On Tue, Apr 22, 2014 at 11:15 AM, Richard Sandiford
<rdsandif...@googlemail.com> wrote:
> Richard Biener <richard.guent...@gmail.com> writes:
>> On Tue, Apr 22, 2014 at 10:43 AM, Richard Sandiford
>> <rdsandif...@googlemail.com> wrote:
>>> Richard Biener <richard.guent...@gmail.com> writes:
>>>> On Sat, Apr 19, 2014 at 9:51 AM, Richard Sandiford
>>>> <rdsandif...@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.
Ok. I clearly used FLOAT_MODE_P (TYPE_MODE ()) because
I assumed that only float-modes will eventually end up in FP
registers (that's what the bug was about - we load into a FP register
and that load can trigger a FP exception).
Richard.
> Thanks,
> Richard
