[Scons-users] env[X'] and signatures

[Bill Deegan]

Albert,

I'm very familiar with the EDA space.
What's the input file?
What environmnent variables changing would trigger a rebuild for you?

-Bill

On Thu, Dec 29, 2016 at 9:58 AM, Albert Arquer <albert.arquer at gmail.com>
wrote:

> I agree it is not the ideal solution. In fact we use scons for hdl
> synthesis/implementation which can take many hours.
>
> Before we used to make the target depend on the sconscript which declared
> the target. However this caused many innecessary rebuilds, as a single
> sconscript often declares multiple targets, and even if a change in the
> sconscript which only afected a single target was modified, scons would
> rebuild all other targets declared in the sconscript and any others which
> depended on them.
>
> By hashing the environment we can isolate sconscript modifications to the
> targets it actually affects. This is not ideal ofcourse, it would be much
> better for each builder to define somehow which items from the environment
> it will be using. However this problem usually causes build to be repeated,
> not missed, and still counting those cases the innecessary builts are
> drastically reduced (at least in our case).
>
>  There is also the issue on the hashing itself, since not everything is
> hashable in python, and even if an object is hashable that hash might not
> change if the object's class definition has changed for example. I opened
> this thread in stackoverflow a while back trying to find a serialization
> module suited for this purpose, turns out dill would be a good choice but
> it has compatibility issues with scons.
>
> http://stackoverflow.com/questions/40640797/compute-
> class-and-instance-hash/40775525#40775525
>
>
> El dj, 29 des 2016 a les 18:51 Bill Deegan <bill at baddogconsulting.com> va
> escriure:
>
>> As a workaround, perhaps add an action string which does nothing, but
>> uses the variables you depend on?
>> Just a thought.
>>
>>
>> Are the builders external python scripts which get run from the shell by
>> SCons? or are the builders python logic run inside scons?
>>
>> On Thu, Dec 29, 2016 at 8:54 AM, Tom Tanner <trtanner at btinternet.com>
>> wrote:
>>
>> On 27/12/16 18:22, Bill Deegan wrote:
>>
>>
>>
>>
>> If every builder depends on all values in env['ENV'], then any change
>> will rebuild everything.
>>
>>
>> This is almost never warranted.
>>
>>
>>
>>
>>
>>
>>
>> But then why does scons clear out the supplied environment? It is
>> precisely because everything behaves in a way that is unclear and not
>> reproducable if you don't know exactly what the environment is.
>>
>>
>>
>>
>> I think, ideally, a builder (Action really), would specify what parts of
>> the Environment() it's sensitive too.
>>
>>
>>
>>
>>
>>
>>
>> This would be better. If every builder specified which parts of
>> env['ENV'] it is interested in, and they (and nothing else) got passed
>> through and added to the build signature
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> Remember, while many users are only building source code which is fairly
>> quick, SCons is used in other "build" systems where steps can take
>> considerable time and resources so extraneous rebuilds are quite costly for
>> them.
>>
>>
>>
>>
>>
>> -Bill
>>
>>
>>
>>
>>
>>
>>
>> I agree but specifying the correct dependencies are how you avoid costly
>> rebuilds. Not rebuilding something that has changed can be just as costly
>> because instead of finding out about it at the build stage, you find out
>> about it when it is on a production machine.
>>
>>
>>
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