On 05/19/2015 12:17 PM, H.J. Lu wrote:
>> But my point is that the only time the compiler should present you with the
>> form of indirect branch you're looking for is when there's no place to hoist
>> the load.
>>
>> At which point, is it really worth adding a new relocation to the ABI? Is it
>> really worth adding new code to the linker that won't be exercised often?
>
> I believe there are plenty of indirect branches via GOT when compiling
> PIE/PIC with -fno-plt:
>
> [hjl@gnu-6 gcc]$ cat /tmp/x.c
> extern void foo (void);
>
> void
> bar (void)
> {
> foo ();
> }
Sure, as I said, when there's no place to hoist the load.
Try anything more complicated,
void bar (void)
{
int i;
for (i = 0; i < 10; ++i)
foo ();
}
void baz (void)
{
foo ();
foo ();
}
and you'll not see the call *foo@GOTPCREL(%rip) form.
Of course there's also plenty of times where combine recreates exactly that
form when perhaps the scheduler might have preferred otherwise. Those are
optimization choices to be addressed under separate cover.
My point that we can already do what you want via LTO, without adding new
relocations, is still relevant.
r~
