Delay slots in GNU sim

Delay slots in GNU sim

[Julius Baxter]

Hi,

I've been working on the OpenRISC CGEN description and have got the
assembler working and am now looking at getting a GNU sim port running
too.

I can see a lot of functionality working OK so far (I've based my
simulator port on the lm32 and m32r ports, which are both CGEN-based
implementations of the simulator) but one thing which is not working
are delay slots. They never appear to b executed when a branch is
going to be taken.

I'm yet to delve into the workings if the simulator to figure out why
it's not executing the delay slots, but I just wanted to ask if this
is a known issue with the particular configuration I'm using. The sim
port has "#define WITH_SCACHE_PBB 1", in the tconfig.in file and the
Makefile calls the mloop.c generation script with:

...
$(SHELL) $(srccom)/genmloop.sh \
                -mono -fast -pbb -full-switch -switch sem-switch.c \
                -cpu or1k32bef -infile $(srcdir)/mloop.in

...

My understanding is that this will generate a main loop for a "fast"
model with pseudo-basic-block processing. This is an "upgrade" over
the default "simple" model without pbb.

I'm wondering if delay slots not working are a known issue when
compiling a fast model with pbb support?

As far as I can tell, my semantic description is correct. For example,
the jump instruction is:

(dni l-j "jump (pc-relative iaddr)"
     ; This function may not be in delay slot
     (NOT-IN-DELAY-SLOT)

     "l.j ${disp-26}"
     (+ OPC_J disp-26)

     ; We execute the delay slot before doin' the real branch
     (delay 1 (set pc disp-26))
     ()
)

Other semantic descriptions of delay slot execution appear to be along
the same lines, but I can't see any CGEN-based sim ports which have
instructions with delay slots, so, as I said, I'm wondering if this is
even supported at all.

Thanks for any help,

    Julius

Re: Delay slots in GNU sim

[Julius Baxter]

On Sun, Jan 15, 2012 at 2:24 PM, Julius Baxter <juliusbaxter@gmail.com> wrote:
> Hi,
>
> I've been working on the OpenRISC CGEN description and have got the
> assembler working and am now looking at getting a GNU sim port running
> too.
>
> I can see a lot of functionality working OK so far (I've based my
> simulator port on the lm32 and m32r ports, which are both CGEN-based
> implementations of the simulator) but one thing which is not working
> are delay slots. They never appear to b executed when a branch is
> going to be taken.
>
> I'm yet to delve into the workings if the simulator to figure out why
> it's not executing the delay slots, but I just wanted to ask if this
> is a known issue with the particular configuration I'm using. The sim
> port has "#define WITH_SCACHE_PBB 1", in the tconfig.in file and the
> Makefile calls the mloop.c generation script with:
>
> ...
> $(SHELL) $(srccom)/genmloop.sh \
>                -mono -fast -pbb -full-switch -switch sem-switch.c \
>                -cpu or1k32bef -infile $(srcdir)/mloop.in
>
> ...
>
> My understanding is that this will generate a main loop for a "fast"
> model with pseudo-basic-block processing. This is an "upgrade" over
> the default "simple" model without pbb.
>
> I'm wondering if delay slots not working are a known issue when
> compiling a fast model with pbb support?
>
> As far as I can tell, my semantic description is correct. For example,
> the jump instruction is:
>
> (dni l-j "jump (pc-relative iaddr)"
>     ; This function may not be in delay slot
>     (NOT-IN-DELAY-SLOT)
>
>     "l.j ${disp-26}"
>     (+ OPC_J disp-26)
>
>     ; We execute the delay slot before doin' the real branch
>     (delay 1 (set pc disp-26))
>     ()
> )
>
> Other semantic descriptions of delay slot execution appear to be along
> the same lines, but I can't see any CGEN-based sim ports which have
> instructions with delay slots, so, as I said, I'm wondering if this is
> even supported at all.

I didn't look hard enough, the cris and iq2000 sim ports implement
instructions using delay slots.

I fixed this issue after looking at their mloop.in files and realised
I was missing a section to handle delay slots. With this code added it
looks like delay slot instructions are getting executed.

Cheers
    Julius