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Re: [PATCH i386] Allow sibcalls in no-PLT PIC
- From: Richard Henderson <rth at redhat dot com>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: Rich Felker <dalias at libc dot org>, Michael Matz <matz at suse dot de>, Jan Hubicka <hubicka at ucw dot cz>, Alexander Monakov <amonakov at ispras dot ru>, GCC Patches <gcc-patches at gcc dot gnu dot org>, Uros Bizjak <ubizjak at gmail dot com>
- Date: Tue, 19 May 2015 12:11:09 -0700
- Subject: Re: [PATCH i386] Allow sibcalls in no-PLT PIC
- Authentication-results: sourceware.org; auth=none
- References: <20150515194824 dot GB14415 at kam dot mff dot cuni dot cz> <CAMe9rOr3yQv-+cJxwp_YfNY=9G+FnV6=JdYh_T6kPDJtSA0fzg at mail dot gmail dot com> <20150515202319 dot GE17573 at brightrain dot aerifal dot cx> <CAMe9rOqRBz7L6Fr1nxDVrTEh3EQ-AVV0dMCC0-xdpq87k=e4EQ at mail dot gmail dot com> <20150515204237 dot GF17573 at brightrain dot aerifal dot cx> <CAMe9rOp1ibVzf0KOt+1boOxb5ag85fe2Rv2Rst=CgAtOTGFBxA at mail dot gmail dot com> <20150515230810 dot GA73210 at kam dot mff dot cuni dot cz> <CAMe9rOr1_r0tvi_JdsCL8w-MMAqhSeVpA6sbksnn1yP224zn_A at mail dot gmail dot com> <20150515234403 dot GG17573 at brightrain dot aerifal dot cx> <alpine dot LSU dot 2 dot 20 dot 1505191637140 dot 27315 at wotan dot suse dot de> <20150519180659 dot GG17573 at brightrain dot aerifal dot cx> <555B87F4 dot 30908 at redhat dot com> <CAMe9rOpPa9edYxaHQ1BOSfq4VuLsEQZHB=hXxdagTXpqv2qc=A at mail dot gmail dot com>
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?
r~