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Re: [PATCH 6/6] Compile sched_getaffinity.c with -fno-builtin-memset
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Joseph Myers <joseph at codesourcery dot com>
- Cc: GNU C Library <libc-alpha at sourceware dot org>
- Date: Fri, 14 Aug 2015 08:55:52 -0700
- Subject: Re: [PATCH 6/6] Compile sched_getaffinity.c with -fno-builtin-memset
- Authentication-results: sourceware.org; auth=none
- References: <20150814121907 dot GF28610 at gmail dot com> <alpine dot DEB dot 2 dot 10 dot 1508141519370 dot 16651 at digraph dot polyomino dot org dot uk> <CAMe9rOrzffv07T31oDX9oVcTpxJ=_kiFNgqcKdgoYbCzvRnEuw at mail dot gmail dot com> <alpine dot DEB dot 2 dot 10 dot 1508141534210 dot 16651 at digraph dot polyomino dot org dot uk>
On Fri, Aug 14, 2015 at 8:46 AM, Joseph Myers <firstname.lastname@example.org> wrote:
> On Fri, 14 Aug 2015, H.J. Lu wrote:
>> On Fri, Aug 14, 2015 at 8:27 AM, Joseph Myers <email@example.com> wrote:
>> > On Fri, 14 Aug 2015, H.J. Lu wrote:
>> >> Since sched_getaffinity.c calls memset which may not be inlined, we
>> >> should compile it with -fno-builtin-memset so that the internal hidden
>> >> memset is called without PLT.
>> >> OK for master?
>> > memset is called all over the place. I don't like hardcoding special
>> > options for particular files based on calls to memset, and especially not
>> > based on what some particular compiler version happens to do with a
>> > particular call. Why doesn't the call to libc_hidden_builtin_proto
>> > (memset) in include/string.h suffice? Can you devise some set of
>> > declarations / toplevel asms to put in a header that will ensure memset
>> > calls go via a hidden alias (ideally with the compiler knowing it's
>> > hidden, not just the assembler / linker, so the compiler can do any
>> > optimizations based on it being an intra-library call)?
>> libc_hidden_builtin_proto (memset)
>> has no impact on memset inlined by compiler.
> Well, I see references to __GI_memset, not memset, in sched_getaffinity.os
> for both x86 and x86_64 (building with GCC 5). So it seems to be working
> OK for me. It's true some of those references use PLT-generating
Yes, it does reference __GI_memset, but via PLT, since GCC doesn't
know the libcall for the builtin memset is hidden.
> relocations, but those relocations shouldn't actually generate PLT entries
> when they are relocations against symbols not exported from the library -
> the linker should convert them to direct calls. If they do generate PLT
> entries, it sounds like a linker optimization is missing.
> Apart from any missing linker optimizations, the compiler should know when
Linker isn't an issue.
> calls are to hidden functions as calls to those may be more efficient than
> calls that only the linker determines don't need a PLT entry. On 32-bit
> x86, that means the compiler knowing it doesn't need to set %ebx up to
%ebx is the issue.
> point to the GOT as it would for a call through the PLT. I don't know
> whether this gets optimized for built-in function calls using
> libc_hidden_builtin_proto, but if it doesn't, that seems like a missed
> optimization that should be addressed in the compiler not in glibc.
It is hard to say it is a GCC issue when we are kind of doing this behind
the back of GCC unless there is a way to tell GCC that the libcall of
a builtin function is hidden.