Differences in results when using the same function in CTFE and Runtime

[Abdulhaq]

On Thursday, 15 August 2024 at 16:21:35 UTC, Abdulhaq wrote:
> On Thursday, 15 August 2024 at 09:13:31 UTC, Carsten Schlote

To clarify a bit more, I'm not just talking about single isolated 
computations, I'm talking about e.g. matrix multiplication. 
Different compilers, even LDC vs DMD for example, could optimise 
the calculation in a different way, loop unrolling, step 
elimination etc. even if the rounding algorithms etc. at the chip 
level are the same, the way the code is compiled and calculations 
sequenced will change the error in the final answer.

Then, variations in pipelining and caching at the processor level 
could also affect the answer.

And if you move on to different computing paradigms such as 
quantum computing and other as yet undiscovered techniques, again 
the way operations and rounding etc is compounded will cause 
divergences in computations.

Now, we could insist that we somehow legislate for the way 
compound calculations are conducted. But that would cripple the 
speed of calculations for some processor architecture / paradigms 
for a goal (reproduceability) which is worthy, but for 99% of 
usages not sufficiently beneficial to pay the big price in 
performance.