[stage1][PATCH] Lower VEC_COND_EXPR into internal functions.

Segher Boessenkool segher@kernel.crashing.org
Fri May 29 17:37:58 GMT 2020

On Fri, May 29, 2020 at 06:26:55PM +0100, Richard Sandiford wrote:
> Segher Boessenkool <segher@kernel.crashing.org> writes:
> > Most patterns *do* FAIL on some target.  We cannot rewind time.
> Sure.  But the point is that FAILing isn't “explicitly allowed” for vcond*.
> In fact it's the opposite.

It has FAILed on rs6000 since 2004.

> If we ignore the docs and look at what the status quo actually is --
> which I agree seems safest for GCC :-) -- then patterns are allowed to
> FAIL if target-independent code provides an expand-time fallback for
> the FAILing case.  But that isn't true for vcond either.

That is a bug in the callers then :-)

> expand_vec_cond_expr does:
>   icode = get_vcond_icode (mode, cmp_op_mode, unsignedp);
>   if (icode == CODE_FOR_nothing)
>     ...
>   comparison = vector_compare_rtx (VOIDmode, tcode, op0a, op0b, unsignedp,
> 				   icode, 4);
>   rtx_op1 = expand_normal (op1);
>   rtx_op2 = expand_normal (op2);
>   create_output_operand (&ops[0], target, mode);
>   create_input_operand (&ops[1], rtx_op1, mode);
>   create_input_operand (&ops[2], rtx_op2, mode);
>   create_fixed_operand (&ops[3], comparison);
>   create_fixed_operand (&ops[4], XEXP (comparison, 0));
>   create_fixed_operand (&ops[5], XEXP (comparison, 1));
>   expand_insn (icode, 6, ops);
>   return ops[0].value;
> which ICEs if the expander FAILs.
> So whether you go from the docs or from what's actually implemented,
> vcond* isn't currently allowed to FAIL.  All Richard's gcc_unreachable
> suggestion would do is change where the ICE happens.

>   icode = get_vcond_icode (mode, cmp_op_mode, unsignedp);
>   if (icode == CODE_FOR_nothing)
>     ...

Of course it is allowed to FAIL, based on this code.  That is: the RTL
pattern is allowed to FAIL.  Whatever optabs do, I never understood :-)

Is this vec_cmp that is used by the fallback?  That will never FAIL
for us (if it is enabled at all, natch, same as for any other target).


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