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.
>
> 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 <xry...@xry111.site>
School of Aerospace Science and Technology, Xidian University
