[tree-ssa] Lazy updating of stmt operands

[Andrew MacLeod]

On Tue, 2003-12-16 at 17:20, Zdenek Dvorak wrote:
> Hello,
> 
> > > > defining stmt's annotation, so given any SSA variable, you can get to
> > > > the immediate uses by looking at the annotation for SSA_NAME_DEF_STMT.
> > > > It needs a marginal extention to deal with the fact that there can be
> > > > multiple defs/vdefs on one stmt, but we need to do that to handle
> > > > virtual defs anyway.  I would prefer to keep this information right with
> > > > the stmt rather than in a table on the side.
> > > 
> > > Immediate use is not a property of the statement, but the property of
> > > the ssa name; so it should be in the SSA_NAME, as my patch does,
> > > not in the statement annotations.
> > 
> > After pondering it, I have concluded you are correct, the immediate uses
> > info should be attached to the SSA_NAME.  The pragmatic reason is that
> > it eliminates the need to have the def stmt of the SSA_NAME in place
> > before the uses are seen... 
> > 
> > ie, keeping the information accurate when you are adding a new ssa_name
> > would *require* you to create the DEF first, or you have no place to put
> > the uses information.  Although its probably good practice to issue the
> > DEF first, enforcing that is pretty lame.  Putting it in the SSA_NAME
> > resolves ths problem since the SSA_NAME must exist in order to actually
> > occur in the stmt :-). 
> 
> my reason for having the information stored in the SSA_NAME actually
> was that there may be multiple definitions in a single statement,
> thus making it necessary to somehow determine of which of them is used
> in the statement recorded as the immediate use; this would be extremely
> cumbersome.
> 
I would have just associated it with the new operand infrastructure..
ie, you ask for the immediate uses info for a def index or vdef index,
since we have that index info available easily. It was maintaining it
that was the clincher for me.

Andrew