linuxaudio.org

On Wed, Jul 20, 2011 at 10:47 AM, Robin Gareus  wrote:

quoted text
>> On gcc/Linux, (gcc 4.5.2) the same code produce a *slow down* of around

>> 2.5x.

>>

>> Well, anybody have an idea of why ?

>>

>> I am actually running linux (Ubuntu 11.04) under a VMWare virtual

>> machine, i do not know is this may have any implications.

>

> Maybe. A better comparison would be: clang/Linux vs. gcc/Linux and

> clang/MacOSX vs gcc/MacOSX compiled binaries.

>

> Also as Dan already pointed out: gcc has a whole lot of optimization

> flags which are not enabled by default. try '-O3 -msse2 -ffast-math'.

>  '-ftree-vectorizer-verbose=2' is handy while optimizing code.

In addition... inspecting the disassembly is helpful (-S -o

myprogram.s).  Rule of thumb is that you should have `movaps` (MOVe

Aligned Packed-Storage) and `mulps` (MULtiply Packed Storage)

instructions for multiplying vectors of single-precision floats.
In addition... profiling with valgrind/callgrind is helpful (esp. if

you have it dump instructions/assembly)...
  $ valgrind --tool=callgrind --dump-instr=yes ./myprogram
Open the output file with kcachegrind and it'll save you a lot of time.
-gabriel

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