From: Szabolcs Nagy <szabolcs.nagy@arm.com>
To: Maxim Kuvyrkov <maxim.kuvyrkov@linaro.org>
Cc: <nd@arm.com>, Torsten Duwe <duwe@suse.de>,
Li Bin <huawei.libin@huawei.com>, Jiri Kosina <jkosina@suse.cz>,
GCC Patches <gcc-patches@gcc.gnu.org>,
Marcus Shawcroft <Marcus.Shawcroft@arm.com>,
Takahiro Akashi <takahiro.akashi@linaro.org>
Subject: Re: [PATCH] add -fprolog-pad=N option to c-family
Date: Thu, 28 Apr 2016 10:58:00 -0000 [thread overview]
Message-ID: <5721ECD1.6090708@arm.com> (raw)
In-Reply-To: <7134BE21-ACFB-4C45-871A-FADFA2973040@linaro.org>
On 28/04/16 09:47, Maxim Kuvyrkov wrote:
>> On Apr 27, 2016, at 7:26 PM, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>>
>> with -mfentry, by default the user only has to
>> implement the fentry call (linux wants nops there, but
>> e.g. glibc could use -pg -mfentry for profiling on
>> aarch64 and the target specific details are easier to
>> document for an -m option than for something general).
>
> I don't understand your point here, could you elaborate, please?
>
if we only provide -mfentry then
- the kernel can use it (they have tools to nop patch the binary),
- others who don't want to fiddle with nops, just have the call,
can also use it (e.g. user-space profiling cannot really use
something that needs binary patching in case the user prefers
-pg -mfentry over the current -pg behaviour).
- it's target specific, so the magic abi of the fentry call can
be documented by the target according to the specific instruction
sequence that is used. (with nop-padding there are psabi and
compiler optimization interactions that may be hard to document
in a generic way and letting the user figure it out may cause
problems later in compiler development.. but i'm just speculating
based on the powerpc toc handling and ipa-ra findings.)
>> the nop-padding is more general, but the size and
>> layout of nops and the call abi will be target
>> specific and the user will most likely need to modify
>> the binary (to get the right sequence) which needs
>> additional tooling. i don't know who might use it
>> other than linux (which already has tools to deal with
>> -mfentry).
>
> Right, but this tooling will require minimal (if any) changes
> to be adapted to nop-pad approach. If I remember correctly,
> recent versions of GCC and kernel for x86_64 generate NOPs,
> not the call sequence in the prologs when -mfentry is used.
i'm trying to find where this happens in the kernel, but
i only see scripts/recordmcount.{c,pl} which are based on
nop patching the fentry/mcount call sites.
without such call sites the tools have to be implemented
differently and the way the kernel records the call site
positions might not match the prolog-pad recording.
next prev parent reply other threads:[~2016-04-28 10:58 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-27 15:22 Torsten Duwe
2016-04-27 16:26 ` Szabolcs Nagy
2016-04-28 8:48 ` Maxim Kuvyrkov
2016-04-28 10:58 ` Szabolcs Nagy [this message]
2016-04-28 11:32 ` Torsten Duwe
2016-05-09 5:53 ` AKASHI Takahiro
2016-09-29 15:18 ` [PATCH v2] " Torsten Duwe
2016-09-30 9:59 ` Jose E. Marchesi
2016-09-30 10:22 ` Torsten Duwe
2016-10-03 9:45 ` AKASHI Takahiro
2016-10-04 21:46 ` Maxim Kuvyrkov
2016-12-06 14:02 ` Maxim Kuvyrkov
2016-04-28 8:40 ` [PATCH] " Maxim Kuvyrkov
2016-04-28 11:18 ` Torsten Duwe
2016-04-28 15:07 ` Jeff Law
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