[Chicken-hackers] [PATCH] Change setjmp/longjmp to _setjmp/_longjmp to avoid overhead and fix signal masking bug

[Peter Bex]

Hi hackers,

While looking over a system call trace while debugging another issue,
I noticed a LOT of system calls to __sigprocmask14 (on NetBSD).

It turns out these are generated by setjmp() and longjmp(), so upon
each minor GC it make do two superfluous system calls.  According
to POSIX, it is actually /unspecified/ if those affect the signal mask:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/longjmp.html

This setting of the signal mask is probably even unintended and may be
the cause of bugs when the mask is set since the GC might reset any
masked signals.  The following program prints "signaling" and
"GOT SIGINT" with Chicken master on NetBSD (using csi):

(use posix)
(set-signal-handler! signal/int (lambda (sig) (print "GOT SIGINT")))
(signal-mask! signal/int)
(let lp ((i 0))
  (if (= i 1000)
      (begin (print "signaling")
             (process-signal (current-process-id) signal/int)
             (lp 0))
      (lp (add1 i))))

With the attached patch it only prints "signaling", which is the same
behavior this program shows on Linux with both versions.  I also made
a small test but I wasn't sure whether the GC is guaranteed to be
triggered so it might not be a very valid test (it doesn't when
you compile it, but for me it does in csi).  This test is attached.
If it's okay, feel free to add it to the testsuite.

So, to solve this, it's more correct to use sigsetjmp/siglongjmp to
explicitly set the mask or not.  However, according to this GNULib
manual http://www.gnu.org/software/gnulib/manual/gnulib.html#sigsetjmp
this function isn't supported by MingW, MSVC and Minix.

That's why the attached patch uses _sigsetjmp and _siglongjmp.
Those are in POSIX, and pretty widely supported (but they're slated
to be *possibly* removed in future POSIX versions).  According to the
aforementioned GNULib manual, _sigsetjmp/_siglongjmp are the fastest
way to set and restore the registers without signal mask.

I've also attached two outputs from the resurrected Chicken benchmarks
by Mario: https://github.com/mario-goulart/chicken-benchmarks
These clearly show about a 5% speed improvement across the board.
(you can see a clear comparison by using the "compare.scm" program from
 that repo).  I compiled Chicken with itself before running this
benchmark.  This may have an effect because according to Mario the
compilation time is included in the measurements.

Cheers,
Peter
-- 
http://sjamaan.ath.cx
--
"The process of preparing programs for a digital computer
 is especially attractive, not only because it can be economically
 and scientifically rewarding, but also because it can be an aesthetic
 experience much like composing poetry or music."
                                                        -- Donald Knuth

0001-Use-_setjmp-_longjmp-instead-of-setjmp-longjmp-to-pr.patch
Description: Text document

with-patch.log
Description: Text document

without-patch.log
Description: Text document

test.scm
Description: Text document