This is the mail archive of the
mailing list for the GCC project.
Re: RFA: tweak integer type used for memcpy folding
- From: Richard Sandiford <rdsandiford at googlemail dot com>
- To: Richard Biener <richard dot guenther at gmail dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Tue, 22 Apr 2014 09:43:51 +0100
- Subject: Re: RFA: tweak integer type used for memcpy folding
- Authentication-results: sourceware.org; auth=none
- References: <87oazxhi4m dot fsf at talisman dot default> <CAFiYyc2f-ynPHuky8Xgbxu6ksKr9g7=zsHUt1yjiGsD54o7vBA at mail dot gmail dot com>
Richard Biener <email@example.com> writes:
> On Sat, Apr 19, 2014 at 9:51 AM, Richard Sandiford
> <firstname.lastname@example.org> 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;
>> desttype = build_nonstandard_integer_type (GET_MODE_BITSIZE (mode),
>> 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;
>> srctype = build_nonstandard_integer_type (GET_MODE_BITSIZE (mode),
>> 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.