[tree-ssa] Lazy updating of stmt operands

[Chris Lattner]

Zdenek Dvorak wrote:
> suppose a statement is changed.  To update the immediate uses of this
> statement, you would need to go to the defining statement for each
> variable, find yourself in a (possibly very long) list of uses and
> make the update.  This would be a disaster for the performance.

This is exactly what I was talking about here:
http://gcc.gnu.org/ml/gcc/2003-12/msg00691.html

Imagine if the value you are using is the constant "0".  If you represent
use chains of the 0, there may be "several" instructions on the list.  The
lists can get large.  In LLVM, this is a constant time operation.

-Chris

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