Re: Misc: Ongoing status...

On Sat, 1 Feb 2025 22:42:39 +0000, BGB wrote:

On 1/31/2025 10:05 PM, MitchAlsup1 wrote:

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Whereas, if performance is dominated by a piece of code that looks like,
say:
   v0=dytf_int2fixnum(123);
   v1=dytf_int2fixnum(456);
   v2=dytf_mul(v0, v1);
   v3=dytf_int2fixnum(789);
   v4=dytf_add(v2, v3);
   v5=dytf_wrapsymbol("x");
   dytf_storeindex(obj, v5, v4);
   ...
With, say, N levels of call-graph in each called function, but with this
sort of code still managing to dominate the total CPU ("Self%" time).
>
This seems to be a situation where callee-save registers are a big win
for performance IME.

With callee save registers, the prologue and epilogue of subroutines
sees all the save/restore memory traffic; sometimes saving a register
that is not "in use" and restoring it later.
With caller save registers, the caller saves exactly the registers
it needs preserved, while the callee saves/restores none. Moreover
it only saves registers currently "in use" and may defer restoring
since it does not need that value in that register for a while.
So, the instruction path length has a better story in caller saves
than callee saves. Nothing that was "Not live" is ever saved or
restored.
The arguments for callee save have to do with I cache footprint.