Hi all,
this long patch provides a fix for a very annoying fact, that GDB on 
x86-64 can't do backtraces from hand-optimized assembler functions (that 
applies for example to glibc's memset, str*, etc as well as to syscall 
wrappers).
This is caused by the lack of a valid debug_frame/eh_frame FDE entry for 
such a function (noone really writes .debug_frame section in his 
assembler code :-)

My approach to fix this behaviour is based on the fortunate fact, that 
most of those affected glibc's functions don't touch the stack at all, 
so creating an artifical FDE for them is easy.

Since this debug info is hardware dependent, I introduce a new gdbarch 
item: dwarf2_create_fde. Any architecture using Dwarf2 CFI that will 
like to profit from it must have it's own <whtever>_dwarf2_create_fde() 
function, very similar to the one I did for x86-64.

In x86_64_dwarf2_create_fde() I handle two cases: functions that have a 
prologue and those that don't (the most usual case). Soon I'll try to 
create FDE for signal handler caller there as well.

Advantages of this approach are simplicity and cleanliness. There is no 
need to "manually" fill in values in CFI's context structure - just 
provide a valid, simple FDE and everything else will be done 
automagically. This new FDE is of course joined to all other FDEs read 
from the file before and is automatically used when needed again.

The patch is transparent to architectures that are not prepared to have 
advantage of it and shouldn't hurt anything.

OK to commit to branch and mainline?

Michal Ludvig
-- 
* SuSE CR, s.r.o     * mludvig@suse.cz
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