Re: [PATCH v4] i386: Allow -mlarge-data-threshold with -mcmodel=large

[Uros Bizjak <ubizjak@gmail.com>]

On Tue, Aug 1, 2023 at 9:51 PM Fangrui Song <maskray@google.com> wrote:
>
> When using -mcmodel=medium, large data objects larger than the
> -mlarge-data-threshold threshold are placed into large data sections
> (.lrodata, .ldata, .lbss and some variants).  GNU ld and ld.lld 17 place
> .l* sections into separate output sections.  If small and medium code
> model object files are mixed, the .l* sections won't exert relocation
> overflow pressure on sections in object files built with -mcmodel=small.
>
> However, when using -mcmodel=large, -mlarge-data-threshold doesn't
> apply.  This means that the .rodata/.data/.bss sections may exert
> relocation overflow pressure on sections in -mcmodel=small object files.
>
> This patch allows -mcmodel=large to generate .l* sections and drops an
> unneeded documentation restriction that the value must be the same.
>
> Link: https://groups.google.com/g/x86-64-abi/c/jnQdJeabxiU
> ("Large data sections for the large code model")
>
> Signed-off-by: Fangrui Song <maskray@google.com>
>
> ---
> Changes from v1 (https://gcc.gnu.org/pipermail/gcc-patches/2023-April/616947.html):
> * Clarify commit message. Add link to https://groups.google.com/g/x86-64-abi/c/jnQdJeabxiU
>
> Changes from v2
> * Drop an uneeded limitation in the documentation.
>
> Changes from v3
> * Change scan-assembler directives to use \. to match literal .
> ---
>  gcc/config/i386/i386.cc                    | 15 +++++++++------
>  gcc/config/i386/i386.opt                   |  2 +-
>  gcc/doc/invoke.texi                        |  6 +++---
>  gcc/testsuite/gcc.target/i386/large-data.c | 13 +++++++++++++
>  4 files changed, 26 insertions(+), 10 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.target/i386/large-data.c
>
> diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
> index eabc70011ea..37e810cc741 100644
> --- a/gcc/config/i386/i386.cc
> +++ b/gcc/config/i386/i386.cc
> @@ -647,7 +647,8 @@ ix86_can_inline_p (tree caller, tree callee)
>  static bool
>  ix86_in_large_data_p (tree exp)
>  {
> -  if (ix86_cmodel != CM_MEDIUM && ix86_cmodel != CM_MEDIUM_PIC)
> +  if (ix86_cmodel != CM_MEDIUM && ix86_cmodel != CM_MEDIUM_PIC &&
> +      ix86_cmodel != CM_LARGE && ix86_cmodel != CM_LARGE_PIC)

Please split multi-line expression before the operator, not after it,
as instructed in GNU Coding Standards [1] ...

[1] https://www.gnu.org/prep/standards/html_node/Formatting.html

>      return false;
>
>    if (exp == NULL_TREE)
> @@ -858,8 +859,9 @@ x86_elf_aligned_decl_common (FILE *file, tree decl,
>                         const char *name, unsigned HOST_WIDE_INT size,
>                         unsigned align)
>  {
> -  if ((ix86_cmodel == CM_MEDIUM || ix86_cmodel == CM_MEDIUM_PIC)
> -      && size > (unsigned int)ix86_section_threshold)
> +  if ((ix86_cmodel == CM_MEDIUM || ix86_cmodel == CM_MEDIUM_PIC ||
> +      ix86_cmodel == CM_LARGE || ix86_cmodel == CM_LARGE_PIC) &&
> +     size > (unsigned int)ix86_section_threshold)

... also here ...

>      {
>        switch_to_section (get_named_section (decl, ".lbss", 0));
>        fputs (LARGECOMM_SECTION_ASM_OP, file);
> @@ -879,9 +881,10 @@ void
>  x86_output_aligned_bss (FILE *file, tree decl, const char *name,
>                         unsigned HOST_WIDE_INT size, unsigned align)
>  {
> -  if ((ix86_cmodel == CM_MEDIUM || ix86_cmodel == CM_MEDIUM_PIC)
> -      && size > (unsigned int)ix86_section_threshold)
> -    switch_to_section (get_named_section (decl, ".lbss", 0));
> +  if ((ix86_cmodel == CM_MEDIUM || ix86_cmodel == CM_MEDIUM_PIC ||
> +       ix86_cmodel == CM_LARGE || ix86_cmodel == CM_LARGE_PIC) &&
> +      size > (unsigned int)ix86_section_threshold)

... and here.

OK with these formatting changes.

Thanks,
Uros.

> +    switch_to_section(get_named_section(decl, ".lbss", 0));
>    else
>      switch_to_section (bss_section);
>    ASM_OUTPUT_ALIGN (file, floor_log2 (align / BITS_PER_UNIT));
> diff --git a/gcc/config/i386/i386.opt b/gcc/config/i386/i386.opt
> index 1cc8563477a..52fad492353 100644
> --- a/gcc/config/i386/i386.opt
> +++ b/gcc/config/i386/i386.opt
> @@ -282,7 +282,7 @@ Branches are this expensive (arbitrary units).
>
>  mlarge-data-threshold=
>  Target RejectNegative Joined UInteger Var(ix86_section_threshold) Init(DEFAULT_LARGE_SECTION_THRESHOLD)
> --mlarge-data-threshold=<number>        Data greater than given threshold will go into .ldata section in x86-64 medium model.
> +-mlarge-data-threshold=<number>        Data greater than given threshold will go into a large data section in x86-64 medium and large code models.
>
>  mcmodel=
>  Target RejectNegative Joined Enum(cmodel) Var(ix86_cmodel) Init(CM_32)
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 104766f446d..bf6fe3e1a20 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -33207,9 +33207,9 @@ the cache line size.  @samp{compat} is the default.
>
>  @opindex mlarge-data-threshold
>  @item -mlarge-data-threshold=@var{threshold}
> -When @option{-mcmodel=medium} is specified, data objects larger than
> -@var{threshold} are placed in the large data section.  This value must be the
> -same across all objects linked into the binary, and defaults to 65535.
> +When @option{-mcmodel=medium} or @option{-mcmodel=large} is specified, data
> +objects larger than @var{threshold} are placed in large data sections. The
> +default is 65535.
>
>  @opindex mrtd
>  @item -mrtd
> diff --git a/gcc/testsuite/gcc.target/i386/large-data.c b/gcc/testsuite/gcc.target/i386/large-data.c
> new file mode 100644
> index 00000000000..bdd4acd30b8
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/i386/large-data.c
> @@ -0,0 +1,13 @@
> +/* { dg-do compile } */
> +/* { dg-require-effective-target lp64 } */
> +/* { dg-options "-O2 -mcmodel=large -mlarge-data-threshold=4" } */
> +/* { dg-final { scan-assembler {\.lbss} } } */
> +/* { dg-final { scan-assembler {\.bss} } } */
> +/* { dg-final { scan-assembler {\.ldata} } } */
> +/* { dg-final { scan-assembler {\.data} } } */
> +/* { dg-final { scan-assembler {\.lrodata} } } */
> +/* { dg-final { scan-assembler {\.rodata} } } */
> +
> +const char rodata_a[] = "abc", rodata_b[] = "abcd";
> +char data_a[4] = {1}, data_b[5] = {1};
> +char bss_a[4], bss_b[5];
> --
> 2.41.0.585.gd2178a4bd4-goog
>