From: Jan Beulich <jbeulich@suse.com>
To: Binutils <binutils@sourceware.org>
Cc: "H.J. Lu" <hjl.tools@gmail.com>
Subject: Re: [PATCH 00/11] x86: NOP emission adjustments
Date: Wed, 27 Sep 2023 17:59:25 +0200 [thread overview]
Message-ID: <35a99dfb-8e33-00cc-f001-b6a5955cf89d@suse.com> (raw)
In-Reply-To: <7ce54bc2-fef2-d2e4-21fd-202fdead0c20@suse.com>
On 27.09.2023 17:46, Jan Beulich via Binutils wrote:
> I've noticed a number of issues and inefficiencies.
>
> 01: x86: record flag_code in tc_frag_data
> 02: x86: i386_generate_nops() may not derive decisions from global variables
> 03: x86: don't use 32-bit LEA as NOP surrogate in 64-bit code
> 04: x86: don't use operand size override with NOP in 16-bit code
> 05: x86: respect ".arch nonop" when selecting which NOPs to emit
> 06: x86: i686 != PentiumPro
> 07: x86: don't record full i386_cpu_flags in struct i386_tc_frag_data
> 08: x86: add a few more NOP patterns
> 09: x86: fold a few of the "alternative" NOP patterns
> 10: x86: fold NOP testcase expecations where possible
> 11: gas: make .nops output visible in listing
I shall have mentioned one further observation: When we use LEA as NOP-
surrogate, we always use %{,e,r}si as destination. I was suspecting this
might not be optimal when these actually end up executing, and indeed on
one of the three systems I checked (a Skylake) there was a reliably
measurable difference between that and alternating the destination
registers used. Question is whether that's enough of a concern, when
generally we expect people to build 64-bit code and not use .arch .nonop.
Jan
prev parent reply other threads:[~2023-09-27 15:59 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-27 15:46 Jan Beulich
2023-09-27 15:47 ` [PATCH 01/11] x86: record flag_code in tc_frag_data Jan Beulich
2023-09-27 15:48 ` [PATCH 02/11] x86: i386_generate_nops() may not derive decisions from global variables Jan Beulich
2023-09-27 15:48 ` [PATCH 03/11] x86: don't use 32-bit LEA as NOP surrogate in 64-bit code Jan Beulich
2023-09-27 15:49 ` [PATCH 04/11] x86: don't use operand size override with NOP in 16-bit code Jan Beulich
2023-09-27 15:50 ` [PATCH 05/11] x86: respect ".arch nonop" when selecting which NOPs to emit Jan Beulich
2023-09-27 15:50 ` [PATCH 06/11] x86: i686 != PentiumPro Jan Beulich
2023-09-27 15:51 ` [PATCH 07/11] x86: don't record full i386_cpu_flags in struct i386_tc_frag_data Jan Beulich
2023-09-27 15:51 ` [PATCH 08/11] x86: add a few more NOP patterns Jan Beulich
2023-09-27 15:52 ` [PATCH 09/11] x86: fold a few of the "alternative" " Jan Beulich
2023-09-27 15:52 ` [PATCH 10/11] x86: fold NOP testcase expectations where possible Jan Beulich
2023-09-27 15:53 ` [PATCH 11/11] gas: make .nops output visible in listing Jan Beulich
2023-09-27 15:59 ` Jan Beulich [this message]
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