Re: [PATCH i386] Allow sibcalls in no-PLT PIC

[Rich Felker <dalias at libc dot org>]

On Tue, May 19, 2015 at 12:17:18PM -0700, H.J. Lu wrote:
> On Tue, May 19, 2015 at 12:11 PM, Richard Henderson <rth@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 <rth@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

I agree these exist. What I question is whether the savings from the
linker being able to relax this to a direct call in the case where the
programmer failed to let the compiler make it a direct call to begin
with (by using hidden or protected visibility) are worth the cost of
not being able to hoist the load out of loops or schedule it earlier
in cases where relaxation is not possible because the call target is
not defined in the same DSO.

Rich