On Wed, May 10, 2023 at 2:58 AM Uros Bizjak <ubiz...@gmail.com> wrote: > > On Fri, Apr 28, 2023 at 2:47 AM Fangrui Song <mask...@google.com> wrote: > > > > When using -mcmodel=medium, large data is placed into .l* sections. GNU ld > > places .l* sections into separate output sections. If small and medium > > code model object files are mixed, the .l* sections won't cause > > relocation overflow pressure on sections in -mcmodel=small object files. > > > > However, when using -mcmodel=large, -mlarge-data-threshold doesn't apply. > > This > > means that the .rodata/.data/.bss sections may cause relocation overflow > > pressure on sections in -mcmodel=small object files. > > > > This patch allows -mcmodel=large to generate .l* sections. > > The x86_64 psABI does not specify how -mlarge-threshold is handled > with -mcmodel=large and it also doesn't mention that -mcmodel=large > inherits handling from -mcmodel=medium. The ABI does say that the > -mcmodel=large is strictly only required if the text of a single > function becomes larger than what the medium model allows. > > OTOH, the ABI also doesn't prohibit large sections with -mcmodel=large > and IMO, the introduction of -mlarge-threshold with -mcmodel=large > does not create an ABI change. > > I think the best way is to first discuss the issue with the x86_64 > psABI group, to clarify how -mlarge-threshold and large data is > handled under a large code model. > > Uros.
Thank you for the suggestion. I started https://groups.google.com/g/x86-64-abi/c/jnQdJeabxiU ("Large data sections for the large code model") and created https://gitlab.com/x86-psABIs/x86-64-ABI/-/merge_requests/42/ to clarify large data sections usage. In the psABI, "Table 4.4: Additional Special Sections for the Large Code Model" lists the large data sections. Perhaps it should be "Table 4.4: Additional Special Sections for the Medium and Large Code Models". > > > > Signed-off-by: Fangrui Song <mask...@google.com> > > --- > > gcc/config/i386/i386.cc | 15 +++++++++------ > > gcc/config/i386/i386.opt | 2 +- > > gcc/doc/invoke.texi | 7 ++++--- > > gcc/testsuite/gcc.target/i386/large-data.c | 13 +++++++++++++ > > 4 files changed, 27 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 a3db55642e3..c68c66a5567 100644 > > --- a/gcc/config/i386/i386.cc > > +++ b/gcc/config/i386/i386.cc > > @@ -637,7 +637,8 @@ 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) > > return false; > > > > if (exp == NULL_TREE) > > @@ -848,8 +849,9 @@ 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) > > { > > switch_to_section (get_named_section (decl, ".lbss", 0)); > > fputs (LARGECOMM_SECTION_ASM_OP, file); > > @@ -869,9 +871,10 @@ 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) > > + 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 d74f6b1f8fc..de8e722cd62 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 e5ee2d536fc..4a20eef92e5 100644 > > --- a/gcc/doc/invoke.texi > > +++ b/gcc/doc/invoke.texi > > @@ -32927,9 +32927,10 @@ 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. > > This > > +value must be the same across all objects linked into the binary, and > > defaults > > +to 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..09a917431d4 > > --- /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]; > > -- > > 2.40.1.495.gc816e09b53d-goog > > -- 宋方睿
