I'm pushing the envelope in order to use gcc's flags -ffunction-sections and -fdata-sections with ld's --gc-sections for dead function elimination in the Linux kernel. I've encountered a problem with constant strings from unused functions not being optimised away along with the function. In other words, the unused functions get optimised away, but the strings they used do not. I've verified this is a problem at least for PowerPC and x86 targets, and I suspect all others too. The problem occurs because even with -fdata-sections specified, strings go in section ".rodata". To be optimised away by ld with --gc-sections, they need to go in a section like ".rodata.something". Noting that string literals don't have globally unique names, the "something" may need to be the function name it appeared in, or derived from the string contents perhaps via a hash. Either option would allow the section containing unused strings to be optimised away correctly by the linker. Similarly, with -fwritable-strings and -fdata-sections, the string data ends up in ".data", not ".data.something" where I would have expected it. Here's a trivial example showing the problem: char *function() { return "unused string"; } Running this through the latest gcc snapshot on: http://www.codesourcery.com/gcc-compile.html with options: -ffunction-sections -fdata-sections -fwritable-strings yeilds: .file "@2566.1" .version "01.01" gcc2_compiled.: .data #### Oh-oh! Plain old .data .LC0: .string "unused string" .section .text.function,"ax",@progbits .align 4 .globl function .type function,@function function: pushl %ebp movl %esp, %ebp movl $.LC0, %eax popl %ebp ret .Lfe1: .size function,.Lfe1-function .ident "GCC: (GNU) 2.96 20000418 (experimental) Release: 2.95.2 Environment: Red Hat Linux 6.2
State-Changed-From-To: open->suspended State-Changed-Why: Recent changes to the ELF spec allow this to be done in the linker. See SHF_STRINGS. Once this has been implemented in GNU binutils, we can think about doing something with it in the compiler.
From: rth@gcc.gnu.org To: gcc-gnats@gcc.gnu.org, greyham@research.canon.com.au, nobody@gcc.gnu.org Cc: Subject: Re: optimization/192 Date: 16 Jan 2001 12:41:34 -0000 Synopsis: String literals don't obey -fdata-sections State-Changed-From-To: open->suspended State-Changed-By: rth State-Changed-When: Tue Jan 16 04:41:34 2001 State-Changed-Why: Recent changes to the ELF spec allow this to be done in the linker. See SHF_STRINGS. Once this has been implemented in GNU binutils, we can think about doing something with it in the compiler. http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view&pr=192&database=gcc
Still happens on the mainline (20030609).
Will not fix as -fwritable-strings is no longer supported in 3.5.0 and above.
Subject: Re: String literals don't obey -fdata-sections On Sat, 22 May 2004, pinskia at gcc dot gnu dot org wrote: > Will not fix as -fwritable-strings is no longer supported in 3.5.0 and above. The basic feature request has nothing to do with -fwritable-strings, that was just one example. As far as I know this sort of feature would still be just as useful as when this feature request was submitted. (Similarly - mentioned in the projects list rather than being the subject of a bug report - an attribute to specify a section for string constants would also be of use.)
following JSM
This is true of other constants too. For example, on powerpc-linux, compiling the testcase in pr9571: gcc -O2 -m32 -fdata-sections -fno-merge-constants -S /src/tmp/pr9571.c gives: .file "pr9571.c" .globl d .section .sdata.d,"a",@progbits .align 3 .type d, @object .size d, 8 d: .long 1074339512 .long 1374389535 .section .rodata .align 3 .LC0: .long 1074339512 .long 1374389535 .section ".text" .align 2 .p2align 4,,15 .globl f .type f, @function f: lis 9,.LC0@ha lfd 1,.LC0@l(9) blr .size f,.-f .ident "GCC: (GNU) 4.0.0 20041129 (experimental)" .section .note.GNU-stack,"",@progbits The duplication of the constant isn't ideal either.
Created attachment 13319 [details] ned
I am also experiencing the same issue. Is there any solution for it?
(In reply to Rahul from comment #9) > I am also experiencing the same issue. Is there any solution for it? You can wrap a preprocessor macro around string literals that you want to subject to the linker's garbage collection: #define GCSTR(str) ({ static const char __str[] = str; __str; }) void hello() { puts(GCSTR("111")); // NOT in .rodata puts("222"); // in .rodata } int main() { puts(GCSTR("333")); // in .rodata puts("444"); // in .rodata return 0; } $ gcc -ffunction-sections -fdata-sections -Wl,--gc-sections -o gcstr gcstr.c $ objdump -s -j .rodata gcstr gcstr: file format elf64-x86-64 Contents of section .rodata: 4005fd 32323200 34343400 33333300 222.444.333. The downside of this strategy, however, is that these strings then become ineligible for merging, so if you have multiple *reachable* occurrences of the same GCSTR in your code, then you'll have multiple copies of the string data in the .rodata section of your linked binary. These redundant copies would not be present if the compiler were correctly outputting literal-initialized constant character arrays to sections with the "merge" and "strings" flags set (which it should do only if -fmerge-all-constants is set). You can simulate how this could/should work by editing the compiler's assembly output so that it sets the section flags appropriately. Given this program, gcstr.c: #define GCSTR(str) ({ static const char __str[] = str; __str; }) int main() { puts(GCSTR("111")); puts(GCSTR("111")); puts("111"); return 0; } Compile (but do not assemble) the program: $ gcc -S -ffunction-sections -fdata-sections -fmerge-all-constants -o gcstr.s gcstr.c Edit the assembly code so that all .rodata.__str.* sections are declared with the "merge" and "strings" flags and an entity size of 1: $ sed -e 's/\(\.section\t\.rodata\.__str\..*\),"a",\(@progbits\)$/\1,"aMS",\2,1/' -i gcstr.s Now assemble and link the program: $ gcc -Wl,--gc-sections -o gcstr gcstr.s Dumping the .rodata section from the resulting executable reveals that the linker did correctly perform string merging. $ objdump -s -j .rodata gcstr gcstr: file format elf64-x86-64 Contents of section .rodata: 40060d 31313100 111. Compare the above objdump output to that which results when skipping the sed step: 40060d 31313100 31313100 31313100 111.111.111. The needed correction is that the compiler should, when -fmerge-all-constants is set, emit literal-initialized constant character array data to a section with flags "aMS" and entsize==sizeof(T), where T is the type of characters in the array. A further correction (and really the main request in this bug report) would be for the compiler to emit string literals to discrete sections when -fdata-sections is set.
Created attachment 35479 [details] put string literals into unique sections when -fmerge-constants -fdata-sections This patch puts each string literal into a (probably) unique section when compiling with -fmerge-constants -fdata-sections. The section name is constructed from the character width and string alignment (as before) plus a 32-bit hash of the string contents. Consider the following program: void used() { puts("keep me"); puts("common"); puts("string"); puts("tail"); } void not_used() { puts("toss me"); puts("common"); puts("ring"); puts("entail"); } int main() { used(); return 0; } $ gcc -ffunction-sections -fdata-sections -fmerge-constants \ -Wl,--gc-sections -o test test.c Compiling with an unpatched GCC produces a binary whose .rodata contains: 40061d 6b656570 206d6500 636f6d6d 6f6e0073 keep me.common.s 40062d 7472696e 6700746f 7373206d 6500656e tring.toss me.en 40063d 7461696c 00 tail. Compiling with a patched GCC produces a binary whose .rodata contains: 40061d 6b656570 206d6500 636f6d6d 6f6e0073 keep me.common.s 40062d 7472696e 67007461 696c00 tring.tail.
(In reply to Matt Whitlock from comment #11) > Created attachment 35479 [details] > put string literals into unique sections when -fmerge-constants > -fdata-sections > > This patch puts each string literal into a (probably) unique section when > compiling with -fmerge-constants -fdata-sections. The section name is > constructed from the character width and string alignment (as before) plus a > 32-bit hash of the string contents. Would it better to use MD5 checksum on string contents?
(In reply to H.J. Lu from comment #12) > Would it better to use MD5 checksum on string contents? MD5 would be slower for not much gain in uniqueness (assuming its output is truncated to 32 bits). This application doesn't require a cryptographically strong hash function, as the consequence of a collision is merely that a string gets included in the binary when maybe it didn't need to be. Actually, I would favor replacing the very old (1996) Lookup2 hash function (implemented in libiberty/hashtab.c) with a more modern hash function, such as MurmurHash3, CityHash, or even Lookup3, all of which are faster than Lookup2. I would hesitate to use more than 32 bits, as the section names would start getting rather long.
This doesn't really look like a good idea to me. Instead, perhaps ld's --gc-sections or new special option should just remove unused string literals from mergeable sections. With your patch, I bet you lose e.g. all tail merging. Consider: const char *used1 () { return "foo bar baz blah blah"; } in one TU and const char *used2 () { return "bar baz blah blah"; } in another. The linker necessarily knows which strings (or other data) in mergeable sections are used and which are unused.
(In reply to Jakub Jelinek from comment #14) > This doesn't really look like a good idea to me. Instead, perhaps ld's > --gc-sections or new special option should just remove unused string > literals from mergeable sections. I believe (I've read, but I haven't verified) that Gold already does this. > With your patch, I bet you lose e.g. all tail merging. Tail merging still works fine. > Consider: > const char *used1 () { return "foo bar baz blah blah"; } > in one TU and > const char *used2 () { return "bar baz blah blah"; } > in another. Okay, I'll use your example. $ echo 'const char *used1 () { return "foo bar baz blah blah"; }' > tu1.c $ echo 'const char *used2 () { return "bar baz blah blah"; }' > tu2.c $ cat > main.c <<EOF extern const char * used1(), * used2(); int main() { puts(used1()); puts(used2()); return 0; } EOF $ gcc -c -fdata-sections -fmerge-constants -o tu1.o tu1.c $ gcc -c -fdata-sections -fmerge-constants -o tu2.o tu2.c $ gcc -c -fdata-sections -fmerge-constants -o main.o main.c $ objdump -s tu1.o tu2.o | fgrep -A2 .rodata Contents of section .rodata.str1.1.b4d3fd7d: 0000 666f6f20 62617220 62617a20 626c6168 foo bar baz blah 0010 20626c61 6800 blah. -- Contents of section .rodata.str1.1.a07ea0c2: 0000 62617220 62617a20 626c6168 20626c61 bar baz blah bla 0010 6800 h. $ gcc -Wl,--gc-sections -o proof main.o tu1.o tu2.o $ ./proof foo bar baz blah blah bar baz blah blah $ objdump -s proof | fgrep -A2 .rodata Contents of section .rodata: 40061d 666f6f20 62617220 62617a20 626c6168 foo bar baz blah 40062d 20626c61 6800 blah. As you can see, tail merging across translation units works fine.
Author: segher Date: Thu May 7 15:51:01 2015 New Revision: 222880 URL: https://gcc.gnu.org/viewcvs?rev=222880&root=gcc&view=rev Log: PR middle-end/192 PR middle-end/54303 * varasm.c (function_mergeable_rodata_prefix): New function. (mergeable_string_section): Use it. (mergeable_constant_section): Use it. gcc/testsuite/ * gcc.dg/fdata-sections-2.c: New file. Added: trunk/gcc/testsuite/gcc.dg/fdata-sections-2.c Modified: trunk/gcc/ChangeLog trunk/gcc/testsuite/ChangeLog trunk/gcc/varasm.c
(In reply to Segher Boessenkool from comment #16) Thanks for the fix, Segher. Your patch seems more "right" than mine, although I will point out that it doesn't precisely address this bug report, as it places string literal data into unique sections only if -ffunction-sections is set, whereas -fdata-sections has no impact. I can see arguments both ways, and personally this distinction is irrelevant to me, as I always use for -ffunction-sections and -fdata-sections, but the new behavior does seem somewhat counter-intuitive to me. Anyway, I tested your new patch (backported to GCC 4.9.2) with the use cases in Comment 11 and Comment 15, and both produced the desired results (after I added -ffunction-sections to the command lines in Comment 15). So I'm appeased.
I'm closing this bug. If there is some other (still supported) case we do not support well, please open a new bug report.
(In reply to Jakub Jelinek from comment #14) > This doesn't really look like a good idea to me. Instead, perhaps ld's > --gc-sections or new special option should just remove unused string > literals from mergeable sections. > With your patch, I bet you lose e.g. all tail merging. Consider: > const char *used1 () { return "foo bar baz blah blah"; } > in one TU and > const char *used2 () { return "bar baz blah blah"; } > in another. The linker necessarily knows which strings (or other data) in > mergeable sections are used and which are unused. I second Jakub's idea that the linker should perform the constant merge (which is implemented in LLD): the cost of a section header (sizeof(Elf64_Shdr)=64) + a section name (".rodata.xxx.str1.1") is quite large. Created a GNU ld (and gold) feature request: https://sourceware.org/bugzilla/show_bug.cgi?id=26622