On Fri, Dec 7, 2018 at 1:24 AM Iain Sandoe <i...@sandoe.co.uk> wrote:
>
> Hi
>
> This got stuck in my stack of patches for LTO debug support, and I forgot to
> ping it…
>
> > On 22 Aug 2018, at 14:20, Richard Biener <richard.guent...@gmail.com> wrote:
> >
> > On Wed, Aug 22, 2018 at 2:56 PM Iain Sandoe <i...@sandoe.co.uk> wrote:
> >>
> >>
> >>> On 20 Aug 2018, at 11:01, Richard Biener <richard.guent...@gmail.com>
> >>> wrote:
> >>>
> >>> On Sat, Aug 18, 2018 at 9:00 PM Iain Sandoe <i...@sandoe.co.uk> wrote:
> >>>>
> >>
> >>>> I plan on making Darwin default to fno-lto for link unless there’s an
> >>>> “flto*” on the link line, since otherwise there’s a process launch for
> >>>> every object on the c/l to do “nm”, which is quite heavy weight. At
> >>>> present, ISTM if the compiler is configured with —enable-lto, the link
> >>>> line defaults to assuming that every object needs to be checked.
> >>>
> >>> I think we wanted to transparently handle LTO objects (similar to how
> >>> it works with a linker plugin).
> >>
> >> At present, we are not only calling nm for every object, but also dsymutil
> >> sometimes unnecessarily - since the assumption on the “maybe_lto” path is
> >> that an temporary file will be generated.
> >>
> >> as a headline - when I did the simplistic “default is to assume fno-lto”
> >> that took 10% off the bootstrap time (with a stage#1 compiler without the
> >> optimisation).
> >>
> >> - I have a patch under test as below + better fidelity of avoiding
> >> dsymutil runs.
> >>
> >>> Ideally collect2 wouldn't use nm but simple-object to inspect files
> >>> (but that doesn't have symbol table query support
> >>
> >> Hrm.my WIP had grow symtab awareness to support debug-section copy on
> >> mach-o
> >> (and the ELF impl. looks like it understands both symtab and relocs)
> >> - so maybe that’s not as far away as we might think.
> >>
> >>> though looking for LTO specific sections would have been better in the
> >>> first place - I'd suggest .gnu.lto_.symtab).
> >>
> >> I’ve done this noting that the symtab seems to be the last emitted section
> >> - is that why you chose it (for security, rather than just stopping as
> >> soon as we see a .gnu.lto_. section?)
> >
> > Ah, any .gnu.lto_.section would work as well I guess - I just picked
> > one that should be always
> > created. .gnu.lto_.opts is another one.
>
> >> comments?
> >
> > OK. Feel free to stop when recognizing any LTO section. Btw, as you
> > include lto-section-names.h
> > you should probably use LTO_SECTION_NAME_PREFIX instead of hard-coding
> > .gnu.lto_.
>
> So I just look for the LTO_SECTION_NAME_PREFIX
>
> > I'm not sure if/how we should handle offload sections and if those
> > work at all without a
> > linker plugin currently?
>
> In this version, I look for the OFFLOAD_SECTION_NAME_PREFIX as well,
> although AFAICS collect2 itself has no specific handling of offload, the
> detection
> is done in lto-wrapper.
>
> OK for trunk now?
OK.
Thanks,
Richard.
> thanks
> Iain
>
>
> From: Iain Sandoe <i...@sandoe.co.uk>
> Date: Tue, 21 Aug 2018 12:55:02 +0100
> Subject: [PATCH] [PATCH, collect2] Use simple-object to find out if objects
> contain LTO.
>
> This replaces the use of nm to search for the LTO common symbol marker
> and uses simple object to see if there's a section starting with
> ".gnu.lto_." or .gnu.offload_lto_
> ---
> gcc/collect2.c | 120 +++++++++++++++++++++++--------------------------
> 1 file changed, 57 insertions(+), 63 deletions(-)
>
> diff --git a/gcc/collect2.c b/gcc/collect2.c
> index 60269682ec..dcbd3e18a6 100644
> --- a/gcc/collect2.c
> +++ b/gcc/collect2.c
> @@ -30,6 +30,8 @@ along with GCC; see the file COPYING3. If not see
> #include "tm.h"
> #include "filenames.h"
> #include "file-find.h"
> +#include "simple-object.h"
> +#include "lto-section-names.h"
>
> /* TARGET_64BIT may be defined to use driver specific functionality. */
> #undef TARGET_64BIT
> @@ -804,7 +806,9 @@ maybe_run_lto_and_relink (char **lto_ld_argv, char
> **object_lst,
> /* Run the linker again, this time replacing the object files
> optimized by the LTO with the temporary file generated by the LTO.
> */
> fork_execute ("ld", out_lto_ld_argv, HAVE_GNU_LD && at_file_supplied);
> - post_ld_pass (true);
> + /* We assume that temp files were created, and therefore we need to
> take
> + that into account (maybe run dsymutil). */
> + post_ld_pass (/*temp_file*/true);
> free (lto_ld_argv);
>
> maybe_unlink_list (lto_o_files);
> @@ -814,10 +818,11 @@ maybe_run_lto_and_relink (char **lto_ld_argv, char
> **object_lst,
> /* Our caller is relying on us to do the link
> even though there is no LTO back end work to be done. */
> fork_execute ("ld", lto_ld_argv, HAVE_GNU_LD && at_file_supplied);
> - post_ld_pass (false);
> + /* No LTO objects were found, so no new temp file. */
> + post_ld_pass (/*temp_file*/false);
> }
> else
> - post_ld_pass (true);
> + post_ld_pass (false); /* No LTO objects were found, no temp file. */
> }
>
> /* Main program. */
> @@ -1710,7 +1715,7 @@ main (int argc, char **argv)
> if (lto_mode != LTO_MODE_NONE)
> maybe_run_lto_and_relink (ld1_argv, object_lst, object, false);
> else
> - post_ld_pass (false);
> + post_ld_pass (/*temp_file*/false);
>
> return 0;
> }
> @@ -1780,7 +1785,7 @@ main (int argc, char **argv)
> #ifdef COLLECT_EXPORT_LIST
> maybe_unlink (export_file);
> #endif
> - post_ld_pass (false);
> + post_ld_pass (/*temp_file*/false);
>
> maybe_unlink (c_file);
> maybe_unlink (o_file);
> @@ -1874,7 +1879,7 @@ main (int argc, char **argv)
> else
> {
> fork_execute ("ld", ld2_argv, HAVE_GNU_LD && at_file_supplied);
> - post_ld_pass (false);
> + post_ld_pass (/*temp_file*/false);
> }
>
> /* Let scan_prog_file do any final mods (OSF/rose needs this for
> @@ -2332,38 +2337,52 @@ write_aix_file (FILE *stream, struct id *list)
>
> /* Check to make sure the file is an LTO object file. */
>
> +static int
> +has_lto_section (void *data, const char *name ATTRIBUTE_UNUSED,
> + off_t offset ATTRIBUTE_UNUSED,
> + off_t length ATTRIBUTE_UNUSED)
> +{
> + int *found = (int *) data;
> +
> + if (strncmp (name, LTO_SECTION_NAME_PREFIX,
> + sizeof (LTO_SECTION_NAME_PREFIX) - 1) != 0)
> + {
> + if (strncmp (name, OFFLOAD_SECTION_NAME_PREFIX,
> + sizeof (OFFLOAD_SECTION_NAME_PREFIX) - 1) != 0)
> + return 1;
> + }
> +
> + *found = 1;
> +
> + /* Stop iteration. */
> + return 0;
> +}
> +
> static bool
> -maybe_lto_object_file (const char *prog_name)
> +is_lto_object_file (const char *prog_name)
> {
> - FILE *f;
> - unsigned char buf[4];
> - int i;
> + const char *errmsg;
> + int err;
> + int found = 0;
> + off_t inoff = 0;
> + int infd = open (prog_name, O_RDONLY | O_BINARY);
>
> - static unsigned char elfmagic[4] = { 0x7f, 'E', 'L', 'F' };
> - static unsigned char coffmagic[2] = { 0x4c, 0x01 };
> - static unsigned char coffmagic_x64[2] = { 0x64, 0x86 };
> - static unsigned char machomagic[4][4] = {
> - { 0xcf, 0xfa, 0xed, 0xfe },
> - { 0xce, 0xfa, 0xed, 0xfe },
> - { 0xfe, 0xed, 0xfa, 0xcf },
> - { 0xfe, 0xed, 0xfa, 0xce }
> - };
> + if (infd == -1)
> + return false;
>
> - f = fopen (prog_name, "rb");
> - if (f == NULL)
> + simple_object_read *inobj = simple_object_start_read (infd, inoff,
> + LTO_SEGMENT_NAME,
> + &errmsg, &err);
> + if (!inobj)
> return false;
> - if (fread (buf, sizeof (buf), 1, f) != 1)
> - buf[0] = 0;
> - fclose (f);
>
> - if (memcmp (buf, elfmagic, sizeof (elfmagic)) == 0
> - || memcmp (buf, coffmagic, sizeof (coffmagic)) == 0
> - || memcmp (buf, coffmagic_x64, sizeof (coffmagic_x64)) == 0)
> + errmsg = simple_object_find_sections (inobj, has_lto_section,
> + (void *) &found, &err);
> + if (! errmsg && found)
> return true;
> - for (i = 0; i < 4; i++)
> - if (memcmp (buf, machomagic[i], sizeof (machomagic[i])) == 0)
> - return true;
>
> + if (errmsg)
> + fatal_error (0, "%s: %s\n", errmsg, xstrerror (err));
> return false;
> }
>
> @@ -2386,7 +2405,6 @@ scan_prog_file (const char *prog_name, scanpass
> which_pass,
> int err;
> char *p, buf[1024];
> FILE *inf;
> - int found_lto = 0;
>
> if (which_pass == PASS_SECOND)
> return;
> @@ -2394,8 +2412,13 @@ scan_prog_file (const char *prog_name, scanpass
> which_pass,
> /* LTO objects must be in a known format. This check prevents
> us from accepting an archive containing LTO objects, which
> gcc cannot currently handle. */
> - if (which_pass == PASS_LTOINFO && !maybe_lto_object_file (prog_name))
> - return;
> + if (which_pass == PASS_LTOINFO)
> + {
> + if(is_lto_object_file (prog_name)) {
> + add_lto_object (<o_objects, prog_name);
> + }
> + return;
> + }
>
> /* If we do not have an `nm', complain. */
> if (nm_file_name == 0)
> @@ -2450,12 +2473,7 @@ scan_prog_file (const char *prog_name, scanpass
> which_pass,
> fatal_error (input_location, "can't open nm output: %m");
>
> if (debug)
> - {
> - if (which_pass == PASS_LTOINFO)
> - fprintf (stderr, "\nnm output with LTO info marker symbol.\n");
> - else
> - fprintf (stderr, "\nnm output with constructors/destructors.\n");
> - }
> + fprintf (stderr, "\nnm output with constructors/destructors.\n");
>
> /* Read each line of nm output. */
> while (fgets (buf, sizeof buf, inf) != (char *) 0)
> @@ -2466,30 +2484,6 @@ scan_prog_file (const char *prog_name, scanpass
> which_pass,
> if (debug)
> fprintf (stderr, "\t%s\n", buf);
>
> - if (which_pass == PASS_LTOINFO)
> - {
> - if (found_lto)
> - continue;
> -
> - /* Look for the LTO info marker symbol, and add filename to
> - the LTO objects list if found. */
> - for (p = buf; (ch = *p) != '\0' && ch != '\n'; p++)
> - if (ch == ' ' && p[1] == '_' && p[2] == '_'
> - && (strncmp (p + (p[3] == '_' ? 2 : 1), "__gnu_lto_v1", 12)
> == 0)
> - && ISSPACE (p[p[3] == '_' ? 14 : 13]))
> - {
> - add_lto_object (<o_objects, prog_name);
> -
> - /* We need to read all the input, so we can't just
> - return here. But we can avoid useless work. */
> - found_lto = 1;
> -
> - break;
> - }
> -
> - continue;
> - }
> -
> /* If it contains a constructor or destructor name, add the name
> to the appropriate list unless this is a kind of symbol we're
> not supposed to even consider. */
> --
> 2.17.1
>
>
>
