lkml.org 
[lkml]   [2015]   [May]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC] Design for flag bit outputs from asms
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


\
 
 \ /
  Last update: 2015-05-05 20:01    [W:0.130 / U:0.596 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site