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我想在編譯後優化elf的大小。var在printf中聲明不能被GCC收集垃圾
但是，在printf中聲明的字符串被移動到.rodata節，並且不會被鏈接器優化。
#include <stdio.h>
#include <stdlib.h>
static char str1[] = "String1"; //goto .data.str1
const char str2[] = "String2"; //goto .rodata.str2
int main(){
printf("%s",str1); //"%s" in .rodata
printf("%s",str2);
printf("On-the fly string"); //goto .rodata
return 0;
}
void unused_function(){
const char str3[] = "String Not Used"; //goto .text.unused_function and is removed by the linker
printf("I would like to optimize this string"); //goto .rodata and isn't removed by the linker
printf("%s", str3); //
}
我編譯和鏈接這些論點：
gcc -fdata-sections -ffunction-sections -Wl,--gc-sections hello.c -o hello.out
這是編譯後：
Contents of section .data.str1:
0000 53747269 6e673100 String1.
Contents of section .rodata.str2:
0000 53747269 6e673200 String2.
Contents of section .rodata:
0000 2573004f 6e2d7468 6520666c 79207374 %s.On-the fly st
0010 72696e67 00000000 4920776f 756c6420 ring....I would
0020 6c696b65 20746f20 6f707469 6d697a65 like to optimize
0030 20746869 73207374 72696e67 00 this string.
Contents of section .text.main:
0000 554889e5 be000000 00bf0000 0000b800 UH..............
0010 000000e8 00000000 be000000 00bf0000 ................
0020 0000b800 000000e8 00000000 bf000000 ................
0030 00b80000 0000e800 000000b8 00000000 ................
0040 5dc3 ].
Contents of section .text.unused_function:
0000 554889e5 4883ec20 64488b04 25280000 UH..H.. dH..%(..
0010 00488945 f831c048 b8537472 696e6720 .H.E.1.H.String
0020 4e488945 e048b86f 74205573 65640048 NH.E.H.ot Used.H
0030 8945e8bf 00000000 b8000000 00e80000 .E..............
0040 0000488d 45e04889 c6bf0000 0000b800 ..H.E.H.........
0050 000000e8 00000000 488b45f8 64483304 ........H.E.dH3.
0060 25280000 007405e8 00000000 c9c3 %(...t........
和鏈接
Contents of section .text:
400440 31ed4989 d15e4889 e24883e4 f0505449 1.I..^H..H...PTI
400450 c7c0e005 400048c7 c1700540 0048c7c7 [email protected]@.H..
400460 2d054000 e8b7ffff fff4660f 1f440000 [email protected]
400470 b83f1060 0055482d 38106000 4883f80e .?.`.UH-8.`.H...
400480 4889e577 025dc3b8 00000000 4885c074 H..w.]......H..t
400490 f45dbf38 106000ff e00f1f80 00000000 .].8.`..........
4004a0 b8381060 0055482d 38106000 48c1f803 .8.`.UH-8.`.H...
4004b0 4889e548 89c248c1 ea3f4801 d048d1f8 H..H..H..?H..H..
4004c0 75025dc3 ba000000 004885d2 74f45d48 u.]......H..t.]H
4004d0 89c6bf38 106000ff e20f1f80 00000000 ...8.`..........
4004e0 803d510b 20000075 11554889 e5e87eff .=Q. ..u.UH...~.
4004f0 ffff5dc6 053e0b20 0001f3c3 0f1f4000 ..]..>. [email protected]
400500 48833d18 09200000 741eb800 00000048 H.=.. ..t......H
400510 85c07414 55bf200e 60004889 e5ffd05d ..t.U. .`.H....]
400520 e97bffff ff0f1f00 e973ffff ff554889 .{.......s...UH.
400530 e5be3010 6000bff8 054000b8 00000000 ..0.`[email protected]
400540 e8cbfeff ffbef005 4000bff8 054000b8 [email protected]@..
400550 00000000 e8b7feff ffbffb05 4000b800 [email protected]
400560 000000e8 a8feffff b8000000 005dc390 .............]..
400570 41574189 ff415649 89f64155 4989d541 AWA..AVI..AUI..A
400580 544c8d25 88082000 55488d2d 88082000 TL.%.. .UH.-.. .
400590 534c29e5 31db48c1 fd034883 ec08e83d SL).1.H...H....=
4005a0 feffff48 85ed741e 0f1f8400 00000000 ...H..t.........
4005b0 4c89ea4c 89f64489 ff41ff14 dc4883c3 L..L..D..A...H..
4005c0 014839eb 75ea4883 c4085b5d 415c415d .H9.u.H...[]A\A]
4005d0 415e415f c366662e 0f1f8400 00000000 A^A_.ff.........
4005e0 f3c3 ..
Contents of section .rodata:
4005f0 53747269 6e673200 2573004f 6e2d7468 String2.%s.On-th
400600 6520666c 79207374 72696e67 00000000 e fly string....
400610 4920776f 756c6420 6c696b65 20746f20 I would like to
400620 6f707469 6d697a65 20746869 73207374 optimize this st
400630 72696e67 00 ring.
Contents of section .data:
601030 53747269 6e673100 String1.
所以你可以後看到函數「unused_function（）」 d爲const被除去，但我想刪除最後一個字符串中聲明的字符串「我想優化該字符串」
UPDATE第一個例子可使用-fmerge常數，因爲該字符串被容易地除去在printf內部將根據它們的長度放入一個部分，但如果兩個字符串的長度相同並且使用了一個，它將不起作用。
#include <stdio.h>
#include <stdlib.h>
void used_function();
static char str1[] = "String1"; //goto .data.str1 const char str2[] = "String2"; //goto .rodata.str2
int main(){
printf("%s",str1); //"%s" in .rodata printf("%s",str2); printf("On-the fly string"); //goto .rodata
used_function();
return 0;
}
void unused_function(){
const char str3[] = "String Not Used"; //goto .text.unused_function and is removed by the linker
printf("I would like to optimize this string 1"); //goto .rodata.str1.8 and isn't removed by the linker
printf("%s", str3); // }
void used_function(){
printf("A string used here with the same size 2"); //goto .rodata.str1.8 }
鏈接後編譯-02
Contents of section .rodata:
400640 2573004f 6e2d7468 6520666c 79207374 %s.On-the fly st
400650 72696e67 00000000 41207374 72696e67 ring....A string
400660 20757365 64206865 72652077 69746820 used here with
400670 74686520 73616d65 2073697a 65203200 the same size 2.
400680 4920776f 756c6420 6c696b65 20746f20 I would like to
400690 6f707469 6d697a65 20746869 73207374 optimize this st
4006a0 72696e67 20310000 53747269 6e673200 ring 1..String2.
字符串還在這裏！
一種方法是用一般的優化一起啓用鏈接時優化：'GCC -02 -flto的hello.c -o hello.out'，我懷疑它會被淘汰，如果你可以用'-fdata-sections -ffunction-sections ...'來啓用優化，加上'-O2' – nos 2014-09-29 13:31:09