Re: [PATCH] Remove fno-unit-at-a-time make variable

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 04/04/2022 12:57, Fangrui Song wrote:
> 
> On 2022-03-31, Adhemerval Zanella wrote:
>>
>>
>> On 31/03/2022 00:43, Fangrui Song wrote:
>>> On 2022-03-30, Adhemerval Zanella wrote:
>>>>
>>>>
>>>> On 30/03/2022 13:23, Fangrui Song wrote:
>>>>> On 2022-03-30, Adhemerval Zanella wrote:
>>>>>>
>>>>>>
>>>>>> On 30/03/2022 02:07, Fangrui Song wrote:
>>>>>>> 795985e4e751 in 2003 added -fno-unit-at-a-time to errlist.c and
>>>>>>> siglist.c to "avoid reordering assembler output". -fno-toplevel-reorder
>>>>>>> is a rough replacement for this legacy option
>>>>>>> (https://sourceware.org/pipermail/gcc-patches/2006-January/186801.html).
>>>>>>>
>>>>>>> The reordering requirement does not seem to be needed any longer.
>>>>>>
>>>>>> We still need them for otherwise DEFINE_COMPAT_ERRLIST used on errlist-compat.c
>>>>>> does not create _sys_errlist and _sys_siglist with expected sizes defined by
>>>>>> glibc ABI.
>>>>>>
>>>>>> I am trying to fix without resorting to compiler options.
>>>>>
>>>>> DEFINE_COMPAT_ERRLIST does not expand to code/data, just reordeable directives:
>>>>>
>>>>>
>>>>>        .globl  __GLIBC_2_1_sys_errlist
>>>>> .set __GLIBC_2_1_sys_errlist, _sys_errlist_internal
>>>>>         .type   __GLIBC_2_1_sys_errlist,@object
>>>>>         .size   __GLIBC_2_1_sys_errlist, 1000
>>>>>         .globl  __GLIBC_2_1__sys_errlist
>>>>> .set __GLIBC_2_1__sys_errlist, _sys_errlist_internal
>>>>>         .type   __GLIBC_2_1__sys_errlist,@object
>>>>>         .size   __GLIBC_2_1__sys_errlist, 1000
>>>>> .symver __GLIBC_2_1_sys_nerr, sys_nerr@GLIBC_2.2.5
>>>>> .symver __GLIBC_2_1__sys_nerr, _sys_nerr@GLIBC_2.2.5
>>>>> .symver __GLIBC_2_1_sys_errlist, sys_errlist@GLIBC_2.2.5
>>>>> .symver __GLIBC_2_1__sys_errlist, _sys_errlist@GLIBC_2.2.5
>>>>>         .globl  __GLIBC_2_3_sys_errlist
>>>>> .set __GLIBC_2_3_sys_errlist, _sys_errlist_internal
>>>>>         .type   __GLIBC_2_3_sys_errlist,@object
>>>>>         .size   __GLIBC_2_3_sys_errlist, 1008
>>>>>         .globl  __GLIBC_2_3__sys_errlist
>>>>> .set __GLIBC_2_3__sys_errlist, _sys_errlist_internal
>>>>>         .type   __GLIBC_2_3__sys_errlist,@object
>>>>>         .size   __GLIBC_2_3__sys_errlist, 1008
>>>>> .symver __GLIBC_2_3_sys_nerr, sys_nerr@GLIBC_2.3
>>>>> .symver __GLIBC_2_3__sys_nerr, _sys_nerr@GLIBC_2.3
>>>>> .symver __GLIBC_2_3_sys_errlist, sys_errlist@GLIBC_2.3
>>>>> .symver __GLIBC_2_3__sys_errlist, _sys_errlist@GLIBC_2.3
>>>>>         .globl  __GLIBC_2_4_sys_errlist
>>>>>
>>>>> I do not know whether GCC would reorder these macros. Even yes,
>>>>> that'd just change the .symtab entries in the relocatable object file.
>>>>> The linker behavior remains the same with reordering.
>>>>
>>>> It does not seem to, just remove the -fno-unit-at-a-time and issue make
>>>> check-abi and you will see that object size for the compat symbols
>>>> reference to _sys_err_internal instead of the define compat ones.
>>>
>>> I see. I think this is a brittle behavior in GNU assembler. Filed
>>> https://sourceware.org/bugzilla/show_bug.cgi?id=29012 with detailed
>>> information. I have created a patch but I know that will not solve
>>> glibc's problem :(
>>
>> It would be good to have this fixes, but unfortunately we need a way
>> to handle this on older binutils.  I am kind worried that the only
>> possible way to actually fix this without resorting to any compiler
>> flags is coding the array definitions in assembly direct...
> 
> The GNU assembler issue has been fixed https://sourceware.org/bugzilla/show_bug.cgi?id=29012
> (milestone 2.39, way larger than the current required version: 2.25)
> 
> Switching to assembly output doesn't seem bad :-)
> 
> If you keep the compiler driver option but need to refactor the nearby
> code, you may drop -fno-unit-at-a-time. It was added in 2006 (somewhere
> between GCC 4.1 and 4.4), while glibc requires GCC>=6.2.

Good to know we won't need to rely on compiler flags to get the expected
correct asm directives. I am still struggling to get a fix without resorting
to compiler flags, but without much success.  Trying to move it to assembly
might be tricky, I am not sure if the data directives would be architecture
agnostic.