proposal for compilation unil wide alias analyis

[Zack Weinberg]

Mark Mitchell <mark@codesourcery.com> writes:

> I agree with the ease-of-maintenance argument, but I'm not sure it
> should triumph.  For speed of -O0 comilation, avoiding going into SSA
> form at all seems like it would be a win.  In other words, I'd expect
> that having the tree-to-RTL expanders take GENERIC -- or even
> near-generic with callbacks into the front end to expand non-GENERIC
> bits to GENERIC on the fly -- would be a win over forcing everything
> to be converted to SSA GIMPLE first.  The fewer passes we make over
> the tree structure, the better.

It seems to me that going into GIMPLE first would allow us to make the
RTL generator dramatically simpler - a definite maintenance win,
possibly also a performance win.  The same is probably not true of SSA
form, but I could make up a story for why it would be so ... which
converts this to another Laffer-curve argument.  (Cute analogy.)

> The Laffer curve argument (we can remove so much code in the
> optimizers so quickly that generating the code will actually be
> faster) seems to me like it is one that should be proven
> experimentally before we commit to it.

Agreement.

What about the 'we need this to do uninitialized-variable detection at
-O0 anyway' argument?

zw