On Fri, Mar 24, 2017 at 1:22 PM, Peter Zijlstra <pet...@infradead.org> wrote: > On Fri, Mar 24, 2017 at 11:45:46AM -0700, Andy Lutomirski wrote: >> After playing with it a bit, I found some of the problem: you're >> passing val into EXCEPTION_VALUE, which keeps it live. If I get rid >> of that, the generated code is great. > > Right, so I needed that because I land on ud2 through 2 different paths: > > - newly saturated > - use-after-free > > And the exception handler can figure out which of the two by looking at > the variable, but then of course, it needs to be life. > > For the full horror of how to do this, look here: > > http://paste.debian.net/924190/ > > But I didn't just show you that, so you can't blame me for any damage > that might've done you.
Wow, that's horrible. Could this not be done by looking at flags instead of regs? For that matter, you're effectively comparing to -1 and 0. I'm not really sure it would be faster, but you could plausibly add one then subtract one again and get the full picture just from flags and a single comparison?
