Re: AltiVec register ptrace support

[Kumar Gala <kumar.gala@motorola.com>]

Is there any reason that we can not spport both methods.  There are
applications in which having the ability to get all the registers is a
single syscall is a major performance improvement.

_ kumar

On Fri, 7 Dec 2001, Daniel Jacobowitz wrote:

>
> On Fri, Dec 07, 2001 at 03:23:02PM -0700, Kevin Buettner wrote:
> > On Dec 7,  2:57pm, Kumar Gala wrote:
> >
> > > I have two different patches to the ptrace mechanism to add support
> > > for AltiVec registers.
> > >
> > > linux-2.4.8-altivec-ptrace.patch:  Adds support similar to existing
> > > mechanisms to get/set registers via PEEK/POKE calls extending the FPU
> > > model.
> > >
> > > linux-2.4.16-altivec-ptrace.patch: Adds support for new ptrace commands
> > > that match sparc/x86 PTRACE_{GET,SET}*REGS.  These dump the full register
> > > state in a single call.
> > >
> > > Personally, I would like to see the PTRACE_{GET,SET}*REGS method adopted
> > > for 2.4.x.  RedHat is trying to push out some GDB changes for AltiVec that
> > > require closure on this matter.
> >
> > I would like to better understand your reasons for preferring
> > PTRACE_{GET,SET}*REGS.  Is it just because that's what x86 does
> > or do you think that this mechanism improves GDB's performance?
>
> I think that it improves performance and that it is generally cleaner.
>
> > My personal opinion is that GETREGS/SETREGS does not greatly enhance
> > performance.  Try running strace on gdb debugging itself on x86 and on
> > PPC and compare the number of PTRACE_PEEKUSR calls on PPC vs.
> > PTRACE_????  calls on x86.  (The ????  is printed because strace
> > doesn't know about the various PTRACE_{GET,SET}*REGS calls.) When I
> > tried it just a moment ago using gdb to debug itself and running to a
> > breakpoint set on main(), I saw _more_ PTRACE_???? calls on x86 than
> > PEEKUSR/POKUSR calls on PPC.  Now, I admit that my testing wasn't very
> > exhaustive, but even if the number of PEEKUSR/POKEUSR calls were
> > higher, I think you'd find that calls to PEEKTEXT (for prologue
> > analysis) would dominate.  I.e, the majority of the ptrace() traffic
> > is due to reading memory, not reading registers.
>
> You get more because there are three sets, and we gratuitously fetch
> all registers instead of just the needed type of register.  I'd bet a
> lot that a third of the 18 ????'s I see are for SSE registers and a
> third for FP registers.  That would bring it down to 6 vs the 16 on PPC
> using PEEKUSER.
>
> Also, while I think _GETREGS is better than PEEKUSER, we're talking
> here specifically about VRREGS.  It's four ptrace calls per vector
> register, since ptrace() can only transfer a word at a time (so far at
> least; I'm contemplating proposing a change to that).  And when you
> want one vector register the odds are very good that one wants to get
> another.
>
> Also, while single stepping there ought to be no PEEKTEXT calls, only
> PEEKUSER, and at least two of them on PPC (in fact we do a lot of
> gratuitous poking around in the text segment).
>
> > Furthermore, I think that introducing GETREGS/SETREGS will make
> > ppc-linux-nat.c (in the GDB sources) more complicated.  We'll need
> > compile time tests to check for the presence of GETREGS/SETREGS and
> > use these mechanisms if they exist.  If they don't, this code will
> > have to fall back to using the old PEEKUSR/POKEUSR mechanism.  Also,
> > it may be necessary to have runtime tests which attempt to use
> > GETREGS/SETREGS and fall back to using PEEKUSR/POKEUSR.  In order to
> > see just how messy it can get, take a look at i386-linux-nat.c.
>
> This part is definitely true.
>
> --
> Daniel Jacobowitz                           Carnegie Mellon University
> MontaVista Software                         Debian GNU/Linux Developer
>
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>