https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120391
Bug ID: 120391
Summary: Enhancement: deduplicate constexpr char[] arrays with
identical or overlapping contents
Product: gcc
Version: 14.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: nickooo314 at gmail dot com
Target Milestone: ---
In optimized builds, GCC currently does not deduplicate "constexpr char[]"
arrays that have identical or overlapping contents, even when using flags like
-Os or -fmerge-all-constants. This results in unnecessary duplication in
.rodata, especially for embedded or size-constrained targets.
Example (no deduplication):
$ cat merge_test.cpp
#include <stdio.h>
constexpr char c1[] = {'H','e','l','l','o',' ','w','o','r','l','d','!','\0'};
constexpr char c2[] = {'w','o','r','l','d','!','\0'};
int main() {
puts(c1);
puts(c2);
}
$ /test/bin/g++ -Os -fmerge-all-constants -o merge_test merge_test.cpp
$ objdump -s -j .rodata merge_test
merge_test: file format elf64-x86-64
Contents of section .rodata:
402000 01000200 00000000 776f726c 64210a00 ........world!..
402010 48656c6c 6f20776f 726c6421 0a00 Hello world!..
Even though c2 is a suffix of c1, GCC emits both arrays independently in
.rodata, as opposed to string literals which get optimized (in fact even
without "-fmerge-all-constants"):
constexpr auto c1 = "Hello world!";
constexpr auto c2 = "world!";
$ objdump -s -j .rodata merge_test
merge_test: file format elf64-x86-64
Contents of section .rodata:
402000 01000200 48656c6c 6f20776f 726c6421 ....Hello world!
402010 00
Is there any reason GCC doesn't perform this optimization even with
"-fmerge-all-constants" for constexpr char arrays?
