Re: [PATCH] compress .gnu.debuglto_.debug_* sections if requested

["Martin Liška" <mliska@suse.cz>]

[-- Attachment #1: Type: text/plain, Size: 1649 bytes --]

On 9/30/22 02:20, Alan Modra wrote:
> On Thu, Sep 29, 2022 at 03:35:15PM +0200, Martin Liška wrote:
>> +  /* Compress/decompress DWARF debug sections with names: .debug_*,
>> +     .zdebug_*, .gnu.debuglto_.debug_, after the section flags is set.  */
>>    if ((newsect->flags & SEC_DEBUGGING) != 0
>>        && (newsect->flags & SEC_HAS_CONTENTS) != 0
>> -      && ((name[1] == 'd' && name[6] == '_')
>> -	  || (name[1] == 'z' && name[7] == '_')))
>> +      && (startswith (name, ".debug_")
>> +	  || startswith (name, ".zdebug_")
>> +	  || startswith (name, ".gnu.debuglto_.debug_")))
> 
> If you test "(newsect->flags & SEC_ELF_OCTETS) != 0" as well, I think
> you could omit any name tests here.  This would also compress
> .gnu.linkonce.wi.* sections.  (Not that they matter very much, I'm
> just pointing out a code simplification.)

Yep, I like the code simplification!

> 
>> --- a/gas/write.c
>> +++ b/gas/write.c
>> @@ -1481,7 +1481,8 @@ compress_debug (bfd *abfd, asection *sec, void *xxx ATTRIBUTE_UNUSED)
>>      return;
>>  
>>    section_name = bfd_section_name (sec);
>> -  if (!startswith (section_name, ".debug_"))
>> +  if (!startswith (section_name, ".debug_")
>> +      && !startswith (section_name, ".gnu.debuglto_.debug_"))
>>      return;
>>  
>>    bool use_zstd = abfd->flags & BFD_COMPRESS_ZSTD;
> 
> Hmm, it looks to me like this function will transform
> ".gnu.debuglto_.debug_*" to ".zgnu.debuglto_.debug_*" if
> --compress-debug-sections=zlib-gnu is used, which will break your
> elf.c change.  Please check --compress-debug-sections=zlib-gnu.

Again, great comment, fixed in v2.

Ready for master now?
Thanks,
Martin

[-- Attachment #2: 0001-compress-.gnu.debuglto_.debug_-sections-if-requested.patch --]
[-- Type: text/x-patch, Size: 2750 bytes --]

From 8cff32766302af863aa1ddf9ae30a18f7c801eb5 Mon Sep 17 00:00:00 2001
From: Martin Liska <mliska@suse.cz>
Date: Thu, 29 Sep 2022 14:10:30 +0200
Subject: [PATCH] compress .gnu.debuglto_.debug_* sections if requested

Right now, when using LTO, the intermediate object files do contain
debug info in sections starting with .gnu.debuglto_ prefix and are
not compressed when --compress-debug-sections is used.

It's a mistake and we can save quite some disk space. The following
example comes from tramp3d when the corresponding LTO sections
are compressed with zlib:

$ bloaty tramp3d-v4-v2.o -- tramp3d-v4.o
    FILE SIZE        VM SIZE
 --------------  --------------
   +83%     +10  [ = ]       0    [Unmapped]
 -68.0%    -441  [ = ]       0    .gnu.debuglto_.debug_line
 -52.3%    -759  [ = ]       0    .gnu.debuglto_.debug_line_str
 -62.4% -3.24Ki  [ = ]       0    .gnu.debuglto_.debug_abbrev
 -64.8% -1.12Mi  [ = ]       0    .gnu.debuglto_.debug_info
 -88.8% -4.58Mi  [ = ]       0    .gnu.debuglto_.debug_str
 -27.7% -5.70Mi  [ = ]       0    TOTAL

bfd/ChangeLog:

	* elf.c (_bfd_elf_make_section_from_shdr): Compress all debug
	  info sections.

gas/ChangeLog:

	* write.c (compress_debug): Compress also ".gnu.debuglto_.debug_"
	if the compression algorithm is different from zlib-gnu.
---
 bfd/elf.c   | 7 +++----
 gas/write.c | 4 +++-
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/bfd/elf.c b/bfd/elf.c
index d496c1b2983..8cd257fc65c 100644
--- a/bfd/elf.c
+++ b/bfd/elf.c
@@ -1199,12 +1199,11 @@ _bfd_elf_make_section_from_shdr (bfd *abfd,
 	}
     }
 
-  /* Compress/decompress DWARF debug sections with names: .debug_* and
-     .zdebug_*, after the section flags is set.  */
+  /* Compress/decompress DWARF debug sections with names: .debug_*,
+     .zdebug_*, .gnu.debuglto_.debug_, after the section flags is set.  */
   if ((newsect->flags & SEC_DEBUGGING) != 0
       && (newsect->flags & SEC_HAS_CONTENTS) != 0
-      && ((name[1] == 'd' && name[6] == '_')
-	  || (name[1] == 'z' && name[7] == '_')))
+      && (newsect->flags & SEC_ELF_OCTETS) != 0)
     {
       enum { nothing, compress, decompress } action = nothing;
       int compression_header_size;
diff --git a/gas/write.c b/gas/write.c
index 0e49df7c03f..b8d5253006c 100644
--- a/gas/write.c
+++ b/gas/write.c
@@ -1481,7 +1481,9 @@ compress_debug (bfd *abfd, asection *sec, void *xxx ATTRIBUTE_UNUSED)
     return;
 
   section_name = bfd_section_name (sec);
-  if (!startswith (section_name, ".debug_"))
+  if (!startswith (section_name, ".debug_")
+      && (!startswith (section_name, ".gnu.debuglto_.debug_")
+	  || flag_compress_debug == COMPRESS_DEBUG_GNU_ZLIB))
     return;
 
   bool use_zstd = abfd->flags & BFD_COMPRESS_ZSTD;
-- 
2.37.3