On 2023-10-07 18:09, J. Gareth Moreton via fpc-devel 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
OK. My results for the AMD A9 CPU mentioned previously and 32-bit trunk
compiler (Linux) are:
Pascal control case: 2.3 ns/call
Using LEA instruction: 1.2 ns/call
Using ADD instructions: 1.5 ns/call
The same machine, the same operating environment, but a 64-bit trunk
compiler:
Pascal control case: 3.6 ns/call
Using LEA instruction: 0.9 ns/call
Using ADD instructions: 1.3 ns/call
I tried compiling and running the test with all of FPC 2.0.4, 2.2.4,
2.4.4, 2.6.4, 3.0.4 and 3.2.2 on my Athlon machine and realized that all
results (for both the assembler and Pascal versions) compiled with
anything older than 3.2.2 are an order of magnitude faster than with
3.2.2 (i.e. less than 1 ns/call for the older versions compared to 8
ns/call with Pascal / 4 ns/call with assembler versions). This means
that the comparison is obviously spoiled with something unrelated.
Moreover, I noticed that when compiling with the highest level of
optimizations, the Pascal version compiled for i386 is as fast or even
little bit faster than the assembler version. I didn't do that
previously, thus the longer time for the older compiler version probably
isn't relevant. From this point of view, it probably doesn't make sense
to spend time on comparing the generated code.
Tomas
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@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
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