scripts: add stackusage script

scripts: add stackusage script

The current checkstack.pl script has a few problems, stemming from the
overly simplistic attempt at parsing objdump output with regular
expressions: For example, on x86_64 it doesn't take the push
instruction into account, making it consistently underestimate the
real stack use, and it also doesn't capture stack pointer adjustments
of exactly 128 bytes [1].

Since newer gcc (>= 4.6) knows about -fstack-usage, we might as well
take the information straight from the horse's mouth. This patch
introduces scripts/stackusage, which is a simple wrapper for running
make with KCFLAGS set to -fstack-usage. Example use is

scripts/stackusage -o out.su -j8 lib/

The script understands "-o foo" for writing to 'foo' and -h for a
trivial help text; anything else is passed to make.

Afterwards, we find all newly created .su files, massage them a
little, sort by stack use and write the result to a single output
file.

Note that the function names printed by (at least) gcc 4.7 are
sometimes useless. For example, the first three lines of out.su
generated above are

./lib/decompress_bunzip2.c:155  get_next_block  448     static
./lib/decompress_unlzma.c:537   unlzma  336     static
./lib/vsprintf.c:616    8       304     static

That function '8' is really the static symbol_string(), but it has
been subject to 'interprocedural scalar replacement of aggregates', so
its name in the object file is 'symbol_string.isra.8'. gcc 5.0 doesn't
have this problem; it uses the full name as seen in the object file.

[1] Since gcc encodes that by

48 83 c4 80             add    $0xffffffffffffff80,%rsp

and not

48 81 ec 80 00 00 00    sub    $0x80,%rsp

since -128 fits in an imm8.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Michal Marek <mmarek@suse.com>