[0/67] Add wrapper classes for machine_modes

[Jeff Law]

On 12/09/2016 05:48 AM, Richard Sandiford wrote:
> This series includes most of the changes in group C from:
> 
>      https://gcc.gnu.org/ml/gcc/2016-11/msg00033.html
> 
> The idea is to add wrapper classes around machine_mode_enum
> for specific groups of modes, such as scalar integers, scalar floats,
> complex values, etc.  This has two main benefits: one specific to SVE
> and one not.
> 
> The SVE-specific benefit is that it helps to introduce the concept
> of variable-length vectors.  To do that we need to change the size
> of a vector mode from being a known compile-time constant to being
> (possibly) a run-time invariant.  We then need to do the same for
> unconstrained machine_modes, which might or might not be vectors.
> Introducing these new constrained types means that we can continue
> to treat them as having a constant size.
> 
> The other benefit is that it uses static type checking to enforce
> conditions that are easily forgotten otherwise.  The most common
> sources of problems seem to be:
> 
> (a) using VOIDmode or BLKmode where a scalar integer was expected
>      (e.g. when getting the number of bits in the value).
> 
> (b) simplifying vector operations in ways that only make sense for
>      scalars.
> 
> The series helps with both of these, although we don't get the full
> benefit of (b) until variable-sized modes are introduced.
> 
> I know of three specific cases in which the static type checking
> forced fixes for things that turned out to be real bugs (although
> we didn't know that at the time, otherwise we'd have posted patches).
> They were later fixed for trunk by:
> 
>    https://gcc.gnu.org/ml/gcc-patches/2016-07/msg01783.html
>    https://gcc.gnu.org/ml/gcc-patches/2016-11/msg02983.html
>    https://gcc.gnu.org/ml/gcc-patches/2016-11/msg02896.html
> 
> The group C patches in ARM/sve-branch did slow compile time down a little.
> I've since taken steps to avoid that:
> 
> - Make the tailcall pass handle aggregate parameters and return values
>    (already in trunk).
> 
> - Turn some of the new wrapper functions into inline functions.
> 
> - Make all the machmode.h macros that used:
> 
>      __builtin_constant_p (M) ? foo_inline (M) : foo_array[M[
> 
>    forward to an ALWAYS_INLINE function, so that (a) M is only evaluated
>    once and (b) __builtin_constant_p is applied to a variable, and so is
>    deferred until later passes.  This helped the optimisation to fire in
>    more cases and to continue firing when M is a class rather than a
>    raw enum.
> 
> - In a similar vein, make sure that conditions like:
> 
>       SImode == DImode
> 
>    are treated as builtin_constant_p by gencondmd, so that .md patterns
>    with those conditions are dropped.
> 
> With these changes the series is actually a very slight compile-time win.
> That might seem unlikely, but there are several possible reasons:
> 
> 1. The machmode.h macro change above might allow more constant folding.
> 
> 2. The series has a tendency to evaluate modes once, rather than
>     continually fetching them from (sometimes quite deep) rtx nests.
>     Refetching a mode is a particular problem if call comes between
>     two uses, since the compiler then has to re-evaluate the whole thing.
> 
> 3. The series introduces many uses of new SCALAR_*TYPE_MODE macros,
>     as alternatives to TYPE_MODE.  The new macros avoid the usual:
> 
>       (VECTOR_TYPE_P (TYPE_CHECK (NODE)) \
>        ? vector_type_mode (NODE) : (NODE)->type_common.mode)
> 
>     and become direct field accesses in release builds.
> 
>     VECTOR_TYPE_P would be consistently false for these uses,
>     but call-clobbered registers would usually be treated as clobbered
>     by the condition as a whole.
> 
> Maybe (3) is the most likely reason.
> 
> I tested this by compiling the testsuite for:
> 
>      aarch64-linux-gnu alpha-linux-gnu arc-elf arm-linux-gnueabi
>      arm-linux-gnueabihf avr-elf bfin-elf c6x-elf cr16-elf cris-elf
>      epiphany-elf fr30-elf frv-linux-gnu ft32-elf h8300-elf
>      hppa64-hp-hpux11.23 ia64-linux-gnu i686-pc-linux-gnu
>      i686-apple-darwin iq2000-elf lm32-elf m32c-elf m32r-elf
>      m68k-linux-gnu mcore-elf microblaze-elf mips-linux-gnu
>      mipsisa64-linux-gnu mmix mn10300-elf moxie-rtems msp430-elf
>      nds32le-elf nios2-linux-gnu nvptx-none pdp11 powerpc-linux-gnuspe
>      powerpc-eabispe powerpc64-linux-gnu powerpc-ibm-aix7.0 rl78-elf
>      rx-elf s390-linux-gnu s390x-linux-gnu sh-linux-gnu sparc-linux-gnu
>      sparc64-linux-gnu sparc-wrs-vxworks spu-elf tilegx-elf tilepro-elf
>      xstormy16-elf v850-elf vax-netbsdelf visium-elf x86_64-darwin
>      x86_64-linux-gnu xtensa-elf
> 
> and checking that there were no changes in assembly.  Also tested
> in the normal way on aarch64-linux-gnu and x86_64-linux-gnu.
> 
> The series depends on the already-posted:
> 
>    https://gcc.gnu.org/ml/gcc-patches/2016-11/msg01657.html
So can we get the discussion around the prerequisite restarted -- I like 
the core ideas around building wrapper classes around machine modes, but 
obviously we can't really move forward on this without the prereqs.

jeff