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Re: RFA: tweak integer type used for memcpy folding


Richard Biener <richard.guenther@gmail.com> writes:
> On Tue, Apr 22, 2014 at 10:43 AM, Richard Sandiford
> <rdsandiford@googlemail.com> wrote:
>> Richard Biener <richard.guenther@gmail.com> writes:
>>> On Sat, Apr 19, 2014 at 9:51 AM, Richard Sandiford
>>> <rdsandiford@googlemail.com> wrote:
>>>> wide-int fails to build libitm because of a bad interaction between:
>>>>
>>>> /* Keep the OI and XI modes from confusing the compiler into thinking
>>>>    that these modes could actually be used for computation.  They are
>>>>    only holders for vectors during data movement.  */
>>>> #define MAX_BITSIZE_MODE_ANY_INT (128)
>>>>
>>>> and the memcpy folding code:
>>>>
>>>>       /* Make sure we are not copying using a floating-point mode or
>>>>          a type whose size possibly does not match its precision.  */
>>>>       if (FLOAT_MODE_P (TYPE_MODE (desttype))
>>>>           || TREE_CODE (desttype) == BOOLEAN_TYPE
>>>>           || TREE_CODE (desttype) == ENUMERAL_TYPE)
>>>>         {
>>>>           /* A more suitable int_mode_for_mode would return a vector
>>>>              integer mode for a vector float mode or a integer complex
>>>>              mode for a float complex mode if there isn't a regular
>>>>              integer mode covering the mode of desttype.  */
>>>>           enum machine_mode mode = int_mode_for_mode (TYPE_MODE (desttype));
>>>>           if (mode == BLKmode)
>>>>             desttype = NULL_TREE;
>>>>           else
>>>>             desttype = build_nonstandard_integer_type (GET_MODE_BITSIZE (mode),
>>>>                                                        1);
>>>>         }
>>>>       if (FLOAT_MODE_P (TYPE_MODE (srctype))
>>>>           || TREE_CODE (srctype) == BOOLEAN_TYPE
>>>>           || TREE_CODE (srctype) == ENUMERAL_TYPE)
>>>>         {
>>>>           enum machine_mode mode = int_mode_for_mode (TYPE_MODE (srctype));
>>>>           if (mode == BLKmode)
>>>>             srctype = NULL_TREE;
>>>>           else
>>>>             srctype = build_nonstandard_integer_type (GET_MODE_BITSIZE (mode),
>>>>                                                       1);
>>>>         }
>>>>
>>>> The failure occurs for complex long double, which we try to copy as
>>>> a 256-bit integer type (OImode).
>>>>
>>>> This patch tries to do what the comment suggests by introducing a new
>>>> form of int_mode_for_mode that replaces vector modes with vector modes
>>>> and complex modes with complex modes.  The fallback case of using a
>>>> MODE_INT is limited by MAX_FIXED_MODE_SIZE, so can never go above
>>>> 128 bits on x86_64.
>>>>
>>>> The question then is what to do about 128-bit types for i386.
>>>> MAX_FIXED_MODE_SIZE is 64 there, which says that int128_t shouldn't be
>>>> used for optimisation.  However, gcc.target/i386/pr49168-1.c only passes
>>>> for -m32 -msse2 because we use int128_t to copy a float128_t.
>>>>
>>>> I handled that by allowing MODE_VECTOR_INT to be used instead of
>>>> MODE_INT if the mode size is greater than MAX_FIXED_MODE_SIZE,
>>>> even if the original type wasn't a vector.
>>>
>>> Hmm.  Sounds reasonable unless there are very weird targets that
>>> cannot efficiently load/store vectors unaligned but can handle
>>> efficient load/store of unaligned scalars.
>>
>> Yeah, in general there's no guarantee that even int_mode_for_mode
>> will return a mode with the same alignment as the original.  Callers
>> need to check that (like the memcpy folder does).
>>
>>>> It might be that other callers to int_mode_for_mode should use
>>>> the new function too, but I'll look at that separately.
>>>>
>>>> I used the attached testcase (with printfs added to gcc) to check that
>>>> the right modes and types were being chosen.  The patch fixes the
>>>> complex float and complex double cases, since the integer type that we
>>>> previously picked had a larger alignment than the original complex type.
>>>
>>> As of complex int modes - are we positively sure that targets even
>>> try to do sth "optimal" for loads/stores of those?
>>
>> Complex modes usually aren't handled directly by .md patterns,
>> either int or float.  They're really treated as a pair of values.
>> So IMO it still makes sense to fold this case.
>>
>>>> One possibly subtle side-effect of FLOAT_MODE_P (TYPE_MODE (desttype))
>>>> is that vectors are copied as integer vectors if the target supports
>>>> them directly but are copied as float vectors otherwise, since in the
>>>> latter case the mode will be BLKmode.  E.g. the 1024-bit vectors in the
>>>> test are copied as vector floats and vector doubles both before and
>>>> after the patch.
>>>
>>> That wasn't intended ... the folding should have failed if we can't
>>> copy using an integer mode ...
>>
>> Does that mean that the fold give up if TYPE_MODE is BLKmode?
>> I can do that as a separate patch if so.
>
> Looking at the code again it should always choose an integer mode/type
> via setting desttype/srctype to NULL for BLKmode and
>
>       if (!srctype)
>         srctype = desttype;
>       if (!desttype)
>         desttype = srctype;
>       if (!srctype)
>         return NULL_TREE;
>
> no?  Thus if we can't get a integer type for either src or dest then
> we fail.  But we should never end up with srctype or desttype
> being a float mode.  No?

Right, they don't have a float _mode_, because the target doesn't
support 1024-bit modes.  Like I say, they have BLKmode instead.
But they have a float type.

Thanks,
Richard


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