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Subject: [std-proposals] Proposal for Switch(Constraints=Both/Upper/Lower/Neither/NoDefault/Sequenced)- improved assembly.
From: Wesley Oliver (wesley.olis_at_[hidden])
Date: 2021-02-05 06:11:14


I have been looking to optimize an algorithm execution, with some
adaptations and improved logic.
One of the things I can across is that I could improve performance by
handling a few special end condition cases differently and further again by
adding upper bounder range check,
The upper bound range check along for default gave me 5-6% performance
increase, giving me a total of like 10-14% overall, by combination of a if
upper range check and switch.
I am sure with this suggested improvement, the compiler will be able to
improve this even further with a single compound statement switch
statement as I proposed below.
So for example:
case - 2: at times.
case -1: continue/runout/run-on with instructions.
case 0: the same thing.
for these case just run out.
case 1: optimize section of handling .
break or goto
case 2: optimize section of handling.
break or goto
case 3: optimize section of handling.
break or goto
default: complex logic.
break or goto

What I have found is that since switch typically gets compiled down to if
statements or range checks for default. There is a nice optimization here,
which I implemented manual, which speeds things save about 5-6% off, but
may have room for more assemble optimization.
At moment I see sub, he, sub, jne optimization for the switch, but feel
could be more if one included the boundaries checking of default in switch
statement. As there is still another default boundaries checking of cmp,
jle, to the switch.

What I would like to propose is SwitchLower and SwitchUpper, which
how upper bound can be checked, this saving a bounds check statement and
with this new statement it enforcing a constraint that all case have
sequential identifier, no gaps.
The code can still be re-ordered for optimization.

With knowledge of this, typically translate the switch to a jump
switchValue that is already in register that just got ranged checked for
the default case, subtract the sequentialOffset, then jump by the
switchValue or relative memory offset or instruction offset, depending on
instruction set. Then jump one more time to the correct address to continue
executing from for that case.
Essentially this is a const stack array of positional jump offset encoding
directly into the instructions, which should never be exicuted, unless
random mistake jump to this data.

if(switchValue < Upper | Lower | Both range check)
default code
switchValue -= SequentialOffset;
Jump SwitchValue;
Jump currentcaseinstructionvalue.

I would like to recommend/propose
*switch(SwitchValue, SwitchConstrainsModifierHint) *, where
= NoCheck | Both | Upper | Lower | NoDefault | Seqeuenced.*

With this additional switchcontrainsModifierHint to a language, the
compilers can easily understand range and sequence intention, in unsafe
range check type language,
which would allow it to further optimize this code much more easily, than a
massive crazy analysis of the whole program to determine and ensure these

This simple would allow everyone to communicate there intentional uses of
the switch statement, based the large scope of the program they are
writing, which would
allow for it to further optimize the low level code generation and increase

I typically I feel that this would be a minor change, because I am not
awhere of an additional parameter that can be supplied to Switch, the
SwitchConstraints modified would
naturally be backwards compatible with all languages and would be a
progressive improvement. The reason for naturally backwards compatible, is
that #define could define the sequence false, to be non-op, so in old
compilers, SwitchConstraintsModiferHints, would be valid and compile, also
due to
the fact in C/C++ you can specify multiple commands separated by a comma.
switch(switchValue, false) already compiles and the compiler
will optimize away the false value I should hope.

Compilers can progressively take advantage of the hint and improve the
low-level code generation, eventually leading to faster and more precise
and consise code
being written communicate its range behaviours more clearly.

Look forward to hearing what one you guys think.

Kind Regards,

Wesley Oliver

Skype: wezley_oliver
MSN messenger: wesley.olis_at_[hidden]

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