[Bug target/103353] Indefinite recursion when compiling -mmma requiring testcase w/ -maltivec

["segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103353

--- Comment #7 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Kewen Lin from comment #5)
> (In reply to Segher Boessenkool from comment #4)
> > You miss all extra errors the expand_call can generate.  This is the general
> > reason why we try to continue instead of stopping after the first error.  The
> > reason is that later errors may be more obvious to the user.  This of course
> > does no longer work so well because our errors now take 30 lines instead of
> > 1.
> 
> Thanks for the explanation! One consequent question is that this point can
> be applied for the other places where some expected conditions don't hold
> for bif expansion, but I saw the other places are using "return const0_rtx".
> Is there something special causing this difference?

Not really, no.  In general we try to continue a bit longer (like, evaluate
the arguments, as here).  This gives much better diagnostics to the user.  In
a few cases you just have to give up early though, for practical reasons.

> > It probably is best if the generic opaque-mode emit_move code does not try
> > to move it via some other mode_class.  Peter?
> > 
> > Failing that, we can work around it by having move patterns for those modes
> > always, but hard erroring on them (FAIL is no good).
> 
> Yeah, one workround can help the ICE gone: (similar thing needed for XOmode
> as well):
> 
> diff --git a/gcc/config/rs6000/mma.md b/gcc/config/rs6000/mma.md
> index 907c9d6d516..04e887ad147 100644
> --- a/gcc/config/rs6000/mma.md
> +++ b/gcc/config/rs6000/mma.md
> @@ -268,10 +268,12 @@ (define_int_attr avvi4i4i4 [(UNSPEC_MMA_PMXVI8GER4PP  
> "pmxvi8ger4pp")
>  (define_expand "movoo"
>    [(set (match_operand:OO 0 "nonimmediate_operand")
>          (match_operand:OO 1 "input_operand"))]
> -  "TARGET_MMA"
> +  ""
>  {
> -  rs6000_emit_move (operands[0], operands[1], OOmode);
> -  DONE;
> +  if (TARGET_MMA) {
> +    rs6000_emit_move (operands[0], operands[1], OOmode);
> +    DONE;
> +  }
>  })

Like that.  But with a big fat comment, what is done when !TARGET_MMA, and
why we do that.  It is arguably the completely wrong thing to do for opaque
modes.