Re: "Mini" tags to reduce the number of op codes

On 4/9/2024 4:05 PM, MitchAlsup1 wrote:

BGB wrote:
 

On 4/9/2024 1:24 PM, Thomas Koenig wrote:

I wrote:
>

MitchAlsup1 <mitchalsup@aol.com> schrieb:

Thomas Koenig wrote:
>

Maybe one more thing: In order to justify the more complex encoding,
I was going for 64 registers, and that didn't work out too well
(missing bits).
>
Having learned about M-Core in the meantime, pure 32-register,
21-bit instruction ISA might actually work better.

 

For 32-bit instructions at least, 64 GPRs can work out OK.

 

Though, the gain of 64 over 32 seems to be fairly small for most "typical" code, mostly bringing a benefit if one is spending a lot of CPU time in functions that have large numbers of local variables all being used at the same time.

 

Seemingly:
16/32/48 bit instructions, with 32 GPRs, seems likely optimal for code density;
32/64/96 bit instructions, with 64 GPRs, seems likely optimal for performance.

 

Where, 16 GPRs isn't really enough (lots of register spills), and 128 GPRs is wasteful (would likely need lots of monster functions with 250+ local variables to make effective use of this, *, which probably isn't going to happen).

 16 GPRs would be "almost" enough if IP, SP, FP, TLS, GOT were not part of GPRs AND you have good access to constants.
 

On the main ISA's I had tried to generate code for, 16 GPRs was kind of a pain as it resulted in fairly high spill rates.
Though, it would probably be less bad if the compiler was able to use all of the registers at the same time without stepping on itself (such as dealing with register allocation involving scratch registers while also not conflicting with the use of function arguments, ...).
My code generators had typically only used callee save registers for variables in basic blocks which ended in a function call (in my compiler design, both function calls and branches terminating the current basic-block).
On SH, the main way of getting constants (larger than 8 bits) was via PC-relative memory loads, which kinda sucked.
This is slightly less bad on x86-64, since one can use memory operands with most instructions, and the CPU tends to deal fairly well with code that has lots of spill-and-fill. This along with instructions having access to 32-bit immediate values.

*: Where, it appears it is most efficient (for non-leaf functions) if the number of local variables is roughly twice that of the number of CPU registers. If more local variables than this, then spill/fill rate goes up significantly, and if less, then the registers aren't utilized as effectively.

 

Well, except in "tiny leaf" functions, where the criteria is instead that the number of local variables be less than the number of scratch registers. However, for many/most small leaf functions, the total number of variables isn't all that large either.

 The vast majority of leaf functions use less than 16 GPRs, given one has
a SP not part of GPRs {including arguments and return values}. Once one starts placing things like memove(), memset(), sin(), cos(), exp(), log()
in the ISA, it goes up even more.
 

Yeah.
Things like memcpy/memmove/memset/etc, are function calls in cases when not directly transformed into register load/store sequences.
Did end up with an intermediate "memcpy slide", which can handle medium size memcpy and memset style operations by branching into a slide.
As noted, on a 32 GPR machine, most leaf functions can fit entirely in scratch registers. On a 64 GPR machine, this percentage is slightly higher (but, not significantly, since there are few leaf functions remaining at this point).
If one had a 16 GPR machine with 6 usable scratch registers, it is a little harder though (as typically these need to cover both any variables used by the function, and any temporaries used, ...). There are a whole lot more leaf functions that exceed a limit of 6 than of 14.
But, say, a 32 GPR machine could still do well here.
Note that there are reasons why I don't claim 64 GPRs as a large performance advantage:
On programs like Doom, the difference is small at best.
It mostly effects things like GLQuake in my case, mostly because TKRA-GL has a lot of functions with a large numbers of local variables (some exceeding 100 local variables).
Partly though this is due to code that is highly inlined and unrolled and uses lots of variables tending to perform better in my case (and tightly looping code, with lots of small functions, not so much...).

 

Where, function categories:
   Tiny Leaf:
     Everything fits in scratch registers, no stack frame, no calls.
   Leaf:
     No function calls (either explicit or implicit);
     Will have a stack frame.
   Non-Leaf:
     May call functions, has a stack frame.

 You are forgetting about FP, GOT, TLS, and whatever resources are required
to do try-throw-catch stuff as demanded by the source language.
 

Yeah, possibly true.
In my case:
   There is no frame pointer, as BGBCC doesn't use one;
     All stack-frames are fixed size, VLA's and alloca use the heap;
   GOT, N/A in my ABI (stuff is GBR relative, but GBR is not a GPR);
   TLS, accessed via TBR.
Try/throw/catch:
   Mostly N/A for leaf functions.
Any function that can "throw", is in effect no longer a leaf function.
Implicitly, any function which uses "variant" or similar is also, no longer a leaf function.
Need for GBR save/restore effectively excludes a function from being tiny-leaf. This may happen, say, if a function accesses global variables and may be called as a function pointer.

There is a "static assign everything" case in my case, where all of the variables are statically assigned to registers (for the scope of the function). This case typically requires that everything fit into callee save registers, so (like the "tiny leaf" category, requires that the number of local variables is less than the available registers).

 

On a 32 register machine, if there are 14 available callee-save registers, the limit is 14 variables. On a 64 register machine, this limit might be 30 instead. This seems to have good coverage.

 The apparent number of registers goes up when one does not waste a register
to hold a use-once constant.

Possibly true. In the "static assign everything" case, each constant used is also assigned a register.
One "TODO" here would be to merge constants with the same "actual" value into the same register. At present, they will be duplicated if the types are sufficiently different (such as integer 0 vs NULL).
For functions with dynamic assignment, immediate values are more likely to be used. If the code-generator were clever, potentially it could exclude assigning registers to constants which are only used by instructions which can encode them directly as an immediate. Currently, BGBCC is not that clever.
Or, say:
   y=x+31;  //31 only being used here, and fits easily in an Imm9.
Ideally, compiler could realize 31 does not need a register here.
Well, and another weakness is with temporaries that exist as function arguments:
If static assigned, the "target variable directly to argument register" optimization can't be used (it ends up needing to go into a callee-save register and then be MOV'ed into the argument register; otherwise the compiler breaks...).
Though, I guess possible could be that the compiler could try to partition temporaries that are used exclusively as function arguments into a different category from "normal" temporaries (or those whose values may cross a basic-block boundary), and then avoid statically-assigning them (and somehow not cause this to effectively break the full-static-assignment scheme in the process).
Though, IIRC, I had also considered the possibility of a temporary "virtual assignment", allowing the argument value to be temporarily assigned to a function argument register, then going "poof" and disappearing when the function is called. Hadn't yet thought of a good way to add this logic to the register allocator though.
But, yeah, compiler stuff is really fiddly...