Extended Description

Consider a function like this:

float foo(float X) { return X + 1234.4123f; }

The FP constant ends up in the constant pool, so we need to get the LR register.
This ends up producing code like this:

_foo:
.LBB_foo_0: ; entry
mflr r11
*** stw r11, 8(r1)
bl "L00000$pb"
"L00000$pb":
mflr r2
addis r2, r2, ha16(.CPI_foo_0-"L00000$pb")
lfs f0, lo16(.CPI_foo_0-"L00000$pb")(r2)
fadds f1, f1, f0
*** lwz r11, 8(r1)
mtlr r11
blr

This is functional, but there is no reason to spill the LR register all the way
to the stack (the two marked instrs): spilling it to a GPR is quite enough.

Implementing this will require some codegen improvements. Nate writes:

"So basically what we need to support the "no stack frame save and restore" is a
generalization of the LR optimization to "callee-save regs".

Currently, we have LR marked as a callee-save reg. The register allocator sees
that it's callee save, and spills it directly to the stack.

Ideally, something like this would happen:

LR would be in a separate register class from the GPRs. The class of LR would be
marked "unspillable". When the register allocator came across an unspillable
reg, it would ask "what is the best class to copy this into that I can spill"
If it gets a class back, which it will in this case (the gprs), it grabs a free
register of that class. If it is then later necessary to spill that reg, so be it.
"

This makes an incredible amount of sense to me. :)

-Chris