From: Vineet Gupta <vineetg@rivosinc.com>
To: Jeff Law <jeffreyalaw@gmail.com>, gcc@gcc.gnu.org
Cc: Kito Cheng <kito.cheng@sifive.com>,
Philipp Tomsich <philipp.tomsich@vrull.eu>
Subject: Re: Redundant constants in coremark crc8 for RISCV/aarch64 (no-if-conversion)
Date: Tue, 18 Oct 2022 19:09:50 -0700 [thread overview]
Message-ID: <f97b82d4-305e-6e38-ffac-6c214ad310b4@rivosinc.com> (raw)
In-Reply-To: <53dcbef4-7aef-5f63-9bd8-e11c614b0be8@gmail.com>
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.
>
>>
>> FWIW -fno-unroll-loops only seems to work at -O2. At -O3 it always
>> unrolls. Is that expected ?
>
> The only case I'm immediately aware of where this wouldn't work would
> be if -O3 came after -fno-unroll-oops.
Weird that gcc-12, gcc-11, gcc-10 all seem to be silently ignoring
-funroll-loops despite following -O3. Perhaps a different toggle is
needed to supress the issue.
Thx,
-Vineet
next prev parent reply other threads:[~2022-10-19 2:09 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-14 15:56 Vineet Gupta
2022-10-14 16:54 ` Jeff Law
2022-10-18 21:51 ` Vineet Gupta
2022-10-18 23:36 ` Jeff Law
2022-10-19 2:09 ` Vineet Gupta [this message]
2022-10-19 3:42 ` Jeff Law
2022-10-19 7:46 ` Richard Biener
2022-10-19 13:30 ` Jeff Law
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