From: sfuld@alumni.cmu.edu.invalid
On 12/28/2025 1:32 PM, MitchAlsup wrote:
>
> Stephen Fuld posted:
>
>> On 12/28/2025 8:22 AM, John Savard wrote:
>>> On Mon, 22 Dec 2025 20:00:06 +0000, Waldek Hebisch wrote:
>>>> John Savard wrote:
>>>>> On Thu, 18 Dec 2025 22:25:08 +0000, Anton Ertl wrote:
>>>>>
>>>>>> It is certainly possible to decode potential instructions at every
>>>>>> starting position in parallel, and later select the ones that actually
>>>>>> correspond to the end of the previous instruction,
>>>>>
>>>>> Oh, yes, I had always realized that, but dismissed it as far too
>>>>> wasteful.
>>>>
>>>> Well, Mitch claims average 35 bits per instructions, that means about
>>>> 90% utilization of decoders, so not bad.
>>>
>>> His minimum instruction size is 32 bits, but I was going for 16 bits.
>>>
>>>> Also, consider that alternative to variable length instructions is to
>>>> use longer instructions or more of them.
>>>
>>> What I did instead was use variable-length instructions, but add a prefix
>>> at the beginning of any 256-bit block of instructions that contained them
>>> which directly showed where each instruction began.
>>>
>>> My intent was to avoid the disadvantages you identify for fixed-length
>>> instructions, but avoid the disadvantage of variable-length instructions
>>> too.
>>
>> I understand your goal, however . . .
>>
>> How many bits do you "waste" on the prefix?
>
> For code like:: (undoctored compiler output)
>
> .LBB1_34:
> mov r1,#0
> br .LBB1_35
> .LBB1_39:
> mov r1,#28
> exit r16,r0,0,32
> .LBB1_36:
> mov r1,#0
> exit r16,r0,0,32
> .LBB1_37:
> call processRestart
> beq0 r1,.LBB1_38
> .LBB1_35:
> exit r16,r0,0,32
> .LBB1_38:
> lduh r1,[ip,gRestartsLeft]
> br .LBB1_2
> .Lfunc_end1:
>
> You are going to eat a lot of headers for 2 instruction BBs.
>
> Also:: Can you return to the middle of a block ??
> Can you make multiple CALLs from a single block ??
> Can you fit an entire loop in a single block ??
> Can you fit a loop with calls in a single block ??
> Does each unique label of a switch(i) require its own block ??
>
>> Since I think any branch must target the beginning of a block, and in
>> general, a routine will not end on a block boundary, there will be
>> "wasted" bits at the end of the last block before a "label". Have you
>> determined for a "typical" program, how many bits are wasted due to this?
>>
>> The point I am making is that you will "cancel" at least some of the
>> savings of 16 bit instructions. You should take this into account
>> before committing to your plan.
>
> I suspect the block-boundaries will consume more space than the 16-bit
> instructions can possibly save.
I agree with your suspicions. I was trying to get John to reach that
same conclusion. :-)
--
- Stephen Fuld
(e-mail address disguised to prevent spam)
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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