Re: [tree-ssa] Lazy updating of stmt operands

[law at redhat dot com]

In message <1070979982.17702.2610.camel@p4>, Andrew MacLeod writes:
 >So use get_immidiate_uses() which does what is required on demand, and
 >if we discover its a serious performance issue down the road, it ought
 >to be easy enough to change. The interface is abstracted out already.
 >since operands are cached, a pass through the IL is actually pretty
 >cheap. Its nothing like the expense of a pass through rtl.
Precisely.


 >The effort and memory required to keep this information up to date
 >throughout all compilations would not be insignificant, and until it is
 >demonstrated that its a win to do so, I wouldn't want to keep it around
 >all the time.
Right.  We already went down that path.  It was completely unusable.


 >CCP uses the information, and one of the big compile time and memory
 >savings on that pass was to optimize the immediate_uses code to only
 >build it for SSA_NAMEs which we actually cared about. It was a
 >significant memory hit to build it and have it around for all stmts. 
 >
 >It would also be possible down the road, if it is an issue, to maintain
 >the immediate uses information only for as long as it is required. 
 >Ie, get_stmt_operands could update the immediate_uses info if its
 >present and we want to keep it up to date. Then if you have a string of
 >optimizations that want the information, you can keep it up to date
 >automatically. 
Precisely.

jeff