On 15 September 2016 at 17:53, Richard Biener <richard.guent...@gmail.com> wrote: > On Thu, Sep 15, 2016 at 1:21 PM, Prathamesh Kulkarni > <prathamesh.kulka...@linaro.org> wrote: >> On 15 September 2016 at 16:31, Richard Sandiford >> <richard.sandif...@arm.com> wrote: >>> Prathamesh Kulkarni <prathamesh.kulka...@linaro.org> writes: >>>> On 15 September 2016 at 04:21, Richard Sandiford >>>> <rdsandif...@googlemail.com> wrote: >>>>> Richard Sandiford <rdsandif...@googlemail.com> writes: >>>>>> Prathamesh Kulkarni <prathamesh.kulka...@linaro.org> writes: >>>>>>> Hi, >>>>>>> I would like to ping the following patch: >>>>>>> https://gcc.gnu.org/ml/gcc-patches/2016-08/msg01015.html >>>>>>> >>>>>>> While implementing divmod transform: >>>>>>> https://gcc.gnu.org/ml/gcc-patches/2016-05/msg01757.html >>>>>>> >>>>>>> I ran into an issue with optab_libfunc(). >>>>>>> It appears optab_libfunc (sdivmod_optab, mode) returns >>>>>>> bogus libfunc for unsupported modes, for instance >>>>>>> on x86_64, optab_libfunc (sdivmod_optab, DImode) returns >>>>>>> a libfunc with name "__divmoddi4", even though such a libfunc >>>>>>> does not exist in libgcc. This happens because in optabs.def >>>>>>> the libfunc entry for sdivmod_optab has gen_int_libfunc, >>>>>>> and call to optab_libfunc (sdivmo_optab, DImode) lazily >>>>>>> creates a bogus libfunc "__divmoddi4" by calling gen_int_libfunc(). >>>>>>> >>>>>>> To work around this issue I set libfunc entry for sdivmod_optab to NULL >>>>>>> and verified that optab_libfunc (sdivmod_optab, DImode) returns NULL_RTX >>>>>>> instead of a bogus libfunc if it's not overriden by the target. >>>>>>> >>>>>>> Bootstrapped and tested on ppc64le-linux-gnu, x86_64-linux-gnu. >>>>>>> Cross tested on arm*-*-*, aarch64*-*-*. >>>>>>> OK for trunk ? >>>>>> >>>>>> I'm not a maintainer for this area, but: >>>>> >>>>> ...in https://gcc.gnu.org/ml/gcc-patches/2016-05/msg01757.html >>>>> you said that c6x follows the return-by-pointer convention. >>>>> I'm no c6x expert, but from a quick look, I think its divrem >>>>> function returns a div/mod pair in A4/A5, which matches the >>>>> ARM convention of returning both results by value. >>>>> >>>>> Does anyone know if the optab function registered by the SPU >>>>> backend is ever called directly? >>>>> >>>>> You mention that this is latent as far as expand_twoval_binop_libfunc >>>>> is concerned. AIUI expand_twoval_binop_libfunc implements the ARM/c6x >>>>> convention and expects the two values to be returned as a pair. >>>>> It then extracts one half of the pair and discards the other. >>>>> So my worry is that we're leaving the udivmod entry intact even though >>>>> the standard __udivmoddi4 doesn't do what expand_twoval_binop_libfunc >>>>> expects. >>>>> >>>>> Would it make sense to set both entries to null and treat __udivmoddi4 >>>>> as a non-optab function? ARM and c6x could then continue to register >>>>> their current optab functions and a non-null optab function would >>>>> indicate a return value pair. >>>> AFAIU, there are only three targets (c6x, spu, arm) that override >>>> optab_libfunc for udivmod_optab for following modes: >>>> ./c6x/c6x.c: set_optab_libfunc (udivmod_optab, SImode, >>>> "__c6xabi_divremu"); >>>> ./c6x/c6x.c: set_optab_libfunc (udivmod_optab, DImode, >>>> "__c6xabi_divremull"); >>>> ./arm/arm.c: set_optab_libfunc (udivmod_optab, DImode, >>>> "__aeabi_uldivmod"); >>>> ./arm/arm.c: set_optab_libfunc (udivmod_optab, SImode, >>>> "__aeabi_uidivmod"); >>>> ./spu/spu.c: set_optab_libfunc (udivmod_optab, DImode, "__udivmoddi4"); >>>> ./spu/spu.c: set_optab_libfunc (udivmod_optab, TImode, "__udivmodti4"); >>>> >>>> Out of these only the arm, and c6x have target-specific divmod libfuncs >>>> which >>>> return <div, mod> pair, while spu merely makes it point to the >>>> standard functions. >>>> >>>> So we could set libfunc entry for udivmod_optab to NULL, thus dropping >>>> support for generic >>>> divmod functions (__udivmoddi4, __udivmodti4). For targets that >>>> require standard divmod libfuncs like __udivmoddi4, >>>> they could explicitly override optab_libfunc and set it to >>>> __udivmoddi4, just as spu does. >>>> >>>> However this implies that non-null optab function doesn't necessarily >>>> follow arm/c6x convention. >>>> (i686-gcc for instance generates call to libgcc routines >>>> __udivdi3/__umoddi3 for DImode division/mod operations >>>> and could profit from divmod transform by calling __udivmoddi4). >>> >>> What I meant was that we shouldn't treat udivmoddi4 as an optab function >>> at all, but handle it with some on-the-side mechanism. That seems like >>> quite a natural fit if we handle the fused div/mod operation as an >>> internal function during gimple. >> Ah right, thanks for pointing out. So if optab function for [us]divmod_optab >> is defined, then it must follow the arm/c6x convention ? >>> >>> I think the current SPU code is wrong, but it looks like a latent bug. >>> (Like I say, does the udivmodti4 function that it registers ever >>> actually get used? It seems unlikely.) >>> >>> In that scenario no other targets should do what SPU does. >> I am testing the following patch which sets libfunc entries for both >> sdivmod_optab, udivmod_optab to NULL. >> This won't change the current (broken) behavior for SPU port since it >> explicitly overrides optab_libfunc for udivmod_optab >> and sets it to __udivmoddi4. > > Just > > -OPTAB_NL(sdivmod_optab, "divmod$a4", UNKNOWN, "divmod", '4', gen_int_libfunc) > -OPTAB_NL(udivmod_optab, "udivmod$a4", UNKNOWN, "udivmod", '4', > gen_int_libfunc) > +OPTAB_NC(sdivmod_optab, "divmod$a4", UNKNOWN) > +OPTAB_NC(udivmod_optab, "udivmod$a4", UNKNOWN) > > If I remember correctly. Thanks for the pointers, I verified with the patch, optab_libfunc (udivmod_optab, DImode) returns NULL for targets that don't override libfunc for udivmod_optab like x86_64, and returns correct libfunc for targets that override libfunc for udivmod_optab like arm.
The patch would make divmod transform more simpler, since now we require that the divmod libfunc has a consistent interface identical to arm/c6x convention (take 2 args and return value <div, mod> pair), so we can get rid of the target hook in divmod patch. However I see one potential issue with SPU port for divmod transform: So far, it appears the only function that could generate call to __udivmoddi4() was expand_twoval_binop_libfunc() which was called from expand_divmod(). Maybe that code-path never got triggered for SPU and the issue remained latent. However with divmod patch, we are introducing another code-path to generate call to __udivmoddi4(), and since __udivmoddi4() and divmod libfunc conventions don't agree, this will result in wrong code with the divmod transform on SPU. We could work around this by SPU port not overriding optab_libfunc for udivmod_optab, by removing following lines in spu_init_libfuncs(): set_optab_libfunc (udivmod_optab, DImode, "__udivmoddi4"); set_optab_libfunc (udivmod_optab, TImode, "__udivmodti4"); If that's not desirable, we could disable the divmod transform for spu for now and revisit the issue later. The patch passes bootstrap+test on x86_64-unknown-linux-gnu, aarch64-linux-gnu, ppc64le-linux-gnu, and cross tested on arm*-*-*, aarch64*-*-*. Would it be OK to commit ? I am happy to do more testing if required and if something breaks due to this patch, I will revert it immediately. Thanks, Prathamesh > > Richard. > > >> Thanks, >> Prathamesh >>> >>> Thanks, >>> Richard
diff --git a/gcc/optabs.def b/gcc/optabs.def index 8875e30..8a64ce1 100644 --- a/gcc/optabs.def +++ b/gcc/optabs.def @@ -116,8 +116,8 @@ OPTAB_NL(ssdiv_optab, "ssdiv$Q$a3", SS_DIV, "ssdiv", '3', gen_signed_fixed_libfu OPTAB_NL(udiv_optab, "udiv$I$a3", UDIV, "udiv", '3', gen_int_unsigned_fixed_libfunc) OPTAB_NX(udiv_optab, "udiv$Q$a3") OPTAB_NL(usdiv_optab, "usdiv$Q$a3", US_DIV, "usdiv", '3', gen_unsigned_fixed_libfunc) -OPTAB_NL(sdivmod_optab, "divmod$a4", UNKNOWN, "divmod", '4', gen_int_libfunc) -OPTAB_NL(udivmod_optab, "udivmod$a4", UNKNOWN, "udivmod", '4', gen_int_libfunc) +OPTAB_NC(sdivmod_optab, "divmod$a4", UNKNOWN) +OPTAB_NC(udivmod_optab, "udivmod$a4", UNKNOWN) OPTAB_NL(smod_optab, "mod$a3", MOD, "mod", '3', gen_int_libfunc) OPTAB_NL(umod_optab, "umod$a3", UMOD, "umod", '3', gen_int_libfunc) OPTAB_NL(ftrunc_optab, "ftrunc$F$a2", UNKNOWN, "ftrunc", '2', gen_fp_libfunc)
