On Thu, Oct 03, 2019 at 10:35:15AM +0100, Richard Earnshaw (lists) wrote:
> On 03/10/2019 00:01, Segher Boessenkool wrote:
> >The documentation says
> >
> >@findex ALL_REGS
> >@findex NO_REGS
> >In general, each register will belong to several classes. In fact, one
> >class must be named @code{ALL_REGS} and contain all the registers. Another
> >class must be named @code{NO_REGS} and contain no registers. Often the
> >union of two classes will be another class; however, this is not required.
> >
> >so is the arm port wrong, or is the documentation wrong? I think the arm
> >port simply forgets to include CC_REG, VFPCC_REG, SFP_REG, AFP_REG:
> >
> >#define REG_CLASS_CONTENTS \
> >...
> > { 0x00000000, 0x00000000, 0x00000000, 0x00000010 }, /* CC_REG */ \
> > { 0x00000000, 0x00000000, 0x00000000, 0x00000020 }, /* VFPCC_REG */ \
> > { 0x00000000, 0x00000000, 0x00000000, 0x00000040 }, /* SFP_REG */ \
> > { 0x00000000, 0x00000000, 0x00000000, 0x00000080 }, /* AFP_REG */ \
> > { 0xFFFF7FFF, 0xFFFFFFFF, 0xFFFFFFFF, 0x0000000F } /* ALL_REGS */ \
> >
> >and that last number should be 0x000000ff instead?
>
> Well CC_REG is completely fixed and can't be used for anything other
> than condition flags. The register allocator should never be
> considering it for allocation. So at that level, it really shouldn't
> matter and adding it would just make more useless work for the register
> allocator.
>
> The omission of SFP_REG and AFP_REG is probably also deliberate. These
> are essentially special pseudo regs that MUST be eliminated during frame
> layout. They should certainly never be considered for general allocation.
But ALL_REGS should contain *all* registers. The union of any two register
classes has to be contained in some register class, so every register class
has to be contained in ALL_REGS.
Segher
