From: Rui Ueyama <rui314@gmail.com>
To: Alexander Monakov <amonakov@ispras.ru>
Cc: "Martin Liška" <mliska@suse.cz>,
"GCC Patches" <gcc-patches@gcc.gnu.org>,
"Jan Hubicka" <hubicka@ucw.cz>
Subject: Re: [PATCH] lto-plugin: add support for feature detection
Date: Sun, 15 May 2022 18:01:36 +0800 [thread overview]
Message-ID: <CACKH++Y3pd7945CB4DuR4_wzL=fTpgMEx6f0ptNY-n8WVEtzmw@mail.gmail.com> (raw)
In-Reply-To: <d1d63bdc-1646-5c44-8dad-2c4ae97b93@ispras.ru>
On Sun, May 15, 2022 at 4:51 PM Alexander Monakov <amonakov@ispras.ru> wrote:
>
> On Sun, 15 May 2022, Rui Ueyama wrote:
>
> > > Is that a good tradeoff in the LTO case though? I believe you cannot assume
> > > the plugin to be thread-safe, so you're serializing its API calls, right?
> > > But the plugin is doing a lot of work, so using the index to feed it with as
> > > few LTO objects as possible should be a significant win, no? (even if it
> > > was thread-safe)
> >
> > Oh, I didn't know that claim_file_hook isn't thread-safe. I need to add a
> > lock to guard it then. But is it actually the case?
>
> You can see for yourself at
> https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=lto-plugin/lto-plugin.c
> (e.g. how claim_file_handler increments the global variable num_claimed_files)
>
> > As to the tradeoff, speculatively loading all object files from archives
> > may not be beneficial if the loaded files are LTO object files. But we do
> > this for consistency. We don't have a multi-phase name resolution pass at
> > all in mold; all symbols are resolved at once in parallel. We don't want
> > to implement another name resolution pass just for LTO for the following
> > reasons:
> >
> > 1. It bloats up the linker's code.
> >
> > 2. We don't know whether an archive file contains an LTO object file or
> > not until we actually read archive members, so there's no chance to switch
> > to the multi-pass name resolution algorithm before we read files.
> >
> > 3. If we have two different name resolution algorithms, it is very hard to
> > guarantee that both algorithms produce the same result. As a result, the
> > output with -flto may behave differently without -flto.
>
> Well, -flto can result in observably different results for other reasons
> anyway.
>
> > 4. We still have to handle --start-libs and --end-libs, so feeding an
> > object file that will end up not being included into the output is
> > unavoidable.
>
> Makes sense, but I still don't understand why mold wants to discover in
> advance whether the plugin is going to use get_symbols_v3. How would it
> help with what mold does today to handle the _v2 case?
Currently, mold restarts itself to reset the internal state of the plugin.
If we know in advance that get_symbols_v3 is supported, we can avoid that
restart. That should make the linker a bit faster. Also, restarting
the linker is
a hack, so we want to avoid it if possible.
next prev parent reply other threads:[~2022-05-15 10:01 UTC|newest]
Thread overview: 76+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-02 7:51 [PATCH] Support LDPT_GET_SYMBOLS_V3 Martin Liška
2022-05-04 12:20 ` [PATCH] lto-plugin: add support for feature detection Martin Liška
2022-05-04 12:32 ` Alexander Monakov
2022-05-04 12:41 ` Martin Liška
2022-05-04 13:10 ` Alexander Monakov
2022-05-04 13:31 ` Martin Liška
2022-05-04 15:06 ` Bernhard Reutner-Fischer
2022-05-05 6:15 ` Richard Biener
2022-05-05 6:31 ` Richard Biener
2022-05-05 10:52 ` Alexander Monakov
2022-05-05 12:50 ` Martin Liška
2022-05-06 14:46 ` Alexander Monakov
2022-05-09 9:05 ` Martin Liška
2022-05-15 6:57 ` Rui Ueyama
2022-05-15 7:53 ` Alexander Monakov
2022-05-15 8:07 ` Rui Ueyama
2022-05-15 8:50 ` Alexander Monakov
2022-05-15 10:01 ` Rui Ueyama [this message]
2022-05-15 10:09 ` Alexander Monakov
2022-05-15 10:32 ` Rui Ueyama
2022-05-15 11:37 ` Alexander Monakov
2022-05-15 11:52 ` Rui Ueyama
2022-05-15 12:07 ` Alexander Monakov
2022-05-16 2:41 ` Rui Ueyama
2022-05-16 6:38 ` Alexander Monakov
2022-05-16 8:37 ` Rui Ueyama
2022-05-16 9:10 ` Richard Biener
2022-05-16 9:15 ` Alexander Monakov
2022-05-16 9:25 ` Jan Hubicka
2022-05-16 9:38 ` Martin Liška
2022-05-16 9:50 ` Jan Hubicka
2022-05-16 10:22 ` Richard Biener
2022-05-16 9:58 ` Rui Ueyama
2022-05-16 10:28 ` Richard Biener
2022-05-16 10:44 ` Rui Ueyama
2022-05-16 12:04 ` Martin Liška
2022-05-16 13:07 ` Rui Ueyama
2022-05-16 13:38 ` Alexander Monakov
2022-05-16 15:16 ` Alexander Monakov
2022-05-17 6:20 ` Richard Biener
2022-05-17 13:44 ` Martin Liška
2022-06-16 6:59 ` [PATCH 1/3] lto-plugin: support LDPT_GET_SYMBOLS_V3 Martin Liška
2022-06-20 9:23 ` Richard Biener
2022-06-16 7:01 ` [PATCH 2/3] lto-plugin: make claim_file_handler thread-safe Martin Liška
2022-06-20 9:32 ` Richard Biener
2022-06-20 10:20 ` Martin Liška
2022-06-21 7:56 ` Richard Biener
2022-06-21 8:43 ` Martin Liška
2022-06-24 8:37 ` Richard Biener
2022-06-16 7:01 ` [PATCH 3/3] lto-plugin: implement LDPT_GET_API_VERSION Martin Liška
2022-06-16 8:00 ` Alexander Monakov
2022-06-16 12:25 ` Martin Liška
2022-06-20 9:35 ` Richard Biener
2022-06-20 13:01 ` Martin Liška
2022-06-30 6:43 ` Rui Ueyama
2022-06-30 8:42 ` Martin Liška
2022-07-01 6:36 ` Richard Biener
2022-07-04 14:17 ` Martin Liška
2022-07-07 2:19 ` Rui Ueyama
2022-07-08 8:42 ` Martin Liška
2022-07-08 12:41 ` Alexander Monakov
2022-07-11 7:23 ` Rui Ueyama
2022-07-11 9:16 ` Alexander Monakov
2022-07-11 9:55 ` Richard Biener
2022-07-11 10:51 ` Martin Liška
2022-07-11 12:24 ` Rui Ueyama
2022-07-11 12:38 ` Alexander Monakov
2022-07-11 12:51 ` Martin Liška
2022-07-12 1:36 ` Rui Ueyama
2022-07-11 16:35 ` Alexander Monakov
2022-07-12 6:28 ` Richard Biener
2022-07-12 7:36 ` Martin Liška
2022-07-12 11:50 ` Rui Ueyama
2022-07-12 13:21 ` Richard Biener
2022-07-12 13:31 ` Martin Liška
2022-07-13 7:44 ` Rui Ueyama
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