Builtins and C++ and such

[Roger Sayle]

> > it delays the optimization until RTL expansion so that tree-based
> > optimizations don't get confused, and it is pretty demonstrably safe
> > by looking at the body of the hook and nothing else.
>
> You don't want to delay things that long.  You want e.g.
> __builtin_abs to turn in to the ABS_EXPR tree code.

We also already perform several built-in optimizations at the tree
level.  For example, strlen("foo") and "strlen(ptr) > 0" are
transformed by fold().  Eventually, many more could be handled at
the tree level, "qsort" where n is zero or one, or transforming
STL container.size() == 0 calls into container.empty().

Roger
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