Vincent Labrecque wrote:
>
> On Oct 7, 2009, at 7:54 AM, Vincent Labrecque wrote:
>>
>> And as I'm writing this, with regards to optimization flags, maybe the
>> sage build system should provide an abstraction over -On. As I'm
>> sure you know, these have widely differing behaviors across compilers.
>> Sometimes -O3 will do nothing, sometimes it will break your code.
>> ``high'', ``medium'' and ``low'' can be translated by an expert on
>> the particular platform (os/compiler/compiler-version).
>
> This probably should even be per-package. In particular with packages
> doing complex operation chains. It could be made easy for a package to
> change the optimization level.
>
> To put what I tried to explain in the last post in practice, here's over
> egineering in the form of a helper program, where packages can downgrade
> optimization levels if they know better:
>
> eval `./interpret-optimization -O0 -O2 -O3`
> #runs CFLAGS_OPTIMIZATION_LOW=-O0; export CFLAGS_OPTIMIZATION_LOW
> # CFLAGS_OPTIMIZATION_MEDIUM=-O2; export
> CFLAGS_OPTIMIZATION_MEDIUM
> # CFLAGS_OPTIMIZATION_HIGH=-O3; export CFLAGS_OPTIMIZATION_MEDIUM
>
> # Here's what could happen if another part of the build system decided
> that anything over -O3 was unsafe.
> CFLAGS_OPTIMIZATION_HIGH=-O2
> eval `./interpret-optimization -O1 -O2 -O4`
> # runs CFLAGS_OPTIMIZATION_LOW=-O1; export CFLAGS_OPTIMIZATION_LOW
> # CFLAGS_OPTIMIZATION_MEDIUM=-O2; export
> CFLAGS_OPTIMIZATION_MEDIUM
>
> Maybe one would want to also make sure low <= medium <= high at all
> times, which would require a few more lines.
> Keep in mind that this was only barely tested. Anyway. Here's how you
> can unintrusively share environment variables.
>
> --Vincent
>
> #!/bin/sh
> new_low=${1-"-O0"}; new_medium=${2-"-O2"}; new_high=${3-"-O3"}
> current_low=${CFLAGS_OPTIMIZATION_LOW-"-O10"}
> current_medium=${CFLAGS_OPTIMIZATION_MEDIUM-"-O10"}
> current_high=${CFLAGS_OPTIMIZATION_HIGH-"-O10"}
>
> new_low_n=`echo $new_low | sed 's/^-O//'`
> new_medium_n=`echo $new_medium | sed 's/^-O//'`
> new_high_n=`echo $new_high | sed 's/^-O//'`
>
> current_low_n=`echo $current_low | sed 's/^-O//'`;
> current_medium_n=`echo $current_medium | sed 's/^-O//'`
> current_high_n=`echo $current_high | sed 's/-O//'`
>
> [ "$current_low_n" -gt "$new_low_n" ] && echo
> 'CFLAGS_OPTIMIZATION_LOW='"$new_low"'; export CFLAGS_OPTIMIZATION_LOW'
> [ "$current_medium_n" -gt "$new_medium_n" ] && echo
> 'CFLAGS_OPTIMIZATION_MEDIUM='"$new_medium"'; export
> CFLAGS_OPTIMIZATION_MEDIUM'
> [ "$current_high_n" -gt "$new_high_n" ] && echo
> 'CFLAGS_OPTIMIZATION_HIGH='"$new_high"'; export CFLAGS_OPTIMIZATION_HIGH'
The problem is -O4 is not necessarily the best if you want high
optimisation. Take for example the Intel compiler for linux and OS X, it
has things like
-mtune=inanium2-p9000
-axsse2
-axeee3
-axss3_ATOM
I believe, if someone wants to optimise the code, then they should set
them just an environment variable to exactly what they want.
But, in all cases, we have sensible defaults.
Dave
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