[kaffe] -O4 jit3 problem

Patrick Tullmann tullmann at cs.utah.edu
Sun Apr 28 22:48:44 PDT 2002

I found one of the problems with Kaffe when compiling with -O4.
Either the linker or the compiler is reordering the functions defined
in jni.c.  Normally this wouldn't matter, but Kaffe tries to identify
if a program counter is in a "JNI entry point method" by testing to
see if the PC is between Kaffe_JNI_estart and Kaffe_JNI_eend.  Those
two values are hardcoded to the first and last functions of interest
defined in jni.c.

When compiled with gcc -O4 (even with debugging and -g enabled, thank
god), some of those methods are moved around.  Specifically, the
method used to invoke each new thread's "run" method uses
Kaffe_CallVoidMethod, and on my machine the compiler moved that
outside of the functions used to delimit the "start" and "end" of

I hacked jni.c to iterate over all the pointers in the JNI entrypoint
table and adjust the Kaffe_JNI_estart and Kaffe_JNI_eend until all of
those methods were included.  This fixed the particular test case
(UncaughtException) but its exceedingly bogus because who knows what
code was eventually included in this coverage.

Unless someone knows of a good way of forcing a compiler/linker to
keep a bunch of functions next to each other in the text segment,
we'll need another approach.  The best I could think of is to expand
the current vmException buffer that's kept in each such method
(they're chained and reachable off the thread object, see
BEGIN_EXCEPTION_HANDLING in jni.c).  If that struct could be expanded
to include the address (and length!) of each the function its
associated with, we could check the pc against that when looking for
JNI exception handlers.  Still for this approach to work, we'd need a
way to get the size of a function's text out of the compiler...

Anyone got other ideas?  (Other than detecting and disabling -O4 in
./configure, of course...)


----- ----- ---- ---  ---  --   -    -      -         -               -
Pat Tullmann                                       tullmann at cs.utah.edu
    Not many people realize just how well known I am -- Anonymous

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