From: Fangrui Song <maskray@google.com>
To: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Cc: Florian Weimer <fweimer@redhat.com>,
Szabolcs Nagy <Szabolcs.Nagy@arm.com>,
Wilco Dijkstra via Libc-alpha <libc-alpha@sourceware.org>
Subject: Re: [PATCH 2/3] Revert "[AArch64][BZ #17711] Fix extern protected data handling"
Date: Tue, 31 May 2022 00:42:37 -0700 [thread overview]
Message-ID: <20220531074237.2ttjnsxyivww7ygu@google.com> (raw)
In-Reply-To: <AM5PR0801MB187563FD4CCC0B7ED114BABF83D99@AM5PR0801MB1875.eurprd08.prod.outlook.com>
On 2022-05-26, Wilco Dijkstra via Libc-alpha wrote:
>Hi Florian,
>
>Sure, something basic like this shows the issues:
>
>int x;
>int f(void) { return ++x; }
>int main(void) { return f(); }
>
>compile with -O2 -fPIC -flto -shared:
>
>00000000000004f0 <f@plt>:
> 4f0: b0000090 adrp x16, 11000 <__cxa_finalize@GLIBC_2.17>
> 4f4: f9400611 ldr x17, [x16, #8]
> 4f8: 91002210 add x16, x16, #0x8
> 4fc: d61f0220 br x17
>
>0000000000000510 <main>:
> 510: 17fffff8 b 4f0 <f@plt>
>
>0000000000000600 <f>:
> 600: 90000081 adrp x1, 10000 <__FRAME_END__+0xf8f8>
> 604: f947e821 ldr x1, [x1, #4048]
> 608: b9400020 ldr w0, [x1]
> 60c: 11000400 add w0, w0, #0x1
> 610: b9000020 str w0, [x1]
> 614: d65f03c0 ret
>
>So f() does not get inlined into main, it is redirected via PLT, and it uses a GOT
>indirection to access the global. The underlying problem is that ELF assumes by
>default that you want interposition/export for all symbols. In reality that is
>almost never needed.
>
>LLVM makes -fno-semantic-interposition the default,
>which solves the PLT and inlining issue, but not the unnecessary GOT indirections.
LLVM/Clang's behavior is more complex, but stating "-fno-semantic-interposition the default" is incorrect.
See https://maskray.me/blog/2021-05-09-fno-semantic-interposition#clang--fno-semantic-interposition
https://fedoraproject.org/wiki/Changes/PythonNoSemanticInterpositionSpeedup#Detailed_Description
used to be incorrect, and I sent them the updates we can see today.
next prev parent reply other threads:[~2022-05-31 7:42 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-24 13:46 Wilco Dijkstra
2022-05-24 17:28 ` maskray
2022-05-24 21:58 ` H.J. Lu
2022-05-25 17:13 ` Wilco Dijkstra
2022-05-25 18:21 ` Florian Weimer
2022-05-25 20:44 ` H.J. Lu
2022-05-26 19:17 ` Wilco Dijkstra
2022-05-26 19:25 ` Florian Weimer
2022-05-26 20:03 ` Wilco Dijkstra
2022-05-26 21:27 ` H.J. Lu
2022-05-27 12:43 ` Florian Weimer
2022-05-31 2:03 ` H.J. Lu
2022-05-31 7:49 ` Fangrui Song
2022-05-31 9:42 ` Wilco Dijkstra
2022-05-31 13:47 ` H.J. Lu
2022-05-31 7:42 ` Fangrui Song [this message]
2022-05-25 20:10 ` maskray
-- strict thread matches above, loose matches on Subject: below --
2022-05-01 6:06 [PATCH 0/3] Simplify ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA and revert aarch64/arm's extern protected data handling Fangrui Song
2022-05-01 6:06 ` [PATCH 2/3] Revert "[AArch64][BZ #17711] Fix extern protected data handling" Fangrui Song
2022-05-23 20:10 ` Szabolcs Nagy
2022-05-23 20:17 ` Fangrui Song
2022-05-24 5:13 ` Fangrui Song
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