Re: RFC: Representing vector lane load/store operations

[Richard Sandiford <richard.sandiford@linaro.org>]

Richard Sandiford <richard.sandiford@linaro.org> writes:
> Richard Guenther <richard.guenther@gmail.com> writes:
>> Well, I meant if the user compiles with -msse, declares such a
>> global var (which means it gets V4SFmode and not BLKmode)
>> and then uses it in a function where he explicitly disables SSE
>> then something is wrong.  If he declares a BLKmode global
>> then generic vector support will happily trigger and make it work.
>
> Ah, OK.  I'm just not sure whether, to take a MIPS example,
> MIPS16 functions in a "-mno-mips16" compile should behave
> differently from unannotated functions in a "-mips16" compile.
>
>> If it's just three element array-of-vector types you need why not expose
>> it via attribute((mode(xyz))) only?  You could alias that mode to BLKmode
>> if neon is not enabled ...
>
> I don't think that really changes anything.  Getting the non-BLK mode
> on the array type seems like the easy part.  The difficult part is
> dealing with the fallout when the array is defined in a Neon context
> and used in a non-Neon context.

As a follow-up to this, I think the current definition of TYPE_MODE
is too restrictive even for the vector case.  Single-element structures
get the modes of their fields, and similarly for arrays.  So if we modify
the original 38240 testcase a bit, we still get a difference:

-------------------------------------------------------------------------
#if STRUCT
typedef struct {
  float x __attribute__ ((__vector_size__ (16), __may_alias__));
} V;
#else
typedef float V __attribute__ ((__vector_size__ (16), __may_alias__));
#endif

V __attribute__((target("sse"))) f(const V *ptr) { return *ptr; }
-------------------------------------------------------------------------

Without -DSTRUCT, this generates the same code regardless of whether
you compile with -msse.  But with -DSTRUCT, you get:

        movaps  (%rdi), %xmm0
        ret

with -msse and:

        movq    (%rdi), %rax
        movq    %rax, -24(%rsp)
        movq    8(%rdi), %rax
        movq    %rax, -16(%rsp)
        movdqa  -24(%rsp), %xmm0
        ret

with -mno-sse.

I think your argument is that most/all uses of TYPE_MODE are a mistake.
But I still think it makes sense to say that types have a natural mode
_in a given context_, just not globally.  So how about replacing it with
a current_mode_of_type function?  That makes it obvious that TYPE_MODE is
not a global property, and that it isn't really a simple accessor any more.
We could then make it recompute the mode for all types, possibly with a
cache if that's necessary for performance reasons.

Richard