Re: [RFC PATCH 4/8] powerpc/ppc_asm: use plain numbers for registers

Excerpts from Daniel Axtens's message of February 26, 2021 10:12 am:

Segher Boessenkool [off-list ref] writes:

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On Thu, Feb 25, 2021 at 02:10:02PM +1100, Daniel Axtens wrote:

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This is dumb but makes the llvm integrated assembler happy.
https://github.com/ClangBuiltLinux/linux/issues/764

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-#define	r0	%r0

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+#define	r0	0

This is a big step back (compare 9a13a524ba37).

If you use a new enough GAS, you can use the -mregnames option and just
say "r0" directly (so not define it at all, or define it to itself).

===
        addi 3,3,3
        addi r3,r3,3
        addi %r3,%r3,3

        addi 3,3,3
        addi r3,r3,r3
        addi %r3,%r3,%r3
===

$ as t.s -o t.o -mregnames
t.s: Assembler messages:
t.s:6: Warning: invalid register expression
t.s:7: Warning: invalid register expression


Many people do not like bare numbers.  It is a bit like not wearing
seatbelts (but so is all assembler code really: you just have to pay
attention).  A better argument is that it is harder to read for people
not used to assembler code like this.

We used to have "#define r0 0" etc., and that was quite problematic.
Like that "addi r3,r3,r3" example, but also, people wrote "r0" where
only a plain 0 is allowed (like in "lwzx r3,0,r3": "r0" would be
misleading there!)

So an overarching comment on all of these patches is that they're not
intended to be ready to merge, nor are they necessarily what I think is
the best solution. I'm just swinging a big hammer to see how far towards
LLVM_IAS=1 I can get on powerpc, and I accept I'm going to have to come
back and clean things up.

Anyway, noted, I'll push harder on trying to get llvm to accept %rN:
there was a patch that went in after llvm-11 that should help.

If you put it under ifdef CONFIG_CC_IS_CLANG in the meantime I think 
that would be okay. Then we get error checking with gcc compiles and
llvm at least builds with its assembler which would be nice.

Thanks,
Nick