Hello,

This patch fixes several memory leaks in the driver, all of which relate
to the handling of static specs. We introduce functions
set_static_spec_{shared,owned}() which are used to enforce proper memory
management when updating the strings in the static_specs table.

This is achieved by making use of the alloc_p field in the table
entries. Similarly to set_spec(), each time we update an entry, we check
whether alloc_p is set, and free the old value if so. We then set
alloc_p correctly based on whether we "own" this memory or whether we're
just taking a pointer to a shared string which we shouldn't free.

The following table shows the number of leaks found by AddressSanitizer
when running a minimal libgccjit program on AArch64. The test program
does the whole libgccjit compilation cycle in a loop (including acquiring
and releasing the context), and the table below shows the number of leaks
for different iterations of that loop.

+--------------+-----+-----+------+---------------+
| # of runs >  | 1   | 2   | 3    | Leaks per run |
+--------------+-----+-----+------+---------------+
| Before patch | 463 | 940 | 1417 | 477           |
+--------------+-----+-----+------+---------------+
| After patch  | 416 | 846 | 1276 | 430           |
+--------------+-----+-----+------+---------------+

Ensuring that we minimize "leaks per run" (ultimately eliminating all of
them) is important in order for long-running applications to be able to
make use of in-process libgccjit.

Testing:
 * Bootstrap and regtest on aarch64-linxu-gnu, x86_64-linux-gnu.
 * Bootstrap and regtest on aarch64-linux-gnu with bootstrap-asan config.
 * Smoke test of libgccjit, ran regressions on a --disable-bootstrap build on
   aarch64-linux-gnu.

OK for master?

Thanks,
Alex

---

gcc/ChangeLog:

2020-07-09  Alex Coplan  <alex.coplan@arm.com>

	* gcc.c (set_static_spec): New.
	(set_static_spec_owned): New.
	(set_static_spec_shared): New.
	(driver::maybe_putenv_COLLECT_LTO_WRAPPER): Use
	set_static_spec_owned() to take ownership of lto_wrapper_file
	such that it gets freed in driver::finalize.
	(driver::maybe_run_linker): Use set_static_spec_shared() to
	ensure that we don't try and free() the static string "ld",
	also ensuring that any previously-allocated string in
	linker_name_spec is freed. Likewise with argv0.
	(driver::finalize): Use set_static_spec_shared() when resetting
	specs that previously had allocated strings; remove if(0)
	around call to free().