[Bug inline-asm/84861] -flto with asm() optimizes too much

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84861

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |marxin at gcc dot gnu.org
         Resolution|---                         |INVALID

--- Comment #2 from Martin Liška <marxin at gcc dot gnu.org> ---
> The only difference is "-flto".

Having a top-level assembly requires to disable LTO for the compilation unit
(-fno-lto). Otherwise
as Andrew mentioned it can screw up LTO partitioning.

> 
> It this a problem of LTO, or is the assembler supposed to be tweaked (how?)
> so that the symbols get exported?
> 
> Having a file containing
>   asm ("");
> does compile, but a file containing only:
>   asm volatile ("");
> fails with "error: expected ‘(’ before ‘volatile’".

asm volatile can be used just within in a function body.
The best approach you can do is to define a normal function
and use asm to implement what you have written in assembly:

...
"        movq   %rax,   0(%rdi)"   "\n"
"        movq   %rbx,   8(%rdi)"   "\n"
"        movq   %rcx,  16(%rdi)"   "\n"
...

Then LTO will work because the assembly will be encapsulated in a function.

> 
> Happens with gcc 8.0.1 20180306 (experimental).