On Sun, Jan 14, 2018 at 08:41:54AM -0800, H.J. Lu wrote:
> They are used in asm statements in kernel:
> 
> extern void (*func_p) (void);
> 
> void
> foo (void)
> {
>   asm ("call __x86_indirect_thunk_%V0" : : "a" (func_p));

Well, using it just with a single register classes wouldn't make much sense,
then you can just use "call __x86_indirect_thunk_rax"
or "call __x86_indirect_thunk_eax" depending on __x86_64__, you wouldn't
need to extend anything.
But supposedly if you use it with "r" or "q" or similar class this will be
different.

        Jakub

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