Re: Redundant constants in coremark crc8 for RISCV/aarch64 (no-if-conversion)

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Oct 19, 2022 at 5:44 AM Jeff Law via Gcc <gcc@gcc.gnu.org> wrote:
>
>
> On 10/18/22 20:09, Vineet Gupta wrote:
> >
> > On 10/18/22 16:36, Jeff Law wrote:
> >>>> There isn't a great place in GCC to handle this right now.  If the
> >>>> constraints were relaxed in PRE, then we'd have a chance, but
> >>>> getting the cost model right is going to be tough.
> >>>
> >>> It would have been better (for this specific case) if loop unrolling
> >>> was not being done so early. The tree pass cunroll is flattening it
> >>> out and leaving for rest of the all tree/rtl passes to pick up the
> >>> pieces and remove any redundancies, if at all. It obviously needs to
> >>> be early if we are injecting 7x more instructions, but seems like a
> >>> lot to unravel.
> >>
> >> Yup.  If that loop gets unrolled, it's going to be a mess.  It will
> >> almost certainly make this problem worse as each iteration is going
> >> to have a pair of constants loaded and no good way to remove them.
> >
> > Thats the original problem that I started this thread with. I'd
> > snipped the disassembly as it would have been too much text but
> > basically on RV, Coremark crc8 loop of const 8 iterations gets
> > unrolled including extraneous 8 insns pairs to load the same constant
> > - which is preposterous. Other arches side-step by using if-conversion
> > / cond moves, latter currently WIP in RV International. x86 w/o
> > if-convert seems OK since the const can be encoded in the xor insn.
> >
> > OTOH given that gimple/tree-pass cunroll is doing the culprit loop
> > unrolling and introducing redundant const 8 times, can it ne addressed
> > there somehow.
> > tree_estimate_loop_size() seems to identify constant expression, not
> > just an operand. Can it be taught to identify a "non-trivial const"
> > and hoist/code-move the expression. Sorry just rambling here, most
> > likely non-sense.

On GIMPLE all constants are "simple".

> Oh, cunroll.  There might be a distinct flag for complete unrolling.

At -O3 we peel completely, there's no flag to disable that.

> I really expect something like Click's work is the way forward.
> Essentially when you VN the function you'll identify those constants and
> collapse them all down to a single instance.  Then the GCM phase will
> kick in and find a place to put the evaluation so that you have one and
> only one.

I'd say postreload gcse would be a place to do that.  At least when
there's no available hardreg CSEing likely isn't going to be a win.

> Some of Bodik's work might catch it as well, though implementing his
> ideas is likely a lot more work.
>
>
> Jeff