Mapping symbol optimization: implicit one at section beginning

[Jan Beulich]

On 29.07.2024 09:55, Fangrui Song wrote:
> On Mon, Jul 22, 2024 at 11:24 PM Jan Beulich <jbeulich@suse.com> wrote:
>> On 22.07.2024 20:37, Fangrui Song wrote:
>>> I have written a detailed analysis at
>>> https://maskray.me/blog/2024-07-21-mapping-symbols-rethinking-for-efficiency
>>> and proposed an alternative scheme to address the size concern:
>>>
>>> * Text sections: Assume an implicit $x at offset 0. Add an ending $x
>>> if the final data isn't instructions.
>>> * Non-text sections: Assume an implicit $d at offset 0. Add an ending
>>> $d only if the final data isn't data commands.
>>>
>>> This approach eliminates most mapping symbols while ensuring correct
>>> disassembly. Here is an illustrated assembler example:
>>>
>>> .section .text.f0,"ax"
>>>   ret
>>> // emit $d
>>>   .long 42
>>> // emit $x. Without this, .text.f1 might be interpreted as data.
>>
>> I don't understand this. Whether such a $x would apply to ...
>>
>>> .section .text.f1,"ax"
>>>   ret
>>
>> ... the section following in source code is entirely unknown until
>> linking time. And that $x doesn't help ...
> 
> Thanks for the response!
> 
> The $x works for the common case where this text section is followed
> by another text section (instead of data section) that starts with
> instructions.

Yet then a proposal like this wants to work not just in the common case,
don't you agree? Unless it was behind a default-off command line option.

Jan