Register variables and optimizations for x86_64
lazarus / 67-Windows

----------------------------------------------------------------------
440bx · 2018-11-29 16:38

Hello,

I'd like to tell FPC to place one or more variables in registers. Is
this possible and if so how ?

The other question I have is, is it possible to tell the compiler to
optimize for the x86_64 architecture ?... those CPUs/mode have more
registers available that the compiler can use to optimize code. 

I tried -Or but, it didn't seem to have any effect  (maybe I missed
something else though)

Basically, what's the "magic" switch sauce that will produce the best
possible code (tight and fast) for Windows 64bit applications ?

Thank you for your help.

----------------------------------------------------------------------
Nitorami · 2018-11-29 17:10

I would expect that with -OoREGVAR set, fpc uses registers for
optimizing function calls. This does make a noticeable difference to
me when calling procedures in tight loops.
You may also tell the compiler to additionally use EBP and RBP, see
here
http://wiki.freepascal.org/Optimization.
Regarding these, I did not notice any difference so far in my
programs, but I guess any effects may be very code specific.

----------------------------------------------------------------------
440bx · 2018-11-29 17:33

Quote from: Nitorami on November 29, 2018, 06:10:28 pmI would expect
that with -OoREGVAR set, fpc uses registers for optimizing function
calls. This does make a noticeable difference to me when calling
procedures in tight loops.

I'll give that -OoREGVAR a try.  Thank you for the suggestion.

I wish there was a way to tell the compiler to put a specific variable
(or variables) in a register.  There are times when that would not
only make the code faster but also, a good bit smaller.