LTO & top-level ASM

[Martin Liška]

Hello,
    I've been solving undefined symbols related to: 
http://gcc.gnu.org/PR57703. In chromium there's a following inline asm:

asm(".type Syscall, @function\n" ...);

intptr_t SandboxSyscall(...)
{
    asm volatile("call SyscallAsm");
}

Where call of SandboxSyscall is inlined in couple of functions. Our 
problem is that top-level asm is streamed to the first ltrans and so 
that undefined symbol is reached:
/tmp/cc9oZmpM.ltrans26.ltrans.o:cc9oZmpM.ltrans26.o:function 
sandbox::Die::ExitGroup(): error: undefined reference to 'SyscallAsm'

There's couple of fixes which can be done:
1) add to build system -fno-lto option for the TU; not easy due to 
chrome's usage of ninja/gyp build system
2) if a compiler sees top-level asm, -flto can be disabled. I was given 
a segfault after I set lto flag to false. Probably there's something 
that relies on the flag
3) put all functions with inline asm to the same partition as top-level 
asm. There are many functions in chrome having inline asm (proto bufs), 
so that the first partition is very big
4) I tried to promote a new flag (top_level_asm) that is used to 
decorate SandboxSyscall. Method works, but I'm not sure if pleasant for 
chromium developers? Patch for this solution is attached.

Thank you for any ideas,
Martin
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