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merged 2 commits into from
Mar 28, 2025
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Avoid wrapping constant allocations in packed structs when not necessary #138503

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a5fa12b
Avoid wrapping constant allocations in packed structs when not necessary
bjorn3 Mar 14, 2025
5c82a59
Add test and comment
bjorn3 Mar 25, 2025
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1 change: 1 addition & 0 deletions compiler/rustc_codegen_gcc/src/consts.rs
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Expand Up @@ -364,6 +364,7 @@ pub fn const_alloc_to_gcc<'gcc>(
llvals.push(cx.const_bytes(bytes));
}

// FIXME(bjorn3) avoid wrapping in a struct when there is only a single element.
cx.const_struct(&llvals, true)
}

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7 changes: 6 additions & 1 deletion compiler/rustc_codegen_llvm/src/consts.rs
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Expand Up @@ -128,7 +128,12 @@ 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)
// 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) }
}

fn codegen_static_initializer<'ll, 'tcx>(
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27 changes: 27 additions & 0 deletions tests/assembly/cstring-merging.rs
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@@ -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"
}
4 changes: 2 additions & 2 deletions tests/codegen/const-array.rs
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Expand Up @@ -2,7 +2,7 @@

#![crate_type = "lib"]

const LUT: [u8; 2] = [1, 1];
const LUT: [u8; 4] = [1, 1, 1, 1];

// CHECK-LABEL: @decode
#[no_mangle]
Expand All @@ -11,5 +11,5 @@ pub fn decode(i: u8) -> u8 {
// CHECK-NEXT: icmp
// CHECK-NEXT: select
// CHECK-NEXT: ret
if i < 2 { LUT[i as usize] } else { 2 }
if i < 4 { LUT[i as usize] } else { 2 }
}
2 changes: 1 addition & 1 deletion tests/codegen/debug-vtable.rs
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Expand Up @@ -15,7 +15,7 @@

// Make sure that vtables don't have the unnamed_addr attribute when debuginfo is enabled.
// This helps debuggers more reliably map from dyn pointer to concrete type.
// CHECK: @vtable.2 = private constant <{
// CHECK: @vtable.2 = private constant [
// CHECK: @vtable.3 = private constant <{
// CHECK: @vtable.4 = private constant <{

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32 changes: 16 additions & 16 deletions tests/codegen/external-no-mangle-statics.rs
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Expand Up @@ -6,72 +6,72 @@
// `#[no_mangle]`d static variables always have external linkage, i.e., no `internal` in their
// definitions

// CHECK: @A = {{(dso_local )?}}local_unnamed_addr constant
// CHECK-DAG: @A = {{(dso_local )?}}local_unnamed_addr constant
#[no_mangle]
static A: u8 = 0;

// CHECK: @B = {{(dso_local )?}}local_unnamed_addr global
// CHECK-DAG: @B = {{(dso_local )?}}local_unnamed_addr global
#[no_mangle]
static mut B: u8 = 0;

// CHECK: @C = {{(dso_local )?}}local_unnamed_addr constant
// CHECK-DAG: @C = {{(dso_local )?}}local_unnamed_addr constant
#[no_mangle]
pub static C: u8 = 0;

// CHECK: @D = {{(dso_local )?}}local_unnamed_addr global
// CHECK-DAG: @D = {{(dso_local )?}}local_unnamed_addr global
#[no_mangle]
pub static mut D: u8 = 0;

mod private {
// CHECK: @E = {{(dso_local )?}}local_unnamed_addr constant
// CHECK-DAG: @E = {{(dso_local )?}}local_unnamed_addr constant
#[no_mangle]
static E: u8 = 0;

// CHECK: @F = {{(dso_local )?}}local_unnamed_addr global
// CHECK-DAG: @F = {{(dso_local )?}}local_unnamed_addr global
#[no_mangle]
static mut F: u8 = 0;

// CHECK: @G = {{(dso_local )?}}local_unnamed_addr constant
// CHECK-DAG: @G = {{(dso_local )?}}local_unnamed_addr constant
#[no_mangle]
pub static G: u8 = 0;

// CHECK: @H = {{(dso_local )?}}local_unnamed_addr global
// CHECK-DAG: @H = {{(dso_local )?}}local_unnamed_addr global
#[no_mangle]
pub static mut H: u8 = 0;
}

const HIDDEN: () = {
// CHECK: @I = {{(dso_local )?}}local_unnamed_addr constant
// CHECK-DAG: @I = {{(dso_local )?}}local_unnamed_addr constant
#[no_mangle]
static I: u8 = 0;

// CHECK: @J = {{(dso_local )?}}local_unnamed_addr global
// CHECK-DAG: @J = {{(dso_local )?}}local_unnamed_addr global
#[no_mangle]
static mut J: u8 = 0;

// CHECK: @K = {{(dso_local )?}}local_unnamed_addr constant
// CHECK-DAG: @K = {{(dso_local )?}}local_unnamed_addr constant
#[no_mangle]
pub static K: u8 = 0;

// CHECK: @L = {{(dso_local )?}}local_unnamed_addr global
// CHECK-DAG: @L = {{(dso_local )?}}local_unnamed_addr global
#[no_mangle]
pub static mut L: u8 = 0;
};

fn x() {
// CHECK: @M = {{(dso_local )?}}local_unnamed_addr constant
// CHECK-DAG: @M = {{(dso_local )?}}local_unnamed_addr constant
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LLVM decided to put this definition before A. Using CHECK-DAG everywhere allows filecheck to match independent of the order.

#[no_mangle]
static M: fn() = x;

// CHECK: @N = {{(dso_local )?}}local_unnamed_addr global
// CHECK-DAG: @N = {{(dso_local )?}}local_unnamed_addr global
#[no_mangle]
static mut N: u8 = 0;

// CHECK: @O = {{(dso_local )?}}local_unnamed_addr constant
// CHECK-DAG: @O = {{(dso_local )?}}local_unnamed_addr constant
#[no_mangle]
pub static O: u8 = 0;

// CHECK: @P = {{(dso_local )?}}local_unnamed_addr global
// CHECK-DAG: @P = {{(dso_local )?}}local_unnamed_addr global
#[no_mangle]
pub static mut P: u8 = 0;
}
2 changes: 1 addition & 1 deletion tests/codegen/link_section.rs
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Expand Up @@ -3,7 +3,7 @@

#![crate_type = "lib"]

// CHECK: @VAR1 = {{(dso_local )?}}constant <{ [4 x i8] }> <{ [4 x i8] c"\01\00\00\00" }>, section ".test_one"
// CHECK: @VAR1 = {{(dso_local )?}}constant [4 x i8] c"\01\00\00\00", section ".test_one"
#[no_mangle]
#[link_section = ".test_one"]
#[cfg(target_endian = "little")]
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2 changes: 1 addition & 1 deletion tests/codegen/remap_path_prefix/main.rs
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Expand Up @@ -12,7 +12,7 @@ mod aux_mod;
include!("aux_mod.rs");

// Here we check that the expansion of the file!() macro is mapped.
// CHECK: @alloc_5761061597a97f66e13ef2ff92712c4b = private unnamed_addr constant <{ [34 x i8] }> <{ [34 x i8] c"/the/src/remap_path_prefix/main.rs" }>
// CHECK: @alloc_5761061597a97f66e13ef2ff92712c4b = private unnamed_addr constant [34 x i8] c"/the/src/remap_path_prefix/main.rs"
pub static FILE_PATH: &'static str = file!();

fn main() {
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6 changes: 3 additions & 3 deletions tests/codegen/uninit-consts.rs
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Expand Up @@ -11,15 +11,15 @@ pub struct PartiallyUninit {
y: MaybeUninit<[u8; 10]>,
}

// CHECK: [[FULLY_UNINIT:@.*]] = private unnamed_addr constant <{ [10 x i8] }> undef
// CHECK: [[FULLY_UNINIT:@.*]] = private unnamed_addr constant [10 x i8] undef

// CHECK: [[PARTIALLY_UNINIT:@.*]] = private unnamed_addr constant <{ [4 x i8], [12 x i8] }> <{ [4 x i8] c"{{\\EF\\BE\\AD\\DE|\\DE\\AD\\BE\\EF}}", [12 x i8] undef }>, align 4

// This shouldn't contain undef, since it contains more chunks
// than the default value of uninit_const_chunk_threshold.
// CHECK: [[UNINIT_PADDING_HUGE:@.*]] = private unnamed_addr constant <{ [32768 x i8] }> <{ [32768 x i8] c"{{.+}}" }>, align 4
// CHECK: [[UNINIT_PADDING_HUGE:@.*]] = private unnamed_addr constant [32768 x i8] c"{{.+}}", align 4

// CHECK: [[FULLY_UNINIT_HUGE:@.*]] = private unnamed_addr constant <{ [16384 x i8] }> undef
// CHECK: [[FULLY_UNINIT_HUGE:@.*]] = private unnamed_addr constant [16384 x i8] undef

// CHECK-LABEL: @fully_uninit
#[no_mangle]
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