This patch is a small usability enhancement from trace frames coming 
from a trace file; if registers have not been collected, then clear them 
all, and guess that $pc must be the same as the tracepoint's address.  
This isn't a good idea for either multi-location tracepoints or stepping 
frames though, and we want to warn the user about those cases.

Stan

2010-04-01  Stan Shebs  <stan@codesourcery.com>

    * tracepoint.c (tfile_fetch_registers): Add fallback case.

    * gdb.texinfo (Tracepoint Restrictions): Document PC inference.

    * gdb.trace/tfile.exp: Sharpen tfind test.