Re: [PATCH] powerpc/dcr: Use cmplwi instead of 3-argument cmpli

Hi!

On Thu, Oct 14, 2021 at 01:44:24PM +1100, Michael Ellerman wrote:

In dcr-low.S we use cmpli with three arguments, instead of four
arguments as defined in the ISA:

	cmpli	cr0,r3,1024

This appears to be a PPC440-ism, looking at the "PPC440x5 CPU Core
User’s Manual" it shows cmpli having no L field, but implied to be 0 due
to the core being 32-bit. It mentions that the ISA defines four
arguments and recommends using cmplwi.

It also corresponds to the old POWER instruction set, which had no L
field there, a reserved bit instead.  It used to be that -many allowed
these insns as well, but not anymore.

Although gas is happy with the 3-argument version when building for
32-bit, the LLVM assembler is not and errors out with:

A GAS targeting powerpc64 isn't happy either, fwiw.

  arch/powerpc/sysdev/dcr-low.S:27:10: error: invalid operand for instruction
   cmpli 0,%r3,1024; ...
           ^

Switching to the four argument version avoids any confusion when reading
the ISA, fixes the issue with the LLVM assembler, and also means the
code could be built 64-bit in future (though that's very unlikely).

You are actually now using to the extended opcode cmpwli (a much better
plan :-) )

Thanks,


Segher