCasts();
+  // Ignore no-op GEPs and bitcasts, but not addrspacecasts that may be runtime
+  // operations.
+  StoredOnceVal = StoredOnceVal->stripPointerCastsSameRepresentation();
 
   // If we are dealing with a pointer global that is initialized to null and
   // only has one (non-null) value stored into it, then we can optimize any
diff --git a/llvm/test/Transforms/GlobalOpt/stored-once-addrspacecast.ll b/llvm/test/Transforms/GlobalOpt/stored-once-addrspacecast.ll
new file mode 100644
index 0000000000000..cb2fcce51a7ae
--- /dev/null
+++ b/llvm/test/Transforms/GlobalOpt/stored-once-addrspacecast.ll
@@ -0,0 +1,31 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals
+; RUN: opt -passes=globalopt < %s -S | FileCheck %s
+
+; Test that we do not fold away addresscasts when optimizing once-stored
+; globals, as these may be runtime operations.
+
+%T = type { ptr }
+
+ at 0 = internal global ptr null
+ at 1 = internal addrspace(1) global i32 0
+
+;.
+; CHECK: @[[GLOB0:[0-9]+]] = internal unnamed_addr global ptr null
+; CHECK: @[[GLOB1:[0-9]+]] = internal addrspace(1) global i32 0
+;.
+define void @a() {
+; CHECK-LABEL: @a(
+; CHECK-NEXT:    [[TMP1:%.*]] = addrspacecast ptr addrspace(1) @[[GLOB1]] to ptr
+; CHECK-NEXT:    store ptr [[TMP1]], ptr @[[GLOB0]], align 8
+; CHECK-NEXT:    [[TMP2:%.*]] = load ptr, ptr @[[GLOB0]], align 8
+; CHECK-NEXT:    [[TMP3:%.*]] = load atomic i64, ptr [[TMP2]] acquire, align 8
+; CHECK-NEXT:    ret void
+;
+  %1 = addrspacecast ptr addrspace(1) @1 to ptr
+  store ptr %1, ptr @0, align 8
+  %2 = load ptr, ptr @0, align 8
+  %3 = load atomic i64, ptr %2 acquire, align 8
+  ret void
+}
+
+declare ptr @_Znwm(i64)