Re: Question on BIGGEST_ALIGNMENT in GCC on NetBSD/m68k

Hi,

On 13.6.2025 4.36, Finn Thain wrote:
> And therein lies the rub -- to identify those workloads which should be
> measured and to afford each one a suitable weight in your decision making.

It's not just workload affecting the results; compiler version, 
optimization options [1], workload & kernel config options and sometimes 
even unrelated code changes [2], can affect how given instruction 
sequence settles into cache.


 > That's why this was always political.

I'd rather keep things technical and fact-based.

Whatever testing is done, the one wider conclusion that *can* be drawn 
from it, is that if there's a noticeable performance difference, such 
differences are possible also in other workloads.

(Very large difference could indicate also functional issues, e.g. bug 
in given compiler build code generation.  That's why it's important to 
have good tooling for pinpointing what exactly is causing the difference.)


	- Eero

[1] One example is -Os vs. -O2 having 2x perf impact on Geert's 
experimental Atari drm fb code.  That would completely hide any impact 
from alignment.

[2] with more complex cache hierarchies than on m68k, adding or removing 
code elsewhere can impact cache line alignment on other parts of the 
resulting binary.  Not a concern for m68k though.