[fpc-devel] What exactly are the differences in what each of the "-Cp", "-Cf", and "-Op" command line flags enable as far as optimizations?

[J. Gareth Moreton]

The gist of it is that you're specifying the earliest CPU that your 
program will run on (if it runs on an older CPU, it might crash with 
SIGILL).  -Cp dictates the instruction set version used overall, while 
-Cf specifies the family of floating-point instructions that will be 
used.  Later versions of SSE have better rounding functions, for 
example, although I think it's more applicable to i386 where the older 
floating-point stack is used by default.

Generally you'd set -Cp and -Op to the same value, and I think -Op is 
set to match -Cp if it's omitted.  I might need Florian to confirm 
though.  I have programmed one or two optimisations that depend on the 
value of -Op - the one that springs to mind is where a group of MOV 
instructions are morphed into an XCHG instruction - XCHG is slower than 
the 3 MOV instructions until relatively recently, so it won't perform 
this optimisation by default unless optimising for size or if -Op is set 
to a particular value (I think at least "CoreI").

When vectorisation starts making a proper appearance in the compiler, 
I'll be very likely developing optimisations for fused multiply-add 
(FMA), which only came about after AVX.

Gareth aka. Kit

On 08/09/2020 00:03, Ben Grasset via fpc-devel wrote:
> For example, a valid FPC command line would be:
>
> fpc -O3 -CfAVX2 -CpCOREAVX2 -OpCOREAVX2 file.pas
>
> To what extent is each of those flags enabling the generation of AVX2 
> instructions? Are they all necessary, or does their functionality 
> overlap to some degree? Which is most important?
>
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