Re: [RFC] Design for flag bit outputs from asms

[Segher Boessenkool]

On Mon, May 04, 2015 at 12:33:38PM -0700, Richard Henderson wrote:
> (1) Each target defines a set of constraint strings,

> (2) A new target hook post-processes the asm_insn, looking for the
>     new constraint strings.  The hook expands the condition prescribed
>     by the string, adjusting the asm_insn as required.

Since it is pre-processed, there is no real reason to overlap this with
the constraints namespace; we could have e.g. "=@[xy]" (and "@[xy]" for
inputs) mean the target needs to do some "xy" transform here.

>   Note that the output constraints are adjusted to a single internal "=j_"
>   which would match the flags register in any mode.  We can collapse
>   several output flags to a single set of the flags hard register.

Many targets would use an already existing contraint that describes the
flags.  Targets that need a fixed register could just insert the hard
register here as far as I see?  (I'm assuming this happens at expand time).

> (3) Note that ppc is both easier and more complicated.
> 
>   There we have 8 4-bit registers, although most of the integer
>   non-comparisons only write to CR0.  And the vector non-comparisons
>   only write to CR1, though of course that's of less interest in the
>   context of kernel code.
> 
>   For the purposes of cr0, the same scheme could certainly work, although
>   the hook would not insert a hard register use, but rather a pseudo to
>   be allocated to cr0 (constaint "x").

And "y" for "any CR field".

>   That said, it's my understanding that "dot insns", setting cr0 are
>   expensive in current processor generations.

They are not.  (Cell BE is not "current" :-) )

PowerPC also has some other bits (the carry bit for example, CA) that
could be usefully exposed via this mechanism.

> Can anyone think of any drawbacks, pitfalls, or portability issues to less
> popular targets that I havn't considered?

I don't like co-opting the constraint names for this; other than that, it
looks quite good :-)


Segher
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