Thanks a lot for the explanation, that's very helpful! On Mon, Sep 7, 2020 at 9:02 PM J. Gareth Moreton via fpc-devel < fpc-devel@lists.freepascal.org> wrote:
> 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? > > _______________________________________________ > fpc-devel maillist - > fpc-devel@lists.freepascal.orghttps://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel > > > > <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient> > Virus-free. > www.avast.com > <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient> > <#m_-4071735898273218845_DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> > _______________________________________________ > fpc-devel maillist - fpc-devel@lists.freepascal.org > https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel >
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