Question about reading LTO function summaries

[Erick Ochoa]

Hello,

just trying again to increase visibility of this question. Many thanks
in advance!


On Fri, 26 Mar 2021 at 13:49, Erick Ochoa <eochoa@gcc.gnu.org> wrote:
>
> Hello,
>
> I already have some experience developing SIMPLE_IPA_PASSes, but I am
> looking to understand IPA_PASSes better. I have made a hello world ipa
> pass that stores "hello world $FUNCTION_NAME" in the function
> summaries; however, I am having trouble reading this information back.
> Can someone help me understand how to use these interfaces correctly?
>
> At the moment, it **seems** to be writing information correctly.
> (I.e., it doesn't segfault when attempting to write data.) However, in
> my read summary function (ipa_hello_world_read_summary (void)) the
> function `lto_get_summary_section_data (file_data,
> LTO_section_ipa_hello_world, &len);` always returns NULL and
> `file_data_vec` is of size 1. This means that at run time, there is
> only one call to `lto_get_summary_section_data` and it returns NULL.
>
> I am copy-pasting the relevant code below.
>
> /* Map from cgraph_node* to "hello world $FUNCTION_NAME". */
> static hash_map<cgraph_node*, char*> *hello_summaries;
>
> static void
> ipa_hello_world_write_summary (void)
> {
>   gcc_assert(hello_summaries);
>   struct output_block *ob = create_output_block (LTO_section_ipa_hello_world);
>   gcc_assert(ob);
>   if (dump_file) fprintf(dump_file, "hello world from write summary\n");
>   lto_symtab_encoder_t encoder = ob->decl_state->symtab_node_encoder;
>   if (dump_file) fprintf(dump_file, "writing %d\n",
> hello_summaries->elements());
>   streamer_write_uhwi (ob, hello_summaries->elements());
>
>   for (auto i = hello_summaries->begin(), e = hello_summaries->end();
> i != e; ++i)
>   {
>     if (dump_file) fprintf(dump_file, "writing %s\n", (*i).second);
>     streamer_write_uhwi(ob, lto_symtab_encoder_encode(encoder, (*i).first));
>     streamer_write_string (ob, ob->main_stream, (*i).second, true);
>   }
>   produce_asm (ob, NULL);
>   destroy_output_block (ob);
>   delete hello_summaries;
> }
>
> static void
> ipa_hello_world_read_summary (void)
> {
>   struct lto_file_decl_data **file_data_vec = lto_get_file_decl_data ();
>   struct lto_file_decl_data *file_data;
>   unsigned int j = 0;
>   if (dump_file) fprintf(dump_file, "hello from read summary\n");
>   while ((file_data = file_data_vec[j++]))
>   {
>     if (dump_file) fprintf(dump_file, "iteration = %d\n", j);
>     size_t len;
>     const char *data =
>       lto_get_summary_section_data (file_data,
> LTO_section_ipa_hello_world, &len);
>     if (!data) continue;
>
>     const struct lto_function_header *header = (const struct
> lto_function_header*) data;
>     gcc_assert(header);
>     gcc_assert(header->cfg_size);
>     const int cfg_offset = sizeof (struct lto_function_header);
>     const int main_offset = cfg_offset + header->cfg_size;
>     const int string_offset = main_offset + header->main_size;
>     class data_in *data_in;
>
>     lto_input_block ib ((const char *) data + main_offset, header->main_size,
>                       file_data->mode_table);
>     data_in
>       = lto_data_in_create (file_data, (const char *) data + string_offset,
>                           header->string_size, vNULL);
>     unsigned int n = streamer_read_uhwi (&ib);
>     //hello_summaries = new hash_map<cgraph_node*, char*>;
>     for (unsigned i = 0; i < n; i++)
>     {
>       unsigned int index = streamer_read_uhwi(&ib);
>       lto_symtab_encoder_t encoder = file_data->symtab_node_encoder;
>       struct cgraph_node *cnode = dyn_cast<cgraph_node *>
> (lto_symtab_encoder_deref(encoder, index));
>       gcc_assert(cnode);
>       const char* string = streamer_read_string (data_in, &ib);
>       if (dump_file) fprintf(dump_file, string);
>     }
>   }
>
> }