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Register cache
- To: gdb at sources dot redhat dot com
- Subject: Register cache
- From: Mark Kettenis <kettenis at wins dot uva dot nl>
- Date: Sat, 10 Feb 2001 23:37:49 +0100
- CC: eliz at delorie dot com
A few days ago Eli proposed the following patch:
http://sources.redhat.com/ml/gdb-patches/2001-02/msg00140.html
It proposes a new interface to supply a single i387 register to GDB's
internal register cache in addition to the existing interface that
supplies the whole FPU state.
In a perhaps overly pedantic mood, I raised some objections to the
patch. I would prefer not to add yet another interface, and I
suggested to Eli that he'd simply use the existing interface, to which
he responded that that would implicate violating the
target_fetch_registers() API. Let me explain:
We have the following comment on target_fetch_registers() in target.h:
/* Fetch register REGNO, or all regs if regno == -1. No result. */
This suggests that target_fetch_registers() will fetch exactly one
register if REGNO != -1. However, quite a few targets do something
different, that is, they fetch at least register REGNO, but actually
fetch a whole bunch of them. This probably started with SVR4-ish
systems that have a /proc filesystem. On those systems it isn't
possible to fetch a single register. Instead one get a whole bunch
and procfs_fetch_registers() supplies them all to GDB's register
cache. Comments in procfs.c suggest that this is an optimization; by
caching the whole register set on possibly saves a few system calls in
the future. Other targets like the modern BSDs and GNU/Linux that
have appropriate ptrace() requests followed this example and do the
same thing.
Is this really a good idea? I think it is, at least on targets where
getting at the registers is relatively expensive since it involves a
system call. Therefore I propose to make this "official" and change
the comment on target_fetch_registers() to:
/* Fetch at least register REGNO, or all regs if REGNO == -1. */
Another question is whether we should make this behaviour (I mean
having target_fetch_registers() supply all registers it managed to get
in one go) recommended practice.
Mark