[LLVMdev] Thoughts about the llvm architecture

[Duncan Sands]

Hi Jochen,

>> Legalize and DAG combine already handle these cases.  Why would we want to duplicate the code?

legalization is a target specific process, thus it is done as part of code
generation.

> But what is the output of legalize and DAG combine? Is it llvm-ir again?

No, the input is a "Selection DAG", a data structure previously generated from
the LLVM IR.  The output is also a SDag.

> For example it seems that ligalization takes place after instcombine,
> but I would think it
> should happen before instcombine.

DAG combine is the selection DAG equivalent of instcombine, and runs after
legalization.

To summarize: target independent transforms occur on the LLVM IR.  Target
specific transforms occur when generating code for the target processor,
and work on completely different data structures.

Ciao,

Duncan.