[PATCH v2 3/3] gas: Add --compress-debug-sections={heuristic-always,heuristic-sizewin}

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

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=heuristic-always that ignores
the size heuristic, such that we have instead:
...
$ gcc hello.c -Wa,-gdwarf-5 -c -Xassembler \
    --compress-debug-sections=zstd,heuristic-always
$ 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
...

Also add an option --compress-debug-sections=heuristic-sizewin that represents
the current default.

Advertised as:
...
$ as --help
  ...
  --compress-debug-sections[={none|zlib|zlib-gnu|zlib-gabi|zstd|heuristic-always|heuristic-sizewin}]
                          compress DWARF debug sections
                            Default: zlib
...

Tested on x86_64-linux.
---
 gas/as.c    | 16 +++++++++++++++-
 gas/as.h    |  8 ++++++++
 gas/write.c |  7 +++++--
 3 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/gas/as.c b/gas/as.c
index c159c5d0d0a..a766f864707 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_heuristic compress_debug_heuristic = cdh_sizewin;
+
 static void
 show_usage (FILE * stream)
 {
@@ -252,7 +254,7 @@ 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|zlib|zlib-gnu|zlib-gabi|zstd|heuristic-sizewin|heuristic-always}]\n\
                           compress DWARF debug sections\n")),
   fprintf (stream, _("\
 		            Default: %s\n"),
@@ -423,6 +425,18 @@ parse_compress_debug_optarg_1 (const char *optarg)
 {
   gas_assert (optarg != NULL);
 
+  if (strcmp (optarg, "heuristic-sizewin") == 0)
+    {
+      compress_debug_heuristic = cdh_sizewin;
+      return;
+    }
+
+  if (strcmp (optarg, "heuristic-always") == 0)
+    {
+      compress_debug_heuristic = cdh_always;
+      return;
+    }
+
   enum compressed_debug_section_type tmp
     = bfd_get_compression_algorithm (optarg);
 
diff --git a/gas/as.h b/gas/as.h
index 4c5fa9ecf7d..cd852500197 100644
--- a/gas/as.h
+++ b/gas/as.h
@@ -331,6 +331,14 @@ COMMON int flag_traditional_format;
 /* Type of compressed debug sections we should generate.   */
 COMMON enum compressed_debug_section_type flag_compress_debug;
 
+/* Debug sections compression heuristics.   */
+enum compress_debug_heuristic
+{
+  cdh_sizewin,
+  cdh_always
+};
+COMMON enum compress_debug_heuristic compress_debug_heuristic;
+
 /* TRUE if .note.GNU-stack section with SEC_CODE should be created */
 COMMON int flag_execstack;
 
diff --git a/gas/write.c b/gas/write.c
index 8273b7a42f1..34ea5746284 100644
--- a/gas/write.c
+++ b/gas/write.c
@@ -1463,9 +1463,11 @@ 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 compress_debug_heuristic_sizewin
+    = compress_debug_heuristic == cdh_sizewin;
 
   if (seginfo == NULL
-      || uncompressed_size < 32
+      || (compress_debug_heuristic_sizewin && uncompressed_size < 32)
       || (flags & SEC_HAS_CONTENTS) == 0)
     return;
 
@@ -1582,7 +1584,8 @@ 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 (compress_debug_heuristic_sizewin
+      && compressed_size >= uncompressed_size)
     return;
 
   /* Replace the uncompressed frag list with the compressed frag list.  */
-- 
2.35.3