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

[Jeff Law]

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.

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


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.

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


Jeff