Add test and comment

Author: bjorn3

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) }
 }
 

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"
+}