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Subject: Re: [std-proposals] Parralized instructions in language in CPU
From: Thiago Macieira (thiago_at_[hidden])
Date: 2021-01-18 10:20:11


On Monday, 18 January 2021 00:12:18 PST Wesley Oliver via Std-Proposals wrote:
> Hi,
>
> I would like to look at how to achieve the same performance that c++ would
> be capable of achieving with characters that are '\0' terminate physically
> by there last data position. Because data that is null terminate, doesn't
> require range checking.

That's an incorrect statement. Just because the bounds are implicit does not
mean range checking is optional. Anywhere where range checking should have
been done, it still needs to be done. The only difference is that you must
calculate the range before doing the checking.

As a consequence of having to iterate to find the boundary, some operations
become different.

But it's not true that implicit termination or termination by sentinel makes
things faster. In fact, from experience, it's quite the opposite. So you'll
need to prove your hypothesis with data if it is the motivation factor for
this.

> So my question is how could we improve things, such that the typically
> conditional bounds checking statements for int array or similar could be
> reduce or written in slightly different form,
> such that we can achieve the same performance as null terminated data.

Please consider that "achieve the same performance as null-terminated data"
currently means "reducing performance". It's not what you want.

> For this to happen, it would require improved compiler and also
> hypothetically invisigaing new cpu wiring or logic, that in 2025 years time
> would give us massive performance boost, as we have figure out how to write
> could that has many performance knocks in better way, by reduction or what
> every that technic is, to reduce the number of instructions required.

Investigating a new CPU is out of scope. Moreover, I don't think you've
investigated CPUs sufficiently, since they do run superscalar and pipelined,
meaning they will run a few instructions ahead. For example, you said:

> So the ideas I have from above, would be conditional statements that could
> be parralized, with out changing the logic of the program. so think that
> both
> numArray[i] == numArray2[j]
> i < maxlen

They ALREADY are parallelised today. Like I said, I don't think you've done
enough investigation of current CPUs before making this proposal. If you want
to talk about how to improve CPUs, aside from locating the right people to
talk to, you need to come with hard data showing where they can do better and
where time is spent in overhead. Similalry, if you want to get compilers to
improve, show some hard numbers on how they are not using the existing CPU
capabilities to the full extent.

>From experience talking to CPU architects (and I have!), they have enough
areas to currently address that can improve code by more than 1%. So your
barrier to get their attention is to show that they could get at least that
much.

> function match(char* str) {
>
> char* matchme = "matchme\0";
>
> let countMatch = 0;
>
> for(char* ch = str, char chd = ch*;chd != '\0';ch++, chd = *ch)
> {
> // could make this an inline function.
>
> char* chs = str, char chds = chs*;
> char* mech = matchme, char mechd = *mech ;
> while(true) { // sure that by now compiler optermized, could just say
> loop
> if (chds != mechd) {
> if(mechd == '\0') {
> break;
> continue; // kicks out of the loop and skip the rest of the
> parent look code, for a case that fail, in the case of successfully match,
> then countMatch++ will execute.
> }
> }
> mech++; mechd = * mech; chs++; chds = *chs ;
> }
> countMatch++;
> }
> }

One other thing: if you're going to post code for which performance can be
imiproved, you have to start with the state of the art. Your code above for
substring matching is not the most optimal today.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
   Software Architect - Intel DPG Cloud Engineering

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