[llvm-dev] llvm intrinsics/libc/libm question

Tue Jun 14 16:40:15 PDT 2016

Unfortunately even TLI.setUnavailable doesn't work for fabs, only for
copysign. I would have thought this would have stopped the conversion to
ISD based on the hasOptimizedCodeGen function.
On Jun 14, 2016 7:20 PM, "Anton Korobeynikov" <anton at korobeynikov.info>
wrote:

> Mostly because there are fabs / fcopysign instructions in many ISAs.
> But nothing like this for floor / trunc.
>
> On Tue, Jun 14, 2016 at 8:15 PM, Ryan Taylor via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > If I do
> >
> > T.getArch() == xxx
> >    TLI.setUnavailable(LibFunc::copysign)
> >
> > then this works at generating a call instead of not being able to select
> the
> > ISD::FCOPYSIGN, but I don't know why I don't need to do this for other
> > LibFunc functions (such as floor, etc... these generate call just fine)?
> >
> > Thanks,
> > Ryan
> >
> >
> >
> > On Tue, Jun 14, 2016 at 11:58 AM, Ryan Taylor <ryta1203 at gmail.com>
> wrote:
> >>
> >> I'm still not sure why copysign and fabs have to be lowered to a call
> when
> >> they are represented as a call in the IR?
> >>
> >> Looks like the DAG makes them into SDNodes.
> >>
> >>
> >> On Thu, Jun 9, 2016 at 2:35 PM, Ahmed Bougacha <
> ahmed.bougacha at gmail.com>
> >> wrote:
> >>>
> >>> On Thu, Jun 9, 2016 at 11:28 AM, Ryan Taylor <ryta1203 at gmail.com>
> wrote:
> >>> > I'm assuming that "__builtin_" is a keyword in LLVM. If I have code
> >>> > like:
> >>> >
> >>> > #define acos __builtin_acos
> >>> >
> >>> > extern double acos(double, double);
> >>> >
> >>> > double a;
> >>> > void foo(float b) {
> >>> >   a = acos(b);
> >>> > }
> >>> >
> >>> > I never see a call to "__builtin_acos", is LLVM removing the prefix
> >>> > __builtin_ ?
> >>>
> >>> Oh, that's a clang thing, not LLVM:  yes, I think clang has special
> >>> handling for __builtin_*, and for libm functions, just turns them into
> >>> a regular function call to the function.  That happens in clang
> >>> CGBuiltin.cpp, getBuiltinLibFunction.
> >>>
> >>> By the time LLVM sees the call, yes, it's just a call to "@acos".
> >>> There's no such thing as "__builtin_" from LLVM's standpoint, it's
> >>> purely a C thing.
> >>>
> >>> -Ahmed
> >>>
> >>> > Thanks,
> >>> >
> >>> > Ryan
> >>> >
> >>> > On Thu, Jun 9, 2016 at 2:10 PM, Ahmed Bougacha
> >>> > <ahmed.bougacha at gmail.com>
> >>> > wrote:
> >>> >>
> >>> >> On Wed, Jun 8, 2016 at 10:43 AM, Ryan Taylor <ryta1203 at gmail.com>
> >>> >> wrote:
> >>> >> > Correct, it does check based on OS and triple, what I meant was
> that
> >>> >> > it
> >>> >> > might be better to have this info in the target specific files and
> >>> >> > have
> >>> >> > the
> >>> >> > LibraryInfo do a look up of that (like most other sections of the
> >>> >> > core
> >>> >> > code
> >>> >> > do, ie have the tablegen or ISelLowering specify the libs etc..)
> >>> >>
> >>> >> I agree it's not the best place, but one difference is that
> >>> >> TargetLibraryInfo is much more about OSes than architectures.
> >>> >>
> >>> >> > I'm not sure I follow about the RTLIB, I'm able to use an
> intrinsic
> >>> >> > for
> >>> >> > floor (def int_floor::Intrinsic in IntrinsicsXXX.td) and still use
> >>> >> > RTLIB
> >>> >> > to
> >>> >> > generate the appropriate name for the function (ie __xxx_floor).
> It
> >>> >> > sounds
> >>> >> > like you're implying either/or, not both?
> >>> >>
> >>> >> No, I'm just saying that RTLIB only solves the codegen problem;
> you'll
> >>> >> need something else (like your intrinsic?) to have better IR
> >>> >> optimizations.
> >>> >>
> >>> >> > I agree, it doesn't seem supported. It looks like I might just
> need
> >>> >> > to
> >>> >> > change 'TLI.has' and 'TLI.getName' in order to make this happen
> >>> >> > (potentially
> >>> >> > removing the prefix here). This goes back to my first point, the
> TLI
> >>> >> > should
> >>> >> > be changed to simply get this info generically from the target
> >>> >> > information,
> >>> >> > you seem to agree with that.
> >>> >>
> >>> >> Hmm, what are you really trying to do?  If you want LLVM to
> recognize
> >>> >> your __xxx functions: yes, the cleanest solution is probably to
> teach
> >>> >> TLI and its users to recognize the "custom" names, and mark the
> >>> >> functions as available with your custom __xxx names.
> >>> >>
> >>> >> HTH,
> >>> >> -Ahmed
> >>> >>
> >>> >> > Thanks,
> >>> >> >
> >>> >> > Ryan
> >>> >> >
> >>> >> > On Tue, Jun 7, 2016 at 7:06 PM, Ahmed Bougacha
> >>> >> > <ahmed.bougacha at gmail.com>
> >>> >> > wrote:
> >>> >> >>
> >>> >> >> On Tue, Jun 7, 2016 at 1:57 PM, Ryan Taylor <ryta1203 at gmail.com>
> >>> >> >> wrote:
> >>> >> >> > Tim,
> >>> >> >> >
> >>> >> >> > Currently, I have to do multiple things:
> >>> >> >> >
> >>> >> >> > 1) create some setLibcallNames in XXXISelLowering.cpp to
> generate
> >>> >> >> > correct
> >>> >> >> > naming for RTLIBS.
> >>> >> >> > 2) lower ISD down to an RTLIB for some calls (and then do
> >>> >> >> > solution 1
> >>> >> >> > on
> >>> >> >> > those to get correct names)
> >>> >> >>
> >>> >> >> These solve a related but different - CodeGen - problem.
> >>> >> >>
> >>> >> >> RTLIB libcalls are used when we're not able to select some IR
> >>> >> >> instruction/intrinsic so have to rely on a runtime library helper
> >>> >> >> function (e.g., the stuff in compiler-rt/lib/builtins/).
> >>> >> >>
> >>> >> >> So, #1 and #2 would make LLVM able to emit calls to __xxx_acos
> when
> >>> >> >> it sees "@llvm.acos.f32", but it won't let LLVM optimize
> (constant
> >>> >> >> fold, transform into the intrinsic, ...) "__xx_acos()" when it
> sees
> >>> >> >> it.
> >>> >> >>
> >>> >> >> It sounds like you also want to recognize and optimize these
> calls.
> >>> >> >> That involves (pre-CodeGen) IR-level optimizations.
> >>> >> >> No, I don't think that's supported today without changing LLVM
> (see
> >>> >> >> the list in my first email).
> >>> >> >>
> >>> >> >> > 3) change TargetLibraryInfo for functions that aren't covered
> in
> >>> >> >> > solutions 1
> >>> >> >> > and 2 (so that they can also be optimized)
> >>> >> >> >
> >>> >> >> > I must be missing something, I'm just not sure what it is.
> >>> >> >> >
> >>> >> >> > Thanks,
> >>> >> >> >
> >>> >> >> > Ryan
> >>> >> >> >
> >>> >> >> >
> >>> >> >> > On Tue, Jun 7, 2016 at 4:45 PM, Ryan Taylor <
> ryta1203 at gmail.com>
> >>> >> >> > wrote:
> >>> >> >> >>
> >>> >> >> >> Tim,
> >>> >> >> >>
> >>> >> >> >>  Are you referring to setLibcallName? That is target specific
> >>> >> >> >> yes
> >>> >> >> >> but
> >>> >> >> >> there isn't RTLIB for most of the libm functions, for example,
> >>> >> >> >> for
> >>> >> >> >> acos
> >>> >> >> >> this
> >>> >> >> >> doesn't apply.
> >>> >> >> >>
> >>> >> >> >>  Ideally what I would like is to create a libc with functions
> >>> >> >> >> like
> >>> >> >> >> acos
> >>> >> >> >> called something like __xxx_acos that can still be recognized
> to
> >>> >> >> >> be
> >>> >> >> >> optimized.
> >>> >> >> >>
> >>> >> >> >>  RTLIB is pretty limited but it works fine, I can just use
> >>> >> >> >> setLibcallName(RTLIB::floor, "__xxx_floor")... but again, the
> >>> >> >> >> functions
> >>> >> >> >> that
> >>> >> >> >> are RTLIB are limited. Using intrinsics make it more difficult
> >>> >> >> >> because
> >>> >> >> >> then
> >>> >> >> >> you have to match the intrinsic (rather than it automatically
> >>> >> >> >> generating a
> >>> >> >> >> lib call). ISD is just as bad (FCOPYSIGN, FABS for example)
> >>> >> >> >> because
> >>> >> >> >> then
> >>> >> >> >> they need to be manually lowered.
> >>> >> >> >> Thanks,
> >>> >> >> >>
> >>> >> >> >> Ryan
> >>> >> >> >>
> >>> >> >> >>
> >>> >> >> >>
> >>> >> >> >> On Tue, Jun 7, 2016 at 4:38 PM, Tim Northover
> >>> >> >> >> <t.p.northover at gmail.com>
> >>> >> >> >> wrote:
> >>> >> >> >>>
> >>> >> >> >>> On 7 June 2016 at 13:24, Ryan Taylor via llvm-dev
> >>> >> >> >>> <llvm-dev at lists.llvm.org> wrote:
> >>> >> >> >>> > Not sure why it's called TargetLibraryInfo if it's not in
> >>> >> >> >>> > target
> >>> >> >> >>> > specific
> >>> >> >> >>> > code? It seems that ALL targets use this code, making it
> >>> >> >> >>> > generic.
> >>> >> >> >>> > Am
> >>> >> >> >>> > I
> >>> >> >> >>> > missing something here?
> >>> >> >>
> >>> >> >> I agree the name "Target" is a bit awkward, but it's not generic
> in
> >>> >> >> that it behaves differently depending on the target triple, which
> >>> >> >> is
> >>> >> >> usually not OK in a "generic" analysis.
> >>> >> >>
> >>> >> >> If you look in TargetLibraryInfo.cpp, there are various checks
> for
> >>> >> >> function availability, usually predicated on OS versions.
> >>> >> >>
> >>> >> >> -Ahmed
> >>> >> >>
> >>> >> >> >>> Some of the names can vary by platform, for example ARM
> >>> >> >> >>> sometimes
> >>> >> >> >>> has
> >>> >> >> >>> __aeabi_memcpy instead of memcpy
> >>> >> >> >>>
> >>> >> >> >>> > ps. The spec also states (albeit unclearly) that you can
> use
> >>> >> >> >>> > "#undef"
> >>> >> >> >>> > to
> >>> >> >> >>> > omit a library function so that a user defined function of
> >>> >> >> >>> > the
> >>> >> >> >>> > same
> >>> >> >> >>> > name can
> >>> >> >> >>> > be used but LLVM doesn't seem to support that.
> >>> >> >> >>>
> >>> >> >> >>> I think it says exactly the opposite: (7.1.2p3):
> >>> >> >> >>>
> >>> >> >> >>>     "If the program removes (with #undef) any macro
> definition
> >>> >> >> >>> of
> >>> >> >> >>> an
> >>> >> >> >>> identifier in the first group listed above, the behavior is
> >>> >> >> >>> undefined."
> >>> >> >> >>>
> >>> >> >> >>> Incidentally, I don't think anyone's mentioned that
> >>> >> >> >>> "-ffreestanding"
> >>> >> >> >>> will probably inhibit the intrinsics substantially if that's
> >>> >> >> >>> what
> >>> >> >> >>> you're after (technically, it's probably a compiler extension
> >>> >> >> >>> that
> >>> >> >> >>> it
> >>> >> >> >>> gives them back to the user, but everyone does it as far as I
> >>> >> >> >>> know).
> >>> >> >> >>>
> >>> >> >> >>> Cheers.
> >>> >> >> >>>
> >>> >> >> >>> Tim.
> >>> >> >> >>
> >>> >> >> >>
> >>> >> >> >
> >>> >> >
> >>> >> >
> >>> >
> >>> >
> >>
> >>
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
>
>
> --
> With best regards, Anton Korobeynikov
> Department of Statistical Modelling, Saint Petersburg State University
>
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