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

Tue Jun 14 08:58:44 PDT 2016

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.
> >> >> >>
> >> >> >>
> >> >> >
> >> >
> >> >
> >
> >
>
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