Re: [PATCH] LoongArch: add -mdirect-extern-access option

[Fangrui Song <maskray@google.com>]

On Fri, Sep 2, 2022 at 4:31 AM Xi Ruoyao via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Thu, 2022-09-01 at 18:54 +0800, Xi Ruoyao wrote:
> > We'd like to introduce a new codegen option to align with the old
> > "-Wa,-mla-global-with-pcrel" and avoid a performance & size regression
> > building the Linux kernel with new-reloc toolchain.  And it should be
> > also useful for building statically linked executables, firmwares (EDK2
> > for example), and other OS kernels.
>
> Some news: get rid of the GOT will also make the implementation of
> relocatable kernel easier, so I hope this can be reviewed quickly.
>
> > OK for trunk?
> >
> > -- >8 --
> >
> > As a new target, LoongArch does not use copy relocation as it's
> > problematic in some circumstances.  One bad consequence is we are
> > emitting GOT for all accesses to all extern objects with default
> > visibility.  The use of GOT is not needed in statically linked
> > executables, OS kernels etc.  The GOT entry just wastes space, and the
> > GOT access just slow down the execution in those environments.
> >
> > Before -mexplicit-relocs, we used "-Wa,-mla-global-with-pcrel" to tell
> > the assembler not to use GOT for extern access.  But with
> > -mexplicit-relocs, we have to opt the logic in GCC.
> >
> > The name "-mdirect-extern-access" is learnt from x86 port.

I think a discussion about the exact option name is useful.

The GCC x86 port generates
GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS for
-mno-direct-extern-access.

The Clang -fdirect-access-external-data (which is added before
-mdirect-extern-access) does not use
GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS.

If a new port avoids copy relocations in the first place,
GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS isn't really useful.

> > gcc/ChangeLog:
> >
> >         * config/loongarch/genopts/loongarch.opt.in: Add
> >         -mdirect-extern-access option.
> >         * config/loongarch/loongarch.opt: Regenerate.
> >         * config/loongarch/loongarch.cc (loongarch_classify_symbol):
> >         Don't use SYMBOL_GOT_DISP if TARGET_DIRECT_EXTERN_ACCESS.
> >         (loongarch_option_override_internal): Complain if
> >         -mdirect-extern-access is used with -fPIC or -fpic.
> >         * doc/invoke.texi: Document -mdirect-extern-access for
> >         LoongArch.
> >
> > gcc/testsuite/ChangeLog:
> >
> >         * gcc.target/loongarch/direct-extern-1.c: New test.
> >         * gcc.target/loongarch/direct-extern-2.c: New test.
> > ---
> >  gcc/config/loongarch/genopts/loongarch.opt.in     |  4 ++++
> >  gcc/config/loongarch/loongarch.cc                 |  5 ++++-
> >  gcc/config/loongarch/loongarch.opt                |  4 ++++
> >  gcc/doc/invoke.texi                               | 15
> > +++++++++++++++
> >  .../gcc.target/loongarch/direct-extern-1.c        |  6 ++++++
> >  .../gcc.target/loongarch/direct-extern-2.c        |  6 ++++++
> >  6 files changed, 39 insertions(+), 1 deletion(-)
> >  create mode 100644 gcc/testsuite/gcc.target/loongarch/direct-extern-
> > 1.c
> >  create mode 100644 gcc/testsuite/gcc.target/loongarch/direct-extern-
> > 2.c
> >
> > diff --git a/gcc/config/loongarch/genopts/loongarch.opt.in
> > b/gcc/config/loongarch/genopts/loongarch.opt.in
> > index ebdd9538d48..e10618777b2 100644
> > --- a/gcc/config/loongarch/genopts/loongarch.opt.in
> > +++ b/gcc/config/loongarch/genopts/loongarch.opt.in
> > @@ -184,3 +184,7 @@ Enum(cmodel) String(@@STR_CMODEL_EXTREME@@)
> > Value(CMODEL_EXTREME)
> >  mcmodel=
> >  Target RejectNegative Joined Enum(cmodel) Var(la_opt_cmodel)
> > Init(CMODEL_NORMAL)
> >  Specify the code model.
> > +
> > +mdirect-extern-access
> > +Target Var(TARGET_DIRECT_EXTERN_ACCESS) Init(0)
> > +Avoid using the GOT to access external symbols.
> > diff --git a/gcc/config/loongarch/loongarch.cc
> > b/gcc/config/loongarch/loongarch.cc
> > index 77e3a105390..2875fa5b0f3 100644
> > --- a/gcc/config/loongarch/loongarch.cc
> > +++ b/gcc/config/loongarch/loongarch.cc
> > @@ -1642,7 +1642,7 @@ loongarch_classify_symbol (const_rtx x)
> >    if (SYMBOL_REF_TLS_MODEL (x))
> >      return SYMBOL_TLS;
> >
> > -  if (!loongarch_symbol_binds_local_p (x))
> > +  if (!TARGET_DIRECT_EXTERN_ACCESS && !loongarch_symbol_binds_local_p
> > (x))
> >      return SYMBOL_GOT_DISP;
> >
> >    tree t = SYMBOL_REF_DECL (x);
> > @@ -6093,6 +6093,9 @@ loongarch_option_override_internal (struct
> > gcc_options *opts)
> >    if (loongarch_branch_cost == 0)
> >      loongarch_branch_cost = loongarch_cost->branch_cost;
> >
> > +  if (TARGET_DIRECT_EXTERN_ACCESS && flag_shlib)
> > +    error ("%qs cannot be used for compiling a shared library",
> > +          "-mdirect-extern-access");
> >
> >    switch (la_target.cmodel)
> >      {
> > diff --git a/gcc/config/loongarch/loongarch.opt
> > b/gcc/config/loongarch/loongarch.opt
> > index 6395234218b..96c811c850b 100644
> > --- a/gcc/config/loongarch/loongarch.opt
> > +++ b/gcc/config/loongarch/loongarch.opt
> > @@ -191,3 +191,7 @@ Enum(cmodel) String(extreme) Value(CMODEL_EXTREME)
> >  mcmodel=
> >  Target RejectNegative Joined Enum(cmodel) Var(la_opt_cmodel)
> > Init(CMODEL_NORMAL)
> >  Specify the code model.
> > +
> > +mdirect-extern-access
> > +Target Var(TARGET_DIRECT_EXTERN_ACCESS) Init(0)
> > +Avoid using the GOT to access external symbols.
> > diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> > index e5eb525a2c1..d4e86682827 100644
> > --- a/gcc/doc/invoke.texi
> > +++ b/gcc/doc/invoke.texi
> > @@ -1016,6 +1016,7 @@ Objective-C and Objective-C++ Dialects}.
> >  -memcpy  -mno-memcpy -mstrict-align -mno-strict-align @gol
> >  -mmax-inline-memcpy-size=@var{n} @gol
> >  -mexplicit-relocs -mno-explicit-relocs @gol
> > +-mdirect-extern-access -mno-direct-extern-access @gol
> >  -mcmodel=@var{code-model}}
> >
> >  @emph{M32R/D Options}
> > @@ -25100,6 +25101,20 @@ GCC build-time by detecting corresponding
> > assembler support:
> >  @code{-mno-explicit-relocs} otherwise.  This option is mostly useful
> > for
> >  debugging, or interoperation with assemblers different from the
> > build-time
> >  one.
> > +
> > +@item -mdirect-extern-access
> > +@itemx -mno-direct-extern-access
> > +@opindex mdirect-extern-access
> > +Do not use or use GOT to access external symbols.  The default is
> > +@option{-mno-direct-extern-access}: GOT is used for external symbols
> > with
> > +default visibility, but not used for other external symbols.
> > +
> > +With @option{-mdirect-extern-access}, GOT is not used and all
> > external
> > +symbols are PC-relatively addressed.  It is @strong{only} suitable
> > for
> > +environments where no dynamic link is performed, like firmwares, OS
> > +kernels, executables linked with @option{-static} or @option{-static-
> > pie}.
> > +@option{-mdirect-extern-access} is not compatible with @option{-fPIC}
> > or
> > +@option{-fpic}.
> >  @end table
> >
> >  @node M32C Options
> > diff --git a/gcc/testsuite/gcc.target/loongarch/direct-extern-1.c
> > b/gcc/testsuite/gcc.target/loongarch/direct-extern-1.c
> > new file mode 100644
> > index 00000000000..85c6c1e8a88
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/loongarch/direct-extern-1.c
> > @@ -0,0 +1,6 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-mexplicit-relocs -mdirect-extern-access" } */
> > +/* { dg-final { scan-assembler-not "got" } } */
> > +
> > +extern int x;
> > +int f() { return x; }
> > diff --git a/gcc/testsuite/gcc.target/loongarch/direct-extern-2.c
> > b/gcc/testsuite/gcc.target/loongarch/direct-extern-2.c
> > new file mode 100644
> > index 00000000000..58d8bd68a06
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/loongarch/direct-extern-2.c
> > @@ -0,0 +1,6 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-mno-explicit-relocs -mdirect-extern-access" } */
> > +/* { dg-final { scan-assembler-not "la.global" } } */
> > +
> > +extern int x;
> > +int f() { return x; }
>
> --
> Xi Ruoyao <xry111@xry111.site>
> School of Aerospace Science and Technology, Xidian University



-- 
宋方睿