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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: Concertlina II: Full Circle
Date: Wed, 19 Jun 2024 18:31:12 +0000
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Thomas Koenig wrote:

> Terje Mathisen <terje.mathisen@tmsw.no> schrieb:
>> MitchAlsup1 wrote:
>>> John Savard wrote:
>>> 
>>>> On Tue, 18 Jun 2024 21:23:57 +0000, mitchalsup@aol.com (MitchAlsup1)
>>>> wrote:
>>>>> John Savard wrote:
>>> 
>>>>>> (1 bit) This indicates whether we're talking about the integer
>>>>>> registers or the floating-point ones.
>>>>>
>>>>> Loops controlled by floating point indexes do not vectorize, however
>>>>> the body of the loop can be any mix of int, logical, memory, or FP
>>>>> instructions.
>>> 
>>>> Oh no, my index is always an integer. This bit applies to the
>>>> "live-in" bits - if the loop performs floating-point computation, it's
>>>> floating-point registers that I want to mark as forwarding nodes.
>>> 
>>> See, I do not have this distinction, there is but one file.
>>> 
>>>> And so you indicate this explicitly in VVM as well. I tended to assume
>>>> only a limited number of registers would be needed to live in, plus I
>>>> have both floating and integer register files, hence the differences.
>>> 
>>> It ends up that the majority of register uses in a loop do not need to
>>> be visible outside of the loop. This is almost the contrapositive of
>>> annotating which registers are temporary in the loop. 90%+ of loops do 
>>> not even need the index register to be live outside of the loop.
>>
>> This is partly due to programming languages that applies lifetimes to 
>> variables, so that an index register which is defined in the
>> scaffolding
>> 
>> of the loop (i.e. for (i = 0; i < limit; i++) {}) is invisible as soon 
>> as the loop terminates.

> This makes things more clear to anybody reading the code (and
> unambiguous to the compiler).  However, lifetime analysis has
> also become very good, and if the value is not used afterwards,
> I expect no difference in practice.

When one writes::

     for( uint64_t i = 0; i < max; i++ )

the lifetime of i is explicit--it terminates with the loop.

>> Without such a restriction, there are many times when it would be very 
>> natural to inspect the index in order to determine if this was a normal
>> 
>> (counting) exit, or an early exit due to some internal test.

> Hmm... do you mean for the programmer, or for the compiler?