[llvm] [GlobalOpt] Do not fold away addrspacecasts which may be runtime operations (PR #153753)

[Owen Anderson via llvm-commits]

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

>From c1a835a7f115a0f16dd8fa1f500f4cc3232faab3 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] [GlobalOpt] 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         |  3 --
 .../GlobalOpt/stored-once-addrspacecast.ll    | 42 +++++++++++++++++++
 2 files changed, 42 insertions(+), 3 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..d7edd1288309b 100644
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -1133,9 +1133,6 @@ static bool
 optimizeOnceStoredGlobal(GlobalVariable *GV, Value *StoredOnceVal,
                          const DataLayout &DL,
                          function_ref<TargetLibraryInfo &(Function &)> GetTLI) {
-  // Ignore no-op GEPs and bitcasts.
-  StoredOnceVal = StoredOnceVal->stripPointerCasts();
-
   // 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
   // users of the loaded value (often calls and loads) that would trap if the
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..35678598032dc
--- /dev/null
+++ b/llvm/test/Transforms/GlobalOpt/stored-once-addrspacecast.ll
@@ -0,0 +1,42 @@
+; 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.
+
+ at g1 = internal global ptr null
+ at g2 = addrspace(1) global i32 0
+
+;.
+; CHECK: @g1 = internal unnamed_addr global ptr null
+; CHECK: @g2 = addrspace(1) global i32 0
+; CHECK: @g4 = local_unnamed_addr addrspace(1) global i32 0
+;.
+define i64 @test1() {
+; CHECK-LABEL: @test1(
+; CHECK-NEXT:    [[TMP1:%.*]] = addrspacecast ptr addrspace(1) @g2 to ptr
+; CHECK-NEXT:    store ptr [[TMP1]], ptr @g1, align 8
+; CHECK-NEXT:    [[TMP2:%.*]] = load ptr, ptr @g1, align 8
+; CHECK-NEXT:    [[TMP3:%.*]] = load i64, ptr [[TMP2]], align 8
+; CHECK-NEXT:    ret i64 [[TMP3]]
+;
+  %l1 = addrspacecast ptr addrspace(1) @g2 to ptr
+  store ptr %l1, ptr @g1, align 8
+  %l2 = load ptr, ptr @g1, align 8
+  %l3 = load i64, ptr %l2, align 8
+  ret i64 %l3
+}
+
+ at g3 = internal global ptr null
+ at g4 = addrspace(1) global i32 0
+
+define i64 @test2() {
+; CHECK-LABEL: @test2(
+; CHECK-NEXT:    [[TMP1:%.*]] = load i64, ptr addrspacecast (ptr addrspace(1) @g4 to ptr), align 8
+; CHECK-NEXT:    ret i64 [[TMP1]]
+;
+  store ptr addrspacecast (ptr addrspace(1) @g4 to ptr), ptr @g3, align 8
+  %l1 = load ptr, ptr @g3, align 8
+  %l2 = load i64, ptr %l1, align 8
+  ret i64 %l2
+}