linux-kernel - Re: [PATCH v2 5/8] objtool: reduce memory usage of struct reloc

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Date:   Thu, 29 Dec 2022 05:57:13 +0000
From:   Thomas Weißschuh <thomas@...ch.de>
To:     Rong Tao <rtoax@...mail.com>
Cc:     jpoimboe@...nel.org, linux-kernel@...r.kernel.org,
        linux@...ssschuh.net, peterz@...radead.org,
        Rong Tao <rongtao@...tc.cn>
Subject: Re: [PATCH v2 5/8] objtool: reduce memory usage of struct reloc

Hi Rong,

On Thu, Dec 29, 2022 at 11:29:07AM +0800, Rong Tao wrote:
> Hi, Thomas:
> 
> In /usr/include/elf.h has:
> 
> #define ELF32_R_TYPE(val)		((val) & 0xff)
> #define ELF64_R_TYPE(i)			((i) & 0xffffffff)
>                                        ^^^^^^^^^^
> 
> So, I still feel that keeping 'unsigned int' is a good option. Can we just
> use __attribute__((packed)) for wasted padding bytes?

As the struct contains addresses, packing it will create a ton of
compiler warnings and will be much more likely to break than reducing
the width of 'type'.

Given that reducing the width of 'type'

* is currently absolutely safe,
* is unlikely to break in the future, as the elf designers seem to be
  actively trying to stay within this range anyways,
* saves 8 bytes from a very heavily allocated struct, having a
  measurable impact on the build process,

I continue to propose this aproach.

Let's let the maintainers decide.

Thomas