From: Palmer Dabbelt <palmer@dabbelt.com>
To: fweimer@redhat.com
Cc: maskray@google.com, l.stelmach@samsung.com,
libc-alpha@sourceware.org, schwab@suse.de,
adhemerval.zanella@linaro.org, joseph@codesourcery.com,
binutils@sourceware.org, m.pikula@partner.samsung.com,
m.szyprowski@samsung.com, k.lewandowsk@samsung.com
Subject: Re: global pointer gets overwritten with dlopen(3) on RISC-V
Date: Fri, 12 May 2023 13:33:25 -0700 (PDT) [thread overview]
Message-ID: <mhng-82428a70-5ed4-4a50-a032-d5dc7fcaf347@palmer-ri-x1c9a> (raw)
In-Reply-To: <877ctdpaww.fsf@oldenburg3.str.redhat.com>
On Fri, 12 May 2023 13:11:43 PDT (-0700), fweimer@redhat.com wrote:
> * Fangrui Song:
>
>>>1. Make -mno-relax the default for ld(1) (on Linux?). We have no
>>>benchmarks whatsoever, but global variables aren't very popular in
>>>application code these days and the gp register allows access to a
>>>single memory page (4kB) only. No big deal really.
>>
>> I do agree that --no-relax-gp is a sensible default choice for GNU ld.
>> https://maskray.me/blog/2021-03-14-the-dark-side-of-riscv-linker-relaxation#global-pointer-relaxation
>>
>> Perhaps you can start a separate topic on binutils? :)
>>
>> According to a doc from SiFive about -static -mcpu=sifive-u74 builds,
>> https://docs.google.com/spreadsheets/d/14V7cPbyc80AcGHzsMaw9hYb232dzRbGCmTApnxj-SpU/edit#gid=0
>> global pointer relaxation saves at best 0.5% size (I guess that refers
>> to .text. If we count all allocable sections, the percentage is likely
>> even smaller.)
>
> For a mature toolchain, 0.5% in code size reduction would be *a lot*,
> so I wouldn't dismiss that.
That's broadly speaking why it sticks around. We've got a bunch of
headaches related to relaxation, GP or otherwise, but they improve
performance and nobody's figured out how to replace that yet.
> Do we have a reproducer? Is the issue actually about gp relaxation for
> the main executable?
In general we don't reference GP from shared libraries as we don't have
a GP save/restore scheme. There may be a bug floating around here
somewhere, in which case we should fix it, but the original post sounds
like it wasn't a supported use case.
> Thanks,
> Florian
next prev parent reply other threads:[~2023-05-12 20:33 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CGME20230512142114eucas1p112d969a89ad2480a0c10a532bd6d8440@eucas1p1.samsung.com>
2023-05-12 14:21 ` Lukasz Stelmach
2023-05-12 15:13 ` Florian Weimer
2023-05-15 13:47 ` Palmer Dabbelt
[not found] ` <CGME20230516065316eucas1p17bffcd25209bb441b9a9f4d263aa8b3c@eucas1p1.samsung.com>
2023-05-16 6:53 ` Lukasz Stelmach
2023-05-16 7:59 ` Szabolcs Nagy
2023-05-17 0:11 ` Palmer Dabbelt
2023-05-12 19:50 ` Fangrui Song
2023-05-12 20:11 ` Florian Weimer
2023-05-12 20:33 ` Palmer Dabbelt [this message]
2023-05-12 21:09 ` Fangrui Song
2023-05-12 21:57 ` Palmer Dabbelt
2023-05-12 22:34 ` Fangrui Song
2023-05-12 22:47 ` Palmer Dabbelt
2023-05-13 0:05 ` Fangrui Song
2023-05-13 0:35 ` Palmer Dabbelt
2023-05-16 3:56 ` Fangrui Song
2023-05-16 22:51 ` Palmer Dabbelt
2023-05-16 23:21 ` Palmer Dabbelt
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