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

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

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.

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.