Re: [PATCH v2] LoongArch: add -mdirect-extern-access option

[Lulu Cheng <chenglulu@loongson.cn>]

LGTM!

Thanks!


在 2022/9/4 下午9:18, Xi Ruoyao 写道:
> v1 -> v2:
>
> Move TARGET_DIRECT_EXTERN_ACCESS check from loongarch_classify_symbol to
> loongarch_symbol_binds_local_p, so "%plt" is not output if
> -mdirect-extern-access used.
>
> -- >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.
>
> gcc/ChangeLog:
>
> 	* config/loongarch/genopts/loongarch.opt.in: Add
> 	-mdirect-extern-access option.
> 	* config/loongarch/loongarch.opt: Regenerate.
> 	* config/loongarch/loongarch.cc
> 	(loongarch_symbol_binds_local_p): Return true 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                 |  6 ++++++
>   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, 41 insertions(+)
>   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..c9187bf81a7 100644
> --- a/gcc/config/loongarch/loongarch.cc
> +++ b/gcc/config/loongarch/loongarch.cc
> @@ -1610,6 +1610,9 @@ loongarch_weak_symbol_p (const_rtx x)
>   bool
>   loongarch_symbol_binds_local_p (const_rtx x)
>   {
> +  if (TARGET_DIRECT_EXTERN_ACCESS)
> +    return true;
> +
>     if (SYMBOL_REF_P (x))
>       return (SYMBOL_REF_DECL (x)
>   	    ? targetm.binds_local_p (SYMBOL_REF_DECL (x))
> @@ -6093,6 +6096,9 @@ 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 dd3302fcd15..9d662e35316 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -1017,6 +1017,7 @@ 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}
> @@ -25090,6 +25091,20 @@ 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; }