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


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.


-- 
H.J.


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