On Sun, Nov 6, 2016 at 8:40 PM, Iain Sandoe <iain_san...@mentor.com> wrote:
> Hi Folks,
>
> (much) Older Darwin linkers needed separate sections marked "coalesce" to
> allow for weak symbol coalescing. This has not been needed for some time
> and is now deprecated, newer assemblers warn if the old coalesced sections
> are used. We need to switch to using the regular sections when a new enough
> linker is available.
>
> There’s a small change to i386.c to make it possible to switch thunk output
> between coalesced and “regular’ sections in response to the relevant linker
> info.
Anything protected by "#if TARGET_MACHO" can be considered as Darwin
specific patch and thus under Darwin port maintainer's approval.
Uros.
> OK for trunk?
> OK for open branches?
> Iain
>
> gcc/
>
> 2016-11-06 Iain Sandoe <i...@codesourcery.com>
>
> PR target/71767
> * config/darwin-sections.def (picbase_thunk_section): New.
> * config/darwin.c (darwin_init_sections): Set up picbase thunk
> section.
> (darwin_rodata_section, darwin_objc2_section, machopic_select_section,
> darwin_asm_declare_constant_name, darwin_emit_weak_or_comdat,
> darwin_function_section): Don’t use coalesced with newer linkers.
> (darwin_override_options): Decide on usage of coalesed sections on the
> basis of the target linker version.
> * config/darwin.h (MIN_LD64_NO_COAL_SECTS): New.
> * config/darwin.opt (mtarget-linker): New.
> * config/i386/i386.c (ix86_code_end): Do not force the thunks into a
> coalesced
> section, instead use a thunks section.
>
>
> ---
> gcc/config/darwin-sections.def | 5 +++
> gcc/config/darwin.c | 77
> +++++++++++++++++++++++++++---------------
> gcc/config/darwin.h | 5 +++
> gcc/config/darwin.opt | 6 ++++
> gcc/config/i386/i386.c | 2 +-
> 5 files changed, 67 insertions(+), 28 deletions(-)
>
> diff --git a/gcc/config/darwin-sections.def b/gcc/config/darwin-sections.def
> index cbf3e41..8a7e985 100644
> --- a/gcc/config/darwin-sections.def
> +++ b/gcc/config/darwin-sections.def
> @@ -31,6 +31,11 @@ along with GCC; see the file COPYING3. If not see
> DEF_SECTION (text_coal_section, SECTION_CODE|SECTION_NO_ANCHOR,
> ".section __TEXT,__textcoal_nt,coalesced,pure_instructions", 0)
>
> +/* We define a picbase thunks section separately, so that we can override the
> + def to be '.text' for versions of ld64 that handle coalescing. */
> +DEF_SECTION (picbase_thunk_section, SECTION_CODE|SECTION_NO_ANCHOR,
> + ".section __TEXT,__textcoal_nt,coalesced,pure_instructions", 0)
> +
> DEF_SECTION (text_hot_section, SECTION_CODE,
> ".section __TEXT,__text_hot,regular,pure_instructions", 0)
> DEF_SECTION (text_cold_section, SECTION_CODE,
> diff --git a/gcc/config/darwin.c b/gcc/config/darwin.c
> index 210c2da..baca46c 100644
> --- a/gcc/config/darwin.c
> +++ b/gcc/config/darwin.c
> @@ -97,6 +97,10 @@ int darwin_running_cxx;
> /* Some code-gen now depends on OS major version numbers (at least). */
> int generating_for_darwin_version ;
>
> +/* For older linkers we need to emit special sections (marked 'coalesced')
> for
> + for weak or single-definition items. */
> +static bool ld_uses_coal_sects = false;
> +
> /* Section names. */
> section * darwin_sections[NUM_DARWIN_SECTIONS];
>
> @@ -220,6 +224,11 @@ darwin_init_sections (void)
> readonly_data_section = darwin_sections[const_section];
> exception_section = darwin_sections[darwin_exception_section];
> eh_frame_section = darwin_sections[darwin_eh_frame_section];
> +
> + /* If our linker is new enough to coalesce weak symbols, then we
> + can just put picbase_thunks into the text section. */
> + if (! ld_uses_coal_sects )
> + darwin_sections[picbase_thunk_section] = text_section;
> }
>
> int
> @@ -1247,9 +1256,9 @@ darwin_mark_decl_preserved (const char *name)
> }
>
> static section *
> -darwin_rodata_section (int weak, bool zsize)
> +darwin_rodata_section (int use_coal, bool zsize)
> {
> - return (weak
> + return (use_coal
> ? darwin_sections[const_coal_section]
> : (zsize ? darwin_sections[zobj_const_section]
> : darwin_sections[const_section]));
> @@ -1418,7 +1427,8 @@ darwin_objc2_section (tree decl ATTRIBUTE_UNUSED, tree
> meta, section * base)
> return darwin_sections[objc2_image_info_section];
>
> else if (!strncmp (p, "V2_EHTY", 7))
> - return darwin_sections[data_coal_section];
> + return ld_uses_coal_sects ? darwin_sections[data_coal_section]
> + : data_section;
>
> else if (!strncmp (p, "V2_CSTR", 7))
> return darwin_sections[objc2_constant_string_object_section];
> @@ -1517,21 +1527,23 @@ machopic_select_section (tree decl,
> int reloc,
> unsigned HOST_WIDE_INT align)
> {
> - bool zsize, one, weak, ro;
> + bool zsize, one, weak, use_coal, ro;
> section *base_section = NULL;
>
> weak = (DECL_P (decl)
> && DECL_WEAK (decl)
> && !lookup_attribute ("weak_import", DECL_ATTRIBUTES (decl)));
>
> - zsize = (DECL_P (decl)
> + zsize = (DECL_P (decl)
> && (TREE_CODE (decl) == VAR_DECL || TREE_CODE (decl) == CONST_DECL)
> && tree_to_uhwi (DECL_SIZE_UNIT (decl)) == 0);
>
> - one = DECL_P (decl)
> + one = DECL_P (decl)
> && TREE_CODE (decl) == VAR_DECL
> && DECL_COMDAT_GROUP (decl);
>
> + use_coal = (weak || one) && ld_uses_coal_sects;
> +
> ro = TREE_READONLY (decl) || TREE_CONSTANT (decl) ;
>
> switch (categorize_decl_for_section (decl, reloc))
> @@ -1542,7 +1554,7 @@ machopic_select_section (tree decl,
>
> case SECCAT_RODATA:
> case SECCAT_SRODATA:
> - base_section = darwin_rodata_section (weak, zsize);
> + base_section = darwin_rodata_section (use_coal, zsize);
> break;
>
> case SECCAT_RODATA_MERGE_STR:
> @@ -1564,7 +1576,7 @@ machopic_select_section (tree decl,
> case SECCAT_DATA_REL_RO_LOCAL:
> case SECCAT_SDATA:
> case SECCAT_TDATA:
> - if (weak || one)
> + if (use_coal)
> {
> if (ro)
> base_section = darwin_sections[const_data_coal_section];
> @@ -1593,7 +1605,7 @@ machopic_select_section (tree decl,
> case SECCAT_BSS:
> case SECCAT_SBSS:
> case SECCAT_TBSS:
> - if (weak || one)
> + if (use_coal)
> base_section = darwin_sections[data_coal_section];
> else
> {
> @@ -2289,14 +2301,18 @@ darwin_asm_declare_constant_name (FILE *file, const
> char *name,
> static void
> darwin_emit_weak_or_comdat (FILE *fp, tree decl, const char *name,
> unsigned HOST_WIDE_INT size,
> + bool use_coal,
> unsigned int align)
> {
> - /* Since the sections used here are coalesed, they will not be eligible
> - for section anchors, and therefore we don't need to break that out. */
> + /* Since the sections used here are coalesced, they will not be eligible
> + for section anchors, and therefore we don't need to break that out.
> + CHECKME: for modern linker on PowerPC. */
> if (TREE_READONLY (decl) || TREE_CONSTANT (decl))
> - switch_to_section (darwin_sections[const_data_coal_section]);
> + switch_to_section (use_coal ? darwin_sections[const_data_coal_section]
> + : darwin_sections[const_data_section]);
> else
> - switch_to_section (darwin_sections[data_coal_section]);
> + switch_to_section (use_coal ? darwin_sections[data_coal_section]
> + : data_section);
>
> /* To be consistent, we'll allow darwin_asm_declare_object_name to assemble
> the align info for zero-sized items... but do it here otherwise. */
> @@ -2515,7 +2531,7 @@ fprintf (fp, "# albss: %s (%lld,%d) ro %d cst %d stat
> %d com %d"
> {
> /* Weak or COMDAT objects are put in mergeable sections. */
> darwin_emit_weak_or_comdat (fp, decl, name, size,
> - DECL_ALIGN (decl));
> + ld_uses_coal_sects, DECL_ALIGN (decl));
> return;
> }
>
> @@ -2629,7 +2645,7 @@ fprintf (fp, "# adcom: %s (%lld,%d) ro %d cst %d stat
> %d com %d pub %d"
> {
> /* Weak or COMDAT objects are put in mergable sections. */
> darwin_emit_weak_or_comdat (fp, decl, name, size,
> - DECL_ALIGN (decl));
> + ld_uses_coal_sects, DECL_ALIGN (decl));
> return;
> }
>
> @@ -2699,7 +2715,7 @@ fprintf (fp, "# adloc: %s (%lld,%d) ro %d cst %d stat
> %d one %d pub %d"
> {
> /* Weak or COMDAT objects are put in mergable sections. */
> darwin_emit_weak_or_comdat (fp, decl, name, size,
> - DECL_ALIGN (decl));
> + ld_uses_coal_sects, DECL_ALIGN (decl));
> return;
> }
>
> @@ -3084,6 +3100,12 @@ darwin_override_options (void)
> /* Earlier versions are not specifically accounted, until required. */
> }
>
> + /* Older Darwin ld could not coalesce weak entities without them being
> + placed in special sections. */
> + if (darwin_target_linker
> + && (strverscmp (darwin_target_linker, MIN_LD64_NO_COAL_SECTS) < 0))
> + ld_uses_coal_sects = true;
> +
> /* In principle, this should be c-family only. However, we really need to
> set sensible defaults for LTO as well, since the section selection stuff
> should check for correctness re. the ABI. TODO: check and provide the
> @@ -3609,12 +3631,13 @@ darwin_function_section (tree decl, enum
> node_frequency freq,
> bool startup, bool exit)
> {
> /* Decide if we need to put this in a coalescable section. */
> - bool weak = (decl
> + bool weak = (decl
> && DECL_WEAK (decl)
> && (!DECL_ATTRIBUTES (decl)
> || !lookup_attribute ("weak_import",
> DECL_ATTRIBUTES (decl))));
>
> + bool use_coal = weak && ld_uses_coal_sects;
> /* If there is a specified section name, we should not be trying to
> override. */
> if (decl && DECL_SECTION_NAME (decl) != NULL)
> @@ -3622,8 +3645,8 @@ darwin_function_section (tree decl, enum node_frequency
> freq,
>
> /* We always put unlikely executed stuff in the cold section. */
> if (freq == NODE_FREQUENCY_UNLIKELY_EXECUTED)
> - return (weak) ? darwin_sections[text_cold_coal_section]
> - : darwin_sections[text_cold_section];
> + return (use_coal) ? darwin_sections[text_cold_coal_section]
> + : darwin_sections[text_cold_section];
>
> /* If we have LTO *and* feedback information, then let LTO handle
> the function ordering, it makes a better job (for normal, hot,
> @@ -3633,23 +3656,23 @@ darwin_function_section (tree decl, enum
> node_frequency freq,
>
> /* Non-cold startup code should go to startup subsection. */
> if (startup)
> - return (weak) ? darwin_sections[text_startup_coal_section]
> - : darwin_sections[text_startup_section];
> + return (use_coal) ? darwin_sections[text_startup_coal_section]
> + : darwin_sections[text_startup_section];
>
> /* Similarly for exit. */
> if (exit)
> - return (weak) ? darwin_sections[text_exit_coal_section]
> - : darwin_sections[text_exit_section];
> + return (use_coal) ? darwin_sections[text_exit_coal_section]
> + : darwin_sections[text_exit_section];
>
> /* Place hot code. */
> if (freq == NODE_FREQUENCY_HOT)
> - return (weak) ? darwin_sections[text_hot_coal_section]
> - : darwin_sections[text_hot_section];
> + return (use_coal) ? darwin_sections[text_hot_coal_section]
> + : darwin_sections[text_hot_section];
>
> /* Otherwise, default to the 'normal' non-reordered sections. */
> default_function_sections:
> - return (weak) ? darwin_sections[text_coal_section]
> - : text_section;
> + return (use_coal) ? darwin_sections[text_coal_section]
> + : text_section;
> }
>
> /* When a function is partitioned between sections, we need to insert a label
> diff --git a/gcc/config/darwin.h b/gcc/config/darwin.h
> index 541bcb3..79fc506 100644
> --- a/gcc/config/darwin.h
> +++ b/gcc/config/darwin.h
> @@ -940,6 +940,11 @@ extern void darwin_driver_init (unsigned int *,struct
> cl_decoded_option **);
> fall-back default. */
> #define DEF_MIN_OSX_VERSION "10.5"
>
> +/* Later versions of ld64 support coalescing weak code/data without requiring
> + that they be placed in specially identified sections. This is the
> earliest
> + _tested_ version known to support this so far. */
> +#define MIN_LD64_NO_COAL_SECTS "236.4"
> +
> #ifndef LD64_VERSION
> #define LD64_VERSION "85.2"
> #else
> diff --git a/gcc/config/darwin.opt b/gcc/config/darwin.opt
> index 5093731..7f2e394 100644
> --- a/gcc/config/darwin.opt
> +++ b/gcc/config/darwin.opt
> @@ -391,5 +391,11 @@ Driver Separate
> sub_umbrella
> Driver Separate
>
> +; Certain aspects of code-gen may be improved / adjusted if the version of
> ld64
> +; is sufficiently modern.
> +mtarget-linker
> +Target RejectNegative Joined Separate Report Var(darwin_target_linker)
> Init(LD64_VERSION)
> +The version of ld64 in use for this toolchain.
> +
> undefined
> Driver Separate
> diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
> index bffba80..9e9fe02 100644
> --- a/gcc/config/i386/i386.c
> +++ b/gcc/config/i386/i386.c
> @@ -11832,7 +11832,7 @@ ix86_code_end (void)
> #if TARGET_MACHO
> if (TARGET_MACHO)
> {
> - switch_to_section (darwin_sections[text_coal_section]);
> + switch_to_section (darwin_sections[picbase_thunk_section]);
> fputs ("\t.weak_definition\t", asm_out_file);
> assemble_name (asm_out_file, name);
> fputs ("\n\t.private_extern\t", asm_out_file);
> --
> 2.8.1
>
>
