Richard Biener <richard.guent...@gmail.com> writes: > On Fri, Dec 9, 2016 at 1:48 PM, Richard Sandiford > <richard.sandif...@arm.com> 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 >> >> Sorry that this is so late, been distracted by other things. Even if >> we're too far into stage 3 for SVE itself to go in, I was hoping this >> part (which was kind-of posted during stage 1) could go in independently. > > What's the benefit of taking this but not SVE?
That's what the third paragraph onwards was addressing. It uses static type checking to avoid a relatively common source of bugs, with the "proof" being that we'd accidentally fixed at least three PRs without knowing about it. Note that the final two of the patches I linked to above were only from the end of last month. There's also the beneefit that it's overall a slight compile-time improvement. Thanks, Richard
