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[PATCH] Add .gnu.lto_.lto section.


Hi.

So this is a patch candidate for the .gnu.lto_.lto ELF section.
As mentioned, the section contains information about bytecode version,
compression algorithm used and slim LTO object flag.

One minor issues is that I need to append random suffix to the section:
  [ 6] .gnu.lto_.lto.36e74acfb145b04b PROGBITS        0000000000000000 000086 000008 00   E  0   0  1

That's because of in WPA we load more object files and the hash is used for mapping.

Is the idea of the patch right?
Martin
>From ec550463df26855982b7b70393766cc8e7c6cc8e Mon Sep 17 00:00:00 2001
From: Martin Liska <mliska@suse.cz>
Date: Fri, 21 Jun 2019 12:14:04 +0200
Subject: [PATCH] Add .gnu.lto_.lto section.

---
 gcc/lto-section-in.c   |  9 +++------
 gcc/lto-section-out.c  |  2 --
 gcc/lto-streamer-out.c | 40 +++++++++++++++++++++++++---------------
 gcc/lto-streamer.h     | 25 +++++++++++++++++++++----
 gcc/lto/lto-common.c   | 15 +++++++++++++++
 5 files changed, 64 insertions(+), 27 deletions(-)

diff --git a/gcc/lto-section-in.c b/gcc/lto-section-in.c
index 4cfc0cad4be..4e7d1181f23 100644
--- a/gcc/lto-section-in.c
+++ b/gcc/lto-section-in.c
@@ -52,10 +52,10 @@ const char *lto_section_name[LTO_N_SECTION_TYPES] =
   "icf",
   "offload_table",
   "mode_table",
-  "hsa"
+  "hsa",
+  "lto"
 };
 
-
 /* Hooks so that the ipa passes can call into the lto front end to get
    sections.  */
 
@@ -146,7 +146,7 @@ lto_get_section_data (struct lto_file_decl_data *file_data,
   /* WPA->ltrans streams are not compressed with exception of function bodies
      and variable initializers that has been verbatim copied from earlier
      compilations.  */
-  if (!flag_ltrans || decompress)
+  if ((!flag_ltrans || decompress) && section_type != LTO_section_lto)
     {
       /* Create a mapping header containing the underlying data and length,
 	 and prepend this to the uncompression buffer.  The uncompressed data
@@ -167,9 +167,6 @@ lto_get_section_data (struct lto_file_decl_data *file_data,
       data = buffer.data + header_length;
     }
 
-  lto_check_version (((const lto_header *)data)->major_version,
-		     ((const lto_header *)data)->minor_version,
-		     file_data->file_name);
   return data;
 }
 
diff --git a/gcc/lto-section-out.c b/gcc/lto-section-out.c
index c91e58f0465..7ae102164ef 100644
--- a/gcc/lto-section-out.c
+++ b/gcc/lto-section-out.c
@@ -285,8 +285,6 @@ lto_destroy_simple_output_block (struct lto_simple_output_block *ob)
   /* Write the header which says how to decode the pieces of the
      t.  */
   memset (&header, 0, sizeof (struct lto_simple_header));
-  header.major_version = LTO_major_version;
-  header.minor_version = LTO_minor_version;
   header.main_size = ob->main_stream->total_size;
   lto_write_data (&header, sizeof header);
 
diff --git a/gcc/lto-streamer-out.c b/gcc/lto-streamer-out.c
index dc68429303c..7dee770aa11 100644
--- a/gcc/lto-streamer-out.c
+++ b/gcc/lto-streamer-out.c
@@ -1974,10 +1974,6 @@ produce_asm (struct output_block *ob, tree fn)
   /* The entire header is stream computed here.  */
   memset (&header, 0, sizeof (struct lto_function_header));
 
-  /* Write the header.  */
-  header.major_version = LTO_major_version;
-  header.minor_version = LTO_minor_version;
-
   if (section_type == LTO_section_function_body)
     header.cfg_size = ob->cfg_stream->total_size;
   header.main_size = ob->main_stream->total_size;
@@ -2270,10 +2266,6 @@ lto_output_toplevel_asms (void)
   /* The entire header stream is computed here.  */
   memset (&header, 0, sizeof (header));
 
-  /* Write the header.  */
-  header.major_version = LTO_major_version;
-  header.minor_version = LTO_minor_version;
-
   header.main_size = ob->main_stream->total_size;
   header.string_size = ob->string_stream->total_size;
   lto_write_data (&header, sizeof header);
@@ -2390,6 +2382,29 @@ prune_offload_funcs (void)
     DECL_PRESERVE_P (fn_decl) = 1;
 }
 
+/* Produce LTO section that contains global information
+   about LTO bytecode.  */
+
+static void
+produce_lto_section ()
+{
+  /* Stream LTO meta section.  */
+  output_block *ob = create_output_block (LTO_section_lto);
+
+  char * section_name = lto_get_section_name (LTO_section_lto, NULL, NULL);
+  lto_begin_section (section_name, false);
+  free (section_name);
+
+  lto_compression compression = ZLIB;
+
+  bool slim_object = flag_generate_lto && !flag_fat_lto_objects;
+  lto_section s
+    = { LTO_major_version, LTO_minor_version, slim_object, compression, 0 };
+  lto_write_data (&s, sizeof s);
+  lto_end_section ();
+  destroy_output_block (ob);
+}
+
 /* Main entry point from the pass manager.  */
 
 void
@@ -2412,6 +2427,8 @@ lto_output (void)
   /* Initialize the streamer.  */
   lto_streamer_init ();
 
+  produce_lto_section ();
+
   n_nodes = lto_symtab_encoder_size (encoder);
   /* Process only the functions with bodies.  */
   for (i = 0; i < n_nodes; i++)
@@ -2827,10 +2844,6 @@ lto_write_mode_table (void)
   struct lto_simple_header_with_strings header;
   memset (&header, 0, sizeof (header));
 
-  /* Write the header.  */
-  header.major_version = LTO_major_version;
-  header.minor_version = LTO_minor_version;
-
   header.main_size = ob->main_stream->total_size;
   header.string_size = ob->string_stream->total_size;
   lto_write_data (&header, sizeof header);
@@ -2901,9 +2914,6 @@ produce_asm_for_decls (void)
       lto_output_decl_state_streams (ob, fn_out_state);
     }
 
-  header.major_version = LTO_major_version;
-  header.minor_version = LTO_minor_version;
-
   /* Currently not used.  This field would allow us to preallocate
      the globals vector, so that it need not be resized as it is extended.  */
   header.num_nodes = -1;
diff --git a/gcc/lto-streamer.h b/gcc/lto-streamer.h
index 3b1e07b4bfc..2f481aebeb2 100644
--- a/gcc/lto-streamer.h
+++ b/gcc/lto-streamer.h
@@ -234,6 +234,7 @@ enum lto_section_type
   LTO_section_offload_table,
   LTO_section_mode_table,
   LTO_section_ipa_hsa,
+  LTO_section_lto,
   LTO_N_SECTION_TYPES		/* Must be last.  */
 };
 
@@ -378,18 +379,31 @@ public:
   unsigned int len;
 };
 
+/* Compression algorithm used for compression of LTO bytecode.  */
 
-/* The is the first part of the record for a function or constructor
-   in the .o file.  */
-struct lto_header
+enum lto_compression
+{
+  ZLIB,
+  ZSTD
+};
+
+/* Structure that represents LTO ELF section with information
+   about the format.  */
+
+struct lto_section
 {
   int16_t major_version;
   int16_t minor_version;
+  unsigned char slim_object: 1;
+  lto_compression compression: 4;
+  int32_t reserved0: 27;
 };
 
+STATIC_ASSERT (sizeof (lto_section) == 8);
+
 /* The is the first part of the record in an LTO file for many of the
    IPA passes.  */
-struct lto_simple_header : lto_header
+struct lto_simple_header
 {
   /* Size of main gimple body of function.  */
   int32_t main_size;
@@ -589,6 +603,9 @@ struct GTY(()) lto_file_decl_data
 
   /* Mode translation table.  */
   const unsigned char *mode_table;
+
+  /* Read LTO section. */
+  lto_section lto_section_header;
 };
 
 typedef struct lto_file_decl_data *lto_file_decl_data_ptr;
diff --git a/gcc/lto/lto-common.c b/gcc/lto/lto-common.c
index 0d38ee6b57b..1cfbda0f554 100644
--- a/gcc/lto/lto-common.c
+++ b/gcc/lto/lto-common.c
@@ -2060,6 +2060,21 @@ lto_file_finalize (struct lto_file_decl_data *file_data, lto_file *file)
 #else
   file_data->mode_table = lto_mode_identity_table;
 #endif
+
+  /* Read and verify LTO section.  */
+  data = lto_get_section_data (file_data, LTO_section_lto, NULL, &len, false);
+  if (data == NULL)
+    {
+      fatal_error (input_location, "bytecode stream in file %qs generated "
+		   "with GCC compiler older than 10.0", file_data->file_name);
+      return;
+    }
+
+  file_data->lto_section_header = *(const lto_section *)data;
+  lto_check_version (file_data->lto_section_header.major_version,
+		     file_data->lto_section_header.minor_version,
+		     file_data->file_name);
+
   data = lto_get_section_data (file_data, LTO_section_decls, NULL, &len);
   if (data == NULL)
     {
-- 
2.21.0


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