[PATCH] Dwarf location expressions, tracing and printing
Stan Shebs
stan@codesourcery.com
Tue Apr 20 19:26:00 GMT 2010
Stan Shebs wrote:
> This patch solves a number of problems related to variables with
> Dwarf2 location expressions - which these days, means just about every
> local in optimized code. :-) Perhaps the most interesting part is
> that I finally got tired of working blind and filled out description
> location functions, so now both "info scope" and "info address" have
> quite a lot of say about variables. Try things like "info scope
> handle_inferior_event" to get some truly awesome verbiage. :-)
>
> Since there's a lot of new code here, I'll leave it up for a couple
> days before committing.
>
Just as a heads-up, this patch exposes a major hole in our tracepoint
machinery when running testsuite with gdbserver - namely that agent
expressions don't have a way to deal with registers that are larger than
64 bits. This crops up in gdb.trace/collection.exp:reglocal_test_func
on x86, in which the local floats are assigned to $stX registers, which
are 10 bytes long. gdbserver properly chokes on any attempt to push
these on the expression stack (although one might argue that it should
be a tracing rather than internal error). Previously, the registers
were incorrectly numbered to 4-byte registers, which resulted in four
FAILs when printing collected floats, but nothing worse.
I went ahead and committed the patch anyway, because the fix/workaround
isn't obvious, and it's a design problem elsewhere.
Alternatives include:
1) Make the expression stack wider. The existing target-side
implementations are designed around the 64-bit assumption, so there
would be some messy fixing-up with sign extensions and the like.
2) Generate bytecodes to handle larger registers as multiple pieces.
3) Disallow larger registers in expressions.
4) Stub out that part of the testsuite. :-)
Stan
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