[fpc-devel] LEA instruction speed

[Nataraj S Narayan]

Hi

[nataraj at dflyHP ~]$ fpc ttt.pas
Free Pascal Compiler version 3.2.2 [2023/07/04] for x86_64
Copyright (c) 1993-2021 by Florian Klaempfl and others
Target OS: DragonFly for x86-64
Compiling ttt.pas
Linking ttt
/usr/local/bin/ld.bfd: warning:
/usr/local/lib/fpc/3.2.2/units/x86_64-dragonfly/rtl/prt0.o: missing
.note.GNU-stack section implies executable stack
/usr/local/bin/ld.bfd: NOTE: This behaviour is deprecated and will be
removed in a future version of the linker
121 lines compiled, 14.9 sec
[nataraj at dflyHP ~]$ ./ttt
   Pascal control case: 6.7 ns/call
 Using LEA instruction: 4.2 ns/call
Using ADD instructions: 4.0 ns/call


Nataraj S Narayan
Synergy Info Systems
Software & Technology Consultants
Ettumanoor, INDIA
Ph:+91 9443211326


On Sat, Oct 7, 2023 at 9:39 PM J. Gareth Moreton via fpc-devel <
fpc-devel at lists.freepascal.org> wrote:

> That's interesting; I am interested to see the assembly output for the
> Pascal control cases.  As for the 64-bit version, that was my fault
> since the assembly language is for Microsoft's ABI rather than the
> System V ABI, so it was checking a register with an undefined value.
> Find attached the fixed test.
>
> Kit
>
> P.S. Results on my Intel(R) Core(TM) i7-10750H
>
>     Pascal control case: 2.0 ns/call
>   Using LEA instruction: 1.7 ns/call
> Using ADD instructions: 1.3 ns/call
>
> On 07/10/2023 16:51, Tomas Hajny via fpc-devel wrote:
> > On 2023-10-07 03:57, J. Gareth Moreton via fpc-devel wrote:
> >
> >
> > Hi Kit,
> >
> >> Do you think this should suffice? Originally it ran for 1,000,000
> >> repetitions but I fear that will take way too long on a 486, so I
> >> reduced it to 10,000.
> >
> > OK, I tried it now. First of all, after turning on the old machine, I
> > realized that it wasn't Intel but AMD 486 DX4 - sorry for my bad
> > memory. :-( I compiled and ran the test under OS/2 there (I was too
> > lazy to boot it to DOS ;-) ), but I assume that it shouldn't make any
> > substantial difference. The ADD and LEA results were basically the
> > same there, both around 100 ns / call. The Pascal result was around
> > twice as long. Interestingly, the Pascal result for FPC 3.2.2 was
> > around 10% longer than the same source compiled with FPC 2.0.3 (the
> > assembler versions were obviously the same for both FPC versions; I
> > tried compiling it also with FPC 1.0.10 and the assembler versions
> > were more than three times slower due to missing support for the
> > nostackframe directive).
> >
> > I tested it under the AMD Athlon 1 GHz machine as well and again, the
> > results for LEA and ADD are basically equal (both 3.1 ns/call) and the
> > result for Pascal slightly more than twice (7.3 ns/call). However,
> > rather surprisingly for me, the overall test run was _much_ longer
> > there?! Finally, I tried compiling the test on a 64-bit machine (AMD
> > A9-9425) with Linux (compiled for 64-bits with FPC 3.2.3 compiled from
> > a fresh 3.2 branch). The Pascal version shows about 4 ns/call, but the
> > assembler version runs forever - well, certainly much longer than my
> > patience lasts. I haven't tried to analyze the reasons, but that's
> > what I get.
> >
> > Tomas
> >
> >
> >
> >>
> >> On 03/10/2023 06:30, Tomas Hajny via fpc-devel wrote:
> >>> On October 3, 2023 03:32:34 +0200, "J. Gareth Moreton via fpc-devel"
> >>> <fpc-devel at lists.freepascal.org> wrote:
> >>>
> >>>
> >>> Hii Kit,
> >>>
> >>>> This is mainly to Florian, but also to anyone else who can answer
> >>>> the question - at which point did a complex LEA instruction (using
> >>>> all three input operands and some other specific circumstances) get
> >>>> slow? Preliminary research suggests the 486 was when it gained
> >>>> extra latency, and then Sandy Bridge when it got particularly bad.
> >>>> Icy Lake seems to be the architecture where faster LEA instructions
> >>>> are reintroduced, but I'm not sure about AMD processors.
> >>> I cannot answer your question, but if you prepare a test program, I
> >>> can run it on an Intel 486 DX2 100 Mhz and AMD Athlon 1 GHz machines
> >>> if it helps you in any way (at least I hope the 486 DX2 machine
> >>> should be still able to start ;-) ).
> >>>
> >>> Tomas
> >>>
> >>> _______________________________________________
> >>> fpc-devel maillist  -  fpc-devel at lists.freepascal.org
> >>> https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
> >>>
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