Re: [PATCH, driver specs][2] Put -flto-partition= on the collect2 c/l

[Iain Sandoe <iain at sandoe dot co dot uk>]

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.guenther@gmail.com> wrote:
> 
> On Wed, Aug 22, 2018 at 2:56 PM Iain Sandoe <iain@sandoe.co.uk> wrote:
>> 
>> 
>>> On 20 Aug 2018, at 11:01, Richard Biener <richard.guenther@gmail.com> wrote:
>>> 
>>> On Sat, Aug 18, 2018 at 9:00 PM Iain Sandoe <iain@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?

thanks
Iain


From: Iain Sandoe <iain@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 (&lto_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 (&lto_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