On Tue, May 19, 2015 at 12:11 PM, Richard Henderson <r...@redhat.com> wrote:
> On 05/19/2015 12:06 PM, H.J. Lu wrote:
>> On Tue, May 19, 2015 at 11:59 AM, Richard Henderson <r...@redhat.com> wrote:
>>> On 05/19/2015 11:06 AM, Rich Felker wrote:
>>>> I'm still mildly worried that concerns for supporting
>>>> relaxation might lead to decisions not to optimize code in ways that
>>>> would be difficult to relax (e.g. certain types of address load
>>>> reordering or hoisting) but I don't understand GCC internals
>>>> sufficiently to know if this concern is warranted or not.
>>>
>>> It is. The relaxation that HJ is working on requires that the reads from
>>> the
>>> got not be hoisted. I'm not especially convinced that what he's working on
>>> is
>>> a win.
>>>
>>> With LTO, the compiler can do the same job that he's attempting in the
>>> linker,
>>> without an extra nop. Without LTO, leaving it to the linker means that you
>>> can't hoist the load and hide the memory latency.
>>>
>>
>> My relax approach won't take away any optimization done by compiler.
>> It simply turns indirect branch into direct branch with a nop prefix at
>> link-time. I am having a hard time to understand why we shouldn't do it.
>
> I well understand what you're doing.
>
> 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 ();
}
[hjl@gnu-6 gcc]$ ./xgcc -B./ -fPIC -O3 -S /tmp/x.c -fno-plt
[hjl@gnu-6 gcc]$ cat x.s
.file "x.c"
.section .text.unlikely,"ax",@progbits
.LCOLDB0:
.text
.LHOTB0:
.p2align 4,,15
.globl bar
.type bar, @function
bar:
.LFB0:
.cfi_startproc
jmp *foo@GOTPCREL(%rip)
.cfi_endproc
.LFE0:
.size bar, .-bar
--
H.J.
