library variant during application startup

Question

I wrote some PoC for load library with best optimization version during application startup. The goal for it check in the _init function code checking by cpuid available opcode extensions and load "the best" variant. I only don't know, how to avoid linking bypass.

Part of selecting procedure:

x86_sse41_plus:
 test ecx, 0x00100000
 jnz x86_sse42_plus
 mov edx, lib_x86_sse41 wrt ..gotoff
 jmp load_lib
x86_sse42_plus:
 test ecx, 0x10000000
 jnz x86_avx_plus
 mov edx, lib_x86_sse42 wrt ..gotoff
 jmp load_lib
x86_avx_plus:
 mov edx, lib_x86_avx wrt ..gotoff
load_lib:
 call _init_pc
_init_pc:
 pop ebx
 lea ebx, [ebx + _GLOBAL_OFFSET_TABLE_+$$-_init_pc wrt ..gotpc]
 lea eax, [ebx + edx]
 sub esp, 8
 push dword 0x0110A
 push eax
 call dlopen wrt ..plt
 add esp, 16
 test eax, eax
 jz _init_end
 sub esp, 12
 push eax
 call dlclose wrt ..plt
 add esp, 16
_init_end:
 pop ebx
 ret

In Makefile I made linking in that way e.g.:

${BINOUT}/libkitdynload_x86_sse42.so: ${OBJECTS_COMMON} ${OBJECTS_X86_SSE42}
 @echo ld libkitdynload_x86_sse42.so
 @ld -m elf_i386 -shared -o $@ ${OBJECTS_COMMON} ${OBJECTS_X86_SSE42} ${LDFLAGS} -T ${SRCDIR}/linker.ld

example application linking:

${BINOUT}/test: ${SRCDIR}/test.c
 @echo cc test
 @${CC} -m32 ${CFLAGS} -L${BINOUT}/link -o ${BINOUT}/test ${SRCDIR}/test.c -lkitdynload

Part of this bypass is that:

-L${BINOUT}/link

I have to link in that directory any of optimized variant and linking with that fs symbolic link.

Simple application:

#include "version.h"
#include <stdio.h>
int main(int argc, char * argv)
{
 printf("%s %s %s\n", get_kit_dynload_name(), get_kit_dynload_version_str(),
 get_kit_dynload_variant_str());
 return 0;
}

During dynamic linking code will select "the best" variant transparent.

Full code version is available here on github

How can it be coded better?

• Sᴀᴍ Onᴇᴌᴀ: Welcome to Code Review! It appears you took the tour (since you earned the informed badge). Did you review the help center pages like What topics can I ask about here? to ensure your question is on-topic?
• Sᴀᴍ Onᴇᴌᴀ: Also: I changed the title so that it describes what the code does per site goals: "State what your code does in your title, not your main concerns about it.". Feel free to edit and give it a different title if there is something more appropriate.
• G. Sliepen: Have a look at function multiversioning and target specific optimization, which might do what you want without resorting to writing your own assembly for variant selection.
• Jakub Juszczakiewicz: @G.Sliepen your solution make that, what I want to. Thanks!