Seems like a good change. A few questions about the implementation:
(maybe just needs more comments)


http://codereview.appspot.com/204046/diff/1/6
File src/liboslexec/exec.cpp (right):

http://codereview.appspot.com/204046/diff/1/6#newcode386
src/liboslexec/exec.cpp:386: Runflag *orig =
context()->m_original_runflags;
Why is this save/restore needed?

http://codereview.appspot.com/204046/diff/1/6#newcode534
src/liboslexec/exec.cpp:534: run (beginop, endop);
Why is m_conditional_level not set for this case?

http://codereview.appspot.com/204046/diff/1/4
File src/liboslexec/opcontrol.cpp (right):

http://codereview.appspot.com/204046/diff/1/4#newcode110
src/liboslexec/opcontrol.cpp:110: exec->enter_conditional ();
Why is this needed? Can't it be handled inside push/pop runflags?

http://codereview.appspot.com/204046/diff/1/2
File src/liboslexec/oslexec_pvt.h (right):

http://codereview.appspot.com/204046/diff/1/2#newcode67
src/liboslexec/oslexec_pvt.h:67: #define NO_DEBUG_ADJUST_VARYING
I would prefer having:

// #define DEBUG_ADJUST_VARYING

http://codereview.appspot.com/204046/show