[Haskell-cafe] GHC optimisations

[Philip Armstrong]

Don's reply didn't reach me for some reason, but pulling it out of the
previous response:

>On 21/08/07, Donald Bruce Stewart <dons at cse.unsw.edu.au> wrote:
>> phil:
>> > The generated assembler suggests (if I've read it correctly) that gcc
>> > is spotting that it can replace the tail call with a jump in the C
>> > version, but for some reason it can't spot it for the Haskell version
>> > when compiling with -fvia-C (and neither does ghc itself using
>> > -fasm). So the haskell version ends up pushing and popping values on
>> > and off the stack for every call to f, which is a bit sad.
>> >
>>
>> That doesn't sound quite right. The C version should get a tail call ,
>> with gcc -O2, the Haskell version should be a tail call anyway.

Just to be clear; the Haskell version is a tail call, but it's pushing
the values to and from memory (well, cache really of course) for every
call to f, which is killing the performance.

>> Let's see:
>>
>> C
>>     $ gcc -O t.c -o t
>>     $ time ./t 1000000000
>>     zsh: segmentation fault (core dumped)  ./t 1000000000
>>     ./t 1000000000  0.02s user 0.22s system 5% cpu 4.640 total
>>
>> Turning on -O2
>>
>>     $ time ./t 1000000000
>>     -243309312
>>     ./t 1000000000  1.89s user 0.00s system 97% cpu 1.940 total

-O3 does better thanks to the loop unrolling, see timings bellow.

>> And GHC:
>>
>>     $ ghc -O2 A.hs -o A
>>     $ time ./A 1000000000
>>     -243309312
>>     ./A 1000000000  3.21s user 0.01s system 97% cpu 3.289 total
>>
>> So, what, 1.6x slower than gcc -O2
>> Seems ok without any tuning.

You're getting much better timings than I am!

$ time -p ./sum-hs 1000000000
-243309312
real 3.75
user 3.70
$ time -p ./sum-c-O2 1000000000
-243309312
real 1.40
user 1.35
$ time -p ./sum-c-O3 1000000000
-243309312
real 1.21
user 1.18

(My box has a AMD Athlon64 3000+ CPU fwiw, but the powerpc version is
even worse when compared to it's respective C binary!)

Phil

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