PAGE_SIZE isn't portable -- e.g. my ARMv8 sysroot doesn't define it. I'm
replacing it in the way the Linux manpage recommends. This should work
on a wide variety of platforms. For example, it works on OS X.
... is that once you put them into the codebase, the odds of them
showing up where you dont want them go up dramatically.
With a gcc 4.7.1 -O0 build I hit a consistent crash in production
where debugBacktrace had stepped too far, thinking that the
first non-vm frame was in fact a vm frame. It did so because
it looked at the word above the frame, and saw that it contained
c_Continuation::kMagic.
It turns out that fixupWork's isVMFrame left kMagic in $rdx if the
frame really was a generator ($rdx was dead, but still). We then
return from the tc via a serviceReq that doesnt need $rdx (so doesnt
set it), and then (with a following wind, so that $rdx hasnt been
smashed yet) enterTCHelper stores $rdx in info.args[1].
By another staggering coincidence, info.args[1] is at exactly the
right address to make the call into enterTCHelper /look/ like a
a continuation (based on kMagic). So if we then have a catch
which re-enters the TC there's a good chance kMagic is still there
and the next debugBacktrace (or uncaught exception) will crash.
This diff rewrites everything in terms of the C++ stack; we
now say that its a VM frame if its not on the C++ stack.
This change is mostly for FB internal organizational reasons.
Building is not effected beyond the fact that the target now
lands in hphp/hhvm/hhvm rather than src/hhvm/hhvm.
aalexandre