[PATCH] gas: Add --compress-debug-sections=force

[Tom de Vries <tdevries@suse.de>]

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[ was: Re: [PATCH] gas: Add --force-compress-debug-sections ]

On 2/27/23 10:03, Jan Beulich wrote:
> On 24.02.2023 15:57, Tom de Vries wrote:
>> On 2/24/23 15:26, Jan Beulich wrote:
>>> On 24.02.2023 15:11, Tom de Vries wrote:
>>>> On 2/24/23 14:23, Jan Beulich wrote:
>>>>> On 24.02.2023 13:21, Tom de Vries wrote:
>>>>>> On 2/24/23 12:28, Jan Beulich wrote:
>>>>>>> I also wouldn't see anything wrong with something
>>>>>>> like "...=force,zstd,none" - the last one(s) win. That's no different
>>>>>>> from specifying a second instance of the option. And without that it
>>>>>>> looks as if the parsing would end up simpler.
>>>>>>
>>>>>> OK, gave that a try.
>>>>>
>>>>> That's still accumulating none and force across the entire sequence
>>>>> (and then giving none priority over force, no matter that force may
>>>>> have been specified last),
>>>>
>>>> Um, so you're saying that none+zstd+force is currently interpreted as none?
>>>>
>>>> Lets try:
>>>> ...
>>>> $ gcc ~/hello.c -c -Wa,-gdwarf-5 -Xassembler
>>>> --compress-debug-sections=none+zstd+force
>>>> $ readelf -S -W hello.o | grep " .debug"
>>>>      [ 9] .debug_line       PROGBITS        0000a8 000064 00   C  0   0  8
>>>>      [11] .debug_line_str   PROGBITS        000110 000046 01 MSC  0   0  8
>>>>      [12] .debug_info       PROGBITS        000158 000046 00   C  0   0  8
>>>>      [14] .debug_abbrev     PROGBITS        0001a0 000049 00   C  0   0  8
>>>>      [15] .debug_aranges    PROGBITS        0001f0 000034 00   C  0   0  8
>>>>      [17] .debug_str        PROGBITS        000228 00005a 01 MSC  0   0  8
>>>>
>>>> ...
>>>>
>>>> So, that doesn't seem to be the case, compression is done, as expected.
>>>
>>> Oh, I've overlooked that you explicitly clear *none when you set *force
>>> (my attention was mainly on the bottom of parse_compress_debug_optarg()).
>>> I think that's more involved than necessary (possibly merely a result of
>>> you having worked incrementally from your earlier version), and less
>>> obviously doing the same as would happen when multiple separate options
>>> were parsed.
>>
>> I've tried to simplify further.
>>
>> Is this more how you want it?
> 
> I have to admit that I'm still puzzled by the presence of
> finalize_parse_compress_debug_optarg() as well as you needing both a new
> static variable and a new global one.

The latter I've fixed, by exporting the static variable.

That eliminates one half of finalize_parse_compress_debug_optarg().

The remaining half is necessary because we accumulate "none" into 
compress_debug_action instead of into flag_compress_debug, so after 
parsing is over we need to assign the accumulation result, if it is 
indeed cda_none, to flag_compress_debug.

And the reason we have that complicated setup is related to preventing 
none from zapping compression type.

> But I guess whether that's really
> needed first of all depends on the semantics we want e.g.
> 
> --nocompress-debug-sections --compress-debug-sections=force
> 
> to have (which, with how you have it presently, could also be expressed
> as
> 
> --compress-debug-sections=none+force
> 
> or
> 
> --compress-debug-sections=none --compress-debug-sections=force
> 
> afaict). I view the present meaning as one sensible one, but I could
> also see "none" (or equivalent) simply zapping the compression type
> (and hence rendering "force" meaningless) as another sensible one.

Sure, that's another possible semantics.

Should I implement that instead?

> A
> change in meaning may then also result in the three option combinations
> above possibly not all doing the same.
> 

> As an aside: As you update the patch, please try to keep the title in
> line with what the patch actually does.
> 

I'm assuming you mean the email title (the patch title looks ok to me), 
and I've updated it.

> Also, ftaod, I don't mean to stand in the way of another maintainer
> approving any of the forms proposed so far. This specifically also
> includes the use of '+' as a separator, which I personally don't
> (currently) intend to approve.

Well, in case that's blocking you from giving approval, I've updated the 
patch to use the ',' separator.

Thanks,
- Tom

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From 0fb4ed6d648a69799cdbf6b19fd36c6125c5fc41 Mon Sep 17 00:00:00 2001
From: Tom de Vries <tdevries@suse.de>
Date: Thu, 23 Feb 2023 12:53:40 +0100
Subject: [PATCH] gas: Add --compress-debug-sections=force

Gas has an option --compress-debug-sections that allows it to generate
compressed debug sections.

That does not guarantee that the debug sections are in fact compressed:
...
$ gcc hello.c -Wa,-gdwarf-5 -c -Wa,--compress-debug-sections=zstd
$ readelf -S -W hello.o | grep " .debug"
  [ 9] .debug_line       PROGBITS         0000a8 000053 00      0   0  1
  [11] .debug_line_str   PROGBITS         0000fb 000025 01  MS  0   0  1
  [12] .debug_info       PROGBITS         000120 000039 00      0   0  1
  [14] .debug_abbrev     PROGBITS         000159 000028 00      0   0  1
  [15] .debug_aranges    PROGBITS         000190 000030 00      0   0 16
  [17] .debug_str        PROGBITS         0001c0 000039 01  MS  0   0  1
...

Sensibly so, they're only compressed if that provides a size benefit.

However, for the purpose of testing components consuming dwarf
we may want the sections to be compressed regardless.

Add a new suboption --compress-debug-sections=force that ignores the size
heuristic, such that we have instead:
...
$ gcc hello.c -Wa,-gdwarf-5 -c -Xassembler \
    --compress-debug-sections=zstd,force
$ readelf -S -W hello.o | grep " .debug"
  [ 9] .debug_line       PROGBITS         0000a8 000064 00   C  0   0  8
  [11] .debug_line_str   PROGBITS         000110 000046 01 MSC  0   0  8
  [12] .debug_info       PROGBITS         000158 000046 00   C  0   0  8
  [14] .debug_abbrev     PROGBITS         0001a0 000049 00   C  0   0  8
  [15] .debug_aranges    PROGBITS         0001f0 000034 00   C  0   0  8
  [17] .debug_str        PROGBITS         000228 00005a 01 MSC  0   0  8
...

Advertised as:
...
$ as --help
  ...
  --compress-debug-sections[={none|<format>|force|force,<format>}]
                          where <format> is {zlib|zlib-gnu|zlib-gabi|zstd}
                          compress DWARF debug sections
                            Default: zstd
...

Tested on x86_64-linux.
---
 gas/as.c        | 106 +++++++++++++++++++++++++++++++++++++++---------
 gas/as.h        |  10 +++++
 gas/doc/as.texi |  10 ++++-
 gas/write.c     |   5 ++-
 4 files changed, 108 insertions(+), 23 deletions(-)

diff --git a/gas/as.c b/gas/as.c
index 598bfd56cf5..acbccfb5dce 100644
--- a/gas/as.c
+++ b/gas/as.c
@@ -230,6 +230,8 @@ enum compressed_debug_section_type flag_compress_debug
   = DEFAULT_COMPRESSED_DEBUG_ALGORITHM;
 #endif
 
+enum compress_debug_action compress_debug_action = cda_default;
+
 static void
 show_usage (FILE * stream)
 {
@@ -252,7 +254,8 @@ Options:\n\
   fprintf (stream, _("\
   --alternate             initially turn on alternate macro syntax\n"));
   fprintf (stream, _("\
-  --compress-debug-sections[={none|zlib|zlib-gnu|zlib-gabi|zstd}]\n\
+  --compress-debug-sections[={none|<format>|force|force,<format>}]\n\
+                          where <format> is {zlib|zlib-gnu|zlib-gabi|zstd}\n\
                           compress DWARF debug sections\n")),
   fprintf (stream, _("\
 		            Default: %s\n"),
@@ -418,6 +421,84 @@ Options:\n\
     fprintf (stream, _("Report bugs to %s\n"), REPORT_BUGS_TO);
 }
 
+static void
+parse_compress_debug_optarg_1 (const char *optarg)
+{
+  gas_assert (optarg != NULL);
+
+  if (strcmp (optarg, "force") == 0)
+    {
+      compress_debug_action = cda_force;
+      return;
+    }
+
+  enum compressed_debug_section_type tmp
+    = bfd_get_compression_algorithm (optarg);
+
+#ifndef HAVE_ZSTD
+  if (tmp == COMPRESS_DEBUG_ZSTD)
+    as_fatal (_ ("--compress-debug-sections=zstd: gas is not "
+		 "built with zstd support"));
+#endif
+
+  if (tmp == COMPRESS_UNKNOWN)
+    as_fatal (_("Invalid --compress-debug-sections option: `%s'"),
+	      optarg);
+
+  if (tmp == COMPRESS_DEBUG_NONE)
+    {
+      compress_debug_action = cda_none;
+      return;
+    }
+
+  compress_debug_action = cda_yes;
+  flag_compress_debug = tmp;
+}
+
+static void
+parse_compress_debug_optarg (const char *optarg)
+{
+#if !defined OBJ_ELF && !defined OBJ_MAYBE_ELF
+  as_fatal (_("--compress-debug-sections=%s is unsupported"),
+	    optarg);
+#endif
+
+  /* Tokenize subopts pass to parse_compress_debug_optarg_1.  */
+  char sep = ',';
+  while (true)
+    {
+      const char *idx = optarg;
+      while (*idx != '\0' && *idx != sep)
+	idx++;
+
+      size_t len = idx - optarg;
+      if (len == 0)
+	{
+	  /* Generate error.  */
+	  parse_compress_debug_optarg_1 ("");
+	  break;
+	}
+
+      char *tmp = xstrndup (optarg, len);
+      parse_compress_debug_optarg_1 (tmp);
+      free (tmp);
+
+      if (*idx == '\0')
+	break;
+
+      /* Step over separator and continue tokenizing.  */
+      gas_assert (*idx == sep);
+      optarg = idx + 1;
+  }
+}
+
+static void
+finalize_parse_compress_debug_optarg (void)
+{
+  if (compress_debug_action == cda_none)
+    flag_compress_debug = COMPRESS_DEBUG_NONE;
+}
+
 /* Since it is easy to do here we interpret the special arg "-"
    to mean "use stdin" and we set that argv[] pointing to "".
    After we have munged argv[], the only things left are source file
@@ -747,28 +828,13 @@ This program has absolutely no warranty.\n"));
 
 	case OPTION_COMPRESS_DEBUG:
 	  if (optarg)
-	    {
-#if defined OBJ_ELF || defined OBJ_MAYBE_ELF
-	      flag_compress_debug = bfd_get_compression_algorithm (optarg);
-#ifndef HAVE_ZSTD
-	      if (flag_compress_debug == COMPRESS_DEBUG_ZSTD)
-		  as_fatal (_ ("--compress-debug-sections=zstd: gas is not "
-			       "built with zstd support"));
-#endif
-	      if (flag_compress_debug == COMPRESS_UNKNOWN)
-		as_fatal (_("Invalid --compress-debug-sections option: `%s'"),
-			  optarg);
-#else
-	      as_fatal (_("--compress-debug-sections=%s is unsupported"),
-			optarg);
-#endif
-	    }
+	    parse_compress_debug_optarg (optarg);
 	  else
-	    flag_compress_debug = COMPRESS_DEBUG_GABI_ZLIB;
+	    parse_compress_debug_optarg ("zlib-gabi");
 	  break;
 
 	case OPTION_NOCOMPRESS_DEBUG:
-	  flag_compress_debug = COMPRESS_DEBUG_NONE;
+	  parse_compress_debug_optarg ("none");
 	  break;
 
 	case OPTION_DEBUG_PREFIX_MAP:
@@ -1136,6 +1202,8 @@ This program has absolutely no warranty.\n"));
   *pargc = new_argc;
   *pargv = new_argv;
 
+  finalize_parse_compress_debug_optarg ();
+
 #ifdef md_after_parse_args
   md_after_parse_args ();
 #endif
diff --git a/gas/as.h b/gas/as.h
index 4c5fa9ecf7d..bc0fe7f5cd9 100644
--- a/gas/as.h
+++ b/gas/as.h
@@ -331,6 +331,16 @@ COMMON int flag_traditional_format;
 /* Type of compressed debug sections we should generate.   */
 COMMON enum compressed_debug_section_type flag_compress_debug;
 
+/* Whether we should compress debug sections.   */
+enum compress_debug_action
+{
+  cda_default,
+  cda_none,
+  cda_force,
+  cda_yes,
+};
+COMMON enum compress_debug_action compress_debug_action;
+
 /* TRUE if .note.GNU-stack section with SEC_CODE should be created */
 COMMON int flag_execstack;
 
diff --git a/gas/doc/as.texi b/gas/doc/as.texi
index bbdfa4bfdca..f0140c09779 100644
--- a/gas/doc/as.texi
+++ b/gas/doc/as.texi
@@ -718,7 +718,8 @@ Begin in alternate macro mode.
 Compress DWARF debug sections using zlib with SHF_COMPRESSED from the
 ELF ABI.  The resulting object file may not be compatible with older
 linkers and object file utilities.  Note if compression would make a
-given section @emph{larger} then it is not compressed.
+given section @emph{larger} then it is not compressed, unless
+@option{--compress-debug-section=force} is used.
 
 @ifset ELF
 @cindex @samp{--compress-debug-sections=} option
@@ -727,6 +728,7 @@ given section @emph{larger} then it is not compressed.
 @itemx --compress-debug-sections=zlib-gnu
 @itemx --compress-debug-sections=zlib-gabi
 @itemx --compress-debug-sections=zstd
+@itemx --compress-debug-sections=force
 These options control how DWARF debug sections are compressed.
 @option{--compress-debug-sections=none} is equivalent to
 @option{--nocompress-debug-sections}.
@@ -738,7 +740,11 @@ using the obsoleted zlib-gnu format.  The debug sections are renamed to begin
 with @samp{.zdebug}.
 @option{--compress-debug-sections=zstd} compresses DWARF debug
 sections using zstd.  Note - if compression would actually make a section
-@emph{larger}, then it is not compressed nor renamed.
+@emph{larger}, then it is not compressed nor renamed, unless
+@option{--compress-debug-section=force} is used.
+@option{--compress-debug-sections=force} compresses DWARF debug sections,
+even if this does not reduce size.  It can be used in conjunction with a format
+selection, for instance @option{--compress-debug-section=zstd,force}.
 
 @end ifset
 
diff --git a/gas/write.c b/gas/write.c
index 8273b7a42f1..1fa3b54a03b 100644
--- a/gas/write.c
+++ b/gas/write.c
@@ -1463,9 +1463,10 @@ compress_debug (bfd *abfd, asection *sec, void *xxx ATTRIBUTE_UNUSED)
   segment_info_type *seginfo = seg_info (sec);
   bfd_size_type uncompressed_size = sec->size;
   flagword flags = bfd_section_flags (sec);
+  bool force_compress_debug = compress_debug_action == cda_force;
 
   if (seginfo == NULL
-      || uncompressed_size < 32
+      || (!force_compress_debug && uncompressed_size < 32)
       || (flags & SEC_HAS_CONTENTS) == 0)
     return;
 
@@ -1582,7 +1583,7 @@ compress_debug (bfd *abfd, asection *sec, void *xxx ATTRIBUTE_UNUSED)
 
   /* PR binutils/18087: If compression didn't make the section smaller,
      just keep it uncompressed.  */
-  if (compressed_size >= uncompressed_size)
+  if (!force_compress_debug && compressed_size >= uncompressed_size)
     return;
 
   /* Replace the uncompressed frag list with the compressed frag list.  */
-- 
2.35.3