From a5fa12b6b908894f59a788641c4b14839e556c5f Mon Sep 17 00:00:00 2001
From: bjorn3 <17426603+bjorn3@users.noreply.github.com>
Date: Fri, 14 Mar 2025 15:56:33 +0000
Subject: Avoid wrapping constant allocations in packed structs when not
 necessary

This way LLVM will set the string merging flag if the alloc is a nul
terminated string, reducing binary sizes.
---
 compiler/rustc_codegen_llvm/src/consts.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

(limited to 'compiler/rustc_codegen_llvm/src')

diff --git a/compiler/rustc_codegen_llvm/src/consts.rs b/compiler/rustc_codegen_llvm/src/consts.rs
index a4e5749b3ac..a1ab6714d37 100644
--- a/compiler/rustc_codegen_llvm/src/consts.rs
+++ b/compiler/rustc_codegen_llvm/src/consts.rs
@@ -128,7 +128,7 @@ pub(crate) fn const_alloc_to_llvm<'ll>(
         append_chunks_of_init_and_uninit_bytes(&mut llvals, cx, alloc, range);
     }
 
-    cx.const_struct(&llvals, true)
+    if let &[data] = &*llvals { data } else { cx.const_struct(&llvals, true) }
 }
 
 fn codegen_static_initializer<'ll, 'tcx>(
-- 
cgit 1.4.1-3-g733a5


From 5c82a59bd30815a942b64fa09e22dbe442edf56d Mon Sep 17 00:00:00 2001
From: bjorn3 <17426603+bjorn3@users.noreply.github.com>
Date: Tue, 25 Mar 2025 12:57:42 +0000
Subject: Add test and comment

---
 compiler/rustc_codegen_llvm/src/consts.rs |  5 +++++
 tests/assembly/cstring-merging.rs         | 27 +++++++++++++++++++++++++++
 2 files changed, 32 insertions(+)
 create mode 100644 tests/assembly/cstring-merging.rs

(limited to 'compiler/rustc_codegen_llvm/src')

diff --git a/compiler/rustc_codegen_llvm/src/consts.rs b/compiler/rustc_codegen_llvm/src/consts.rs
index a1ab6714d37..75b4ff1b6bb 100644
--- a/compiler/rustc_codegen_llvm/src/consts.rs
+++ b/compiler/rustc_codegen_llvm/src/consts.rs
@@ -128,6 +128,11 @@ pub(crate) fn const_alloc_to_llvm<'ll>(
         append_chunks_of_init_and_uninit_bytes(&mut llvals, cx, alloc, range);
     }
 
+    // Avoid wrapping in a struct if there is only a single value. This ensures
+    // that LLVM is able to perform the string merging optimization if the constant
+    // is a valid C string. LLVM only considers bare arrays for this optimization,
+    // not arrays wrapped in a struct. LLVM handles this at:
+    // https://github.com/rust-lang/llvm-project/blob/acaea3d2bb8f351b740db7ebce7d7a40b9e21488/llvm/lib/Target/TargetLoweringObjectFile.cpp#L249-L280
     if let &[data] = &*llvals { data } else { cx.const_struct(&llvals, true) }
 }
 
diff --git a/tests/assembly/cstring-merging.rs b/tests/assembly/cstring-merging.rs
new file mode 100644
index 00000000000..7436e241823
--- /dev/null
+++ b/tests/assembly/cstring-merging.rs
@@ -0,0 +1,27 @@
+//@ only-linux
+//@ assembly-output: emit-asm
+//@ compile-flags: --crate-type=lib -Copt-level=3 --edition 2024
+
+use std::ffi::CStr;
+
+// CHECK: .section .rodata.str1.1,"aMS"
+// CHECK: .Lanon.{{.+}}:
+// CHECK-NEXT: .asciz "foo"
+#[unsafe(no_mangle)]
+static CSTR: &[u8; 4] = b"foo\0";
+
+// CHECK-NOT: .section
+// CHECK: .Lanon.{{.+}}:
+// CHECK-NEXT: .asciz "bar"
+#[unsafe(no_mangle)]
+pub fn cstr() -> &'static CStr {
+    c"bar"
+}
+
+// CHECK-NOT: .section
+// CHECK: .Lanon.{{.+}}:
+// CHECK-NEXT: .asciz "baz"
+#[unsafe(no_mangle)]
+pub fn manual_cstr() -> &'static str {
+    "baz\0"
+}
-- 
cgit 1.4.1-3-g733a5