Eyes Above The Waves

Thursday 22 October 2015

rr 4.0 Released With Reverse Execution

I finally got around to releasing rr 4.0!

rr is Mozilla's low overhead record-and-replay debugging tool for large C++ applications like Firefox. For more information see the rr project page, or watch my Technion talk:

rr 4.0 is the first stable release with reverse-execution enabled. This means gdb's reverse-continue, reverse-step, reverse-next and reverse-finish commands work under rr. Not only do they work, but under rr they're quite efficient and have unlimited scope. It's completely reasonable to reverse-continue from the end of a Firefox run all the way back to the beginning. Furthermore, breakpoints and hardware data watchpoints work with reverse execution. Suppose we're debugging Firefox layout:

Breakpoint 1, nsCanvasFrame::BuildDisplayList (this=0x2aaadd7dbeb0, aBuilder=0x7fffffffaaa0, aDirtyRect=..., aLists=...)
    at /home/roc/mozilla-inbound/layout/generic/nsCanvasFrame.cpp:460
460   if (GetPrevInFlow()) {
(gdp) p mRect.width
12000

(gdb) watch -l mRect.width
(gdb) reverse-cont
Continuing.
Hardware watchpoint 2: -location mRect.width
Old value = 12000
New value = 11220
0x00002aaab100c0fd in nsIFrame::SetRect (this=0x2aaadd7dbeb0, aRect=...)
    at /home/roc/mozilla-inbound/layout/base/../generic/nsIFrame.h:718
718       mRect = aRect;

This release is about as stable as rr has ever been --- meaning it seems to be generally working for people and very few new issues have been coming in. However, rr depends on gnarly details of the kernel, system libraries and CPU so I expect as the number of rr users grows we'll need to keep fixing bugs and updating rr regularly.

By the way, Mozilla is interested in hiring people to work on rr-related technology, more closely integrated with the browser. I'd like to hear from people who're interested!