From: sfuld@alumni.cmu.edu.invalid
On 9/4/2025 8:23 AM, MitchAlsup wrote:
>
> MitchAlsup posted:
>
>>
>> anton@mips.complang.tuwien.ac.at (Anton Ertl) posted:
>>
>>> BGB writes:
>>>> But, it seems to have a few obvious weak points for RISC-V:
>>>> Crappy with arrays;
>>>> Crappy with code with lots of large immediate values;
>>>> Crappy with code which mostly works using lots of global variables;
>>>> Say, for example, a lot of Apogee / 3D Realms code;
>>>> They sure do like using lots of global variables.
>>>> id Software also likes globals, but not as much.
>>>> ...
>>>
>>> Let's see:
>>>
>>> #include
>>>
>>> long arrays(long *v, size_t n)
>>> {
>>> long i, r;
>>> for (i=0, r=0; i>> r+=v[i];
>>> return r;
>>> }
>>
>> arrays:
>> MOV R3,#0
>> MOV R4,#0
>> VEC R5,{}
>> LDD R6,[R1,R3<<3]
>> ADD R4,R4,R6
>> LOOP LT,R3,#1,R2
>> MOV R1,R4
>> RET
>>
>>>
>>> long a, b, c, d;
>>>
>>> void globals(void)
>>> {
>>> a = 0x1234567890abcdefL;
>>> b = 0xcdef1234567890abL;
>>> c = 0x567890abcdef1234L;
>>> d = 0x5678901234abcdefL;
>>> }
>>
>> globals:
>> STD #0x1234567890abcdef,[ip,a-.]
>> STD #0xcdef1234567890ab,[ip,b-.]
>> STD #0x567890abcdef1234,[ip,c-.]
>> STD #0x5678901234abcdef,[ip,d-.]
>> RET
>>>
>> -----------------
>>>
>>> So, the overall sizes (including data size for globals() on RV64GC) are:
>>> Bytes Instructions
>>> arrays globals Architecture arrays globals
>>> 28 66 (34+32) RV64GC 12 9
>>> 27 69 AMD64 11 9
>>> 44 84 ARM A64 11 22
>> 32 68 My 66000 8 5
>
> In light of the above, what do people think is more important, small
> code size or fewer instructions ??
>
> At some scale, smaller code size is beneficial, but once the implementation
> has a GBOoO µarchitecture, I would think that fewer instructions is better
> than smaller code--so long as the code size is less than 150% of the smaller
> AND so long as the ISA does not resort to sequential decode (i.e., VAX).
>
> What say ye !
In general yes, but as you pointed out in another post, if you are
talking about a GBOoO machine, it isn't the absolute number of
instructions (because of parallel execution), but the number of cycles
to execute a particular routine. Of course, this is harder to tell at a
glance from a code listing.
And, of course your "150%" is arbitrary, but I agree that small
differences in code size are not important, except in some small
embedded applications.
And I guess I would add, as a third, much lower priority, power usage.
--
- 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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