Using -frandom-seed=0 for reproducible builds?

[Sebastian Huber <sebastian.huber@embedded-brains.de>]

Hello,

I noticed that the coverage and profiling instrumentation needs the 
-frandom-seed flag to be reproducible. The documentation

https://gcc.gnu.org/onlinedocs/gcc/Developer-Options.html#index-frandom-seed

says:

     The string should be different for every file you compile.

Searching the internet suggests that using -frandom-seed=0 is not 
unusual. Why should the string be different for every file you compile? 
Or, what could happen if two files use the same random seed?

It seems the random seed is only used in two places:

gcc/tree.cc:#include "toplev.h" /* get_random_seed */
gcc/tree.cc:            crc32_string (0, name), get_random_seed (false));

/* Generate a name for a special-purpose function.
    The generated name may need to be unique across the whole link.
    Changes to this function may also require corresponding changes to
    xstrdup_mask_random.
    TYPE is some string to identify the purpose of this function to the
    linker or collect2; it must start with an uppercase letter,
    one of:
    I - for constructors
    D - for destructors
    N - for C++ anonymous namespaces
    F - for DWARF unwind frame information.  */

tree
get_file_function_name (const char *type)

gcc/lto-streamer.cc:    sprintf (post, "." HOST_WIDE_INT_PRINT_HEX_PURE, 
get_random_seed (false));

char *
lto_get_section_name (int section_type, const char *name,
		      int node_order, struct lto_file_decl_data *f)
{
[...]
   /* Make the section name unique so that ld -r combining sections
      doesn't confuse the reader with merged sections.

      For options don't add a ID, the option reader cannot deal with them
      and merging should be ok here. */

I am not sure which bad things happen if some items are not unique 
across the entire link.

For the coverage profiling, the -frandom-seed flag just results in 
"local_tick" being -1. The "local_tick" is only used to initialize the 
random seed and in coverage.cc for a file stamp for notes file.

Would it make sense to add a new option to just control "local_tick" for 
reproducible coverage instrumentation and don't touch the random seed stuff?

-- 
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