Re: update_stmt calls

[Andrew MacLeod <amacleod at redhat dot com>]

Zdenek Dvorak wrote:
> Hello,
>
> during a recent discussion, it was pointed to my attention that
> update_stmt is performance critical.  I wondered why; this is the number
> of update_stmt calls for combine.i (all the other passes have less then
> 1000 calls):
>   

> <...>
>   

> I have a patch that decreases number of update_stmt calls in tree alias
> analysis to 46525; still, is it really that useful to run pass_may_alias
> *six* times during compilation?  Obviously, we need the initial one, and
> there are comments after pass_sra and pass_fold_builtins that indicate
> that the following pass_may_alias cannot be avoided (which seems
> doubtful to me, at the very least in the later case), but the remaining
> three seem to be just placed randomly.
>
> I also have a patch that decreases the number of update_stmt calls
> in VRP to 5229 (which is more or less the number of ASSERT_EXPRs it
> creates, so this cannot be improved significantly).
>   

I can't say I'm suprised, there was a time when the general rule was 
"when in doubt, update", so not a lot of thought has gone into those 
calls, especially in the older passes. Anything which reduces the calls 
to update_stmt() is a probably good thing :-)

I can't speak to the number of calls to pass_may_alias, but that does 
seem a bit excessive to me as well.  To the best of my knowledge, no one 
has recently (if ever) sat down and figured out which passes we actually 
need where and when. They just get added when people think they are a 
good idea, but rarely get removed later when things change. I would 
suspect there are numerous passes that could be eliminated with some 
analysis and shuffling.  Thats something that would be easier to do with 
a dynamic pass manager :-)

Andrew