[LLVMdev] Register Dependencies and Register Allocation

Marc de Kruijf dekruijf at cs.wisc.edu
Tue Dec 23 13:03:22 CST 2008

I'm writing a back-end for an architecture that supports multi-word loads.
As a concrete example, "ldqw r0, [addr]" would load a quadword (4 words)
into 4 registers starting with r0 (implicit writes to r1, r2, and r3).

First, is there any currently supported architecture that has anything like
this?  I suspect not.  If not, I hope someone might help me figure out how
to make this work, particularly with the cooperation of the register
allocator?  In particular, I need the register allocator to understand that
there are multiple, contiguous register assignments, and that their
locations are moreover dependent on the specified initial input register.

I thought about defining a set of special register classes to group
contiguous registers for each load size (2, 4, and 8), but this doesn't feel
very satisfying.  Is this the right approach?  I'm wondering if it would
work, and if so, if it would also still be efficient.

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