In GCC, what are "tree optimizations" vs "RTL optimizations"?

Question

When I file a bug against GCC about a missed optimization, I am always at a loss whether I should file it under "rtl-optimizations" or "tree-optimizations". I looked at the components page on the GCC bugzilla, and it isn't really explained there.

So, what are these two kinds of optimizations, or rather what distinguishes between them?

Answer 1

Partial answer:

The distinction has to do with different internal representations of (semi-compiled) programs within the GCC compiler. This is actually a rather complex issue, but for the purposes of this question:

• There's a abstact-syntax-tree-based representation called GIMPLE. Some optimization passes are made on this representation; see this list. These are known as Tree Optimizations or Tree-SSA Optimizations.
• A later and "lower-level" representation is in the Register Transfer Language (or RTL). Other optimization passes are made on this representation, later in the process than the tree optimizations; here's a list of these. These are call RTL optimizations.

That being said, I haven't given an easy rule-of-thumb for distinguishing which optimizations belong where.

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For a lot more information, you might want to read the GCC internals guide, where I've found the lists of optimization passes.