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.
Thanks, Prathamesh > > Thanks, > Richard
diff --git a/gcc/optabs.def b/gcc/optabs.def index 8875e30..b37ac2e 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_NL(sdivmod_optab, "divmod$a4", UNKNOWN, "divmod", '4', NULL) +OPTAB_NL(udivmod_optab, "udivmod$a4", UNKNOWN, "udivmod", '4', NULL) 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)
