[LLVMdev] Splitting Legalize?

[Chris Lattner]

On Fri, 13 Jul 2007, Dan Gohman wrote:
> While working on vector intrinsics, I found myself thinking about
> splitting the Legalize phase of codegen into two separate phases,
> legalizing types, and then legalizing operations in a separate
> traversal. Has anyone else thought about doing this, or knows of
> a reason why this would or wouldn't be beneficial?

Yes, that is an excellent idea.

One significant advantage of this is that we can then run a dag combine 
pass between the two.  Right now, dag combine sometimes misses some xforms 
because the operation-lowering piece custom lowers some operations to 
target-specific dag nodes that dag combine doesn't understand.  Splitting 
this up would allow an intermediate step for dag combine to simplify code.

Another thing on my long term todo list has been to switch legalize from 
being recursive to being iterative.  Right now, on some very large basic 
blocks, legalize runs out of stack space (also, if you're running the JIT 
in a threaded program, the JIT often runs on thread stacks that are 
comparatively small, which greatly exacerbates the issue).  Doing the 
first change would make the second one easier to do.

-Chris

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