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

["Fāng-ruì Sòng" <maskray@google.com>]

On Tue, Apr 5, 2022 at 10:40 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 05/04/2022 13:22, Fāng-ruì Sòng wrote:
> > On Tue, Apr 5, 2022 at 8:35 AM Adhemerval Zanella
> > <adhemerval.zanella@linaro.org> wrote:
> >>
> >>
> >>
> >> 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.
> >
> > Many directives are architecture-independent:
> > https://sourceware.org/binutils/docs/as/Pseudo-Ops.html#Pseudo-Ops
> > binutils-gdb/gas/read.c:346 The `portable[]` array.
>
> I think we can make it work with asciz directive.
>
> >
> > To support Clang, no refactoring is probably needed: just change
> > fno_unit_at_a_time to only specify -fno-toplevel-reorder (and rename
> > it), not th legacy -fno-unit-at-a-time.
>
> Afaik llvm does not support -fno-toplevel-reorder

It doesn't, but its integrated assembler does not need the option to
have the desired semantics. I have a note that the new GNU as behavior
is quite similar to LLVM's integrated assembler since 2014-03:
https://sourceware.org/bugzilla/show_bug.cgi?id=29012#c1