[LLVMdev] specifying accumulator based load/stores

[Sanjiv Gupta]

Thanks Evan.
I checked into and found that it is spilling the "acc" to stack slot and
reloading it later.
How do I tell it to rematerialize constants into "acc" rather than to spill
/ reload the "acc".


Thanks,
sanjiv

On 1/18/08, Evan Cheng <evan.cheng at apple.com> wrote:
>
>
> On Jan 17, 2008, at 4:12 AM, Sanjiv Gupta wrote:
>
> > I have load / store instructions that require accumulator.
> > So a store looks like..
> >
> >    mov 3, acc
> >    st acc, addr
> >
> > I have specified "acc" as a separate register class containing only
> > one register which is the "acc".
> > The instr patterns are then splitted into:
> >
> >    set imm:$src, ACCClass:$dst  (generating the "mov" above)
> >    set ACCClass:$src, mem:$dst  (generating the "st" above)
> >
> >
> > The problem with the generated code is incorrectly assuming that
> > value in "acc" is still live.
> > for example, for code like below
> >
> >    a = 3;
> >    b = 4;
> >    c = 3;
> >
> >
> > it generates:
> >
> >   mov 3, acc
> >   st  acc, @a
> >   mov 4, acc
> >   st acc, @b
> >   st acc, @c    (Wrong)
> >
> >
> > When I use the GPRRegs, which has more regs, the code is ok.
> > is it because I still haven't implemented the stack/frame related
> > routines and the value in acc is getting spilled to stack and
> > reloaded?
>
> It's very hard to tell what's going on without having access to your
> backend. If you dump out the machineinstrs just before register
> allocation and again after allocation but before spiller rewrite
> (VirtRegMap.cpp), it might be easier to see where things started going
> wrong.
>
> Evan
>
> >
> >
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