[Bug 1322784] [NEW] Firefox crashes in flag_qsort during spellchecker initialization on x86 due to gcc bug

[L. David Baron]

Public bug reported:

The most common Firefox crash on Linux in Mozilla's crash-stats system
is crashes in the function flag_qsort.

These crashes occur:
 * only on x86 architecture
 * only on Ubuntu packages (and not on Mozilla's builds)
 * on precise and saucy and trusty (based on kernel versions reported with the crashes)
and appear to be due to a compiler bug in the compiler used to generate Ubuntu's builds.  (It could be a common compiler bug triggered by different compiler options or a compiler bug specific to Ubuntu's gcc.)

The analysis that leads to the conclusion that this is a compiler bug is
in https://bugzilla.mozilla.org/show_bug.cgi?id=983817 .  In particular,
the compiler is miscompiling an access to an element of an array of
unsigned short as a 32-bit read, and when the unsigned short in question
is the last one in the allocation and that allocation is aligned so that
the byte following has a different 0x100000 bit, this can lead to
crashes.

The most recent (whenever you follow the link) 7 days of crash reports
are available at: https://crash-
stats.mozilla.com/report/list?signature=flag_qsort&product=Firefox&query_type=contains&range_unit=weeks

** Affects: firefox
     Importance: Unknown
         Status: Unknown

** Affects: firefox (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: gcc-defaults (Ubuntu)
     Importance: Undecided
         Status: New

** Also affects: gcc-defaults (Ubuntu)
   Importance: Undecided
       Status: New

** Bug watch added: Mozilla Bugzilla #983817
   https://bugzilla.mozilla.org/show_bug.cgi?id=983817

** Also affects: firefox via
   https://bugzilla.mozilla.org/show_bug.cgi?id=983817
   Importance: Unknown
       Status: Unknown

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https://bugs.launchpad.net/bugs/1322784

Title:
  Firefox crashes in flag_qsort during spellchecker initialization on
  x86 due to gcc bug

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