[llvm-dev] [cfe-dev] need help adding arch-specific builtin

[Craig Topper via llvm-dev]

The C to IR conversion happens in CGBuiltin.cpp where it calls
Intrinsic::getIntrinsicForGCCBuiltin.

The error your getting is coming from SelectionDAG. The error seems to
indicate that i32 is not a legal type for the AVR and
LegalizeIntegerTypes.cpp does know how to make it legal. It wants to split
into 2 i16s but it doesn’t know how. Intrinsics with illegal types are
usually handled by adding setOperationAction(ISD::INTRINSIC_W_CHAIN,
MVT::i32, Custom) to the AVRTargetLowering constructor. And then adding a
case for INTRINSIC_W_CHAIN to AVRTargetLowering::LowerOperation to detect
the specific intrinsic and handle it.

Alternatively you chang the intrinsic to use i16 instead of i32 if that
meets your needs.

On Sun, Oct 24, 2021 at 9:30 AM Mara Sophie Grosch via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> that's good to know, thank you. Where is the code generation for those
> intrinsics then?
>
> (forwarded to list because I don't use mailings lists very often and
> answered Craig only..)
>
> Am Sun, Oct 24, 2021 at 09:23:34AM -0700 schrieb Craig Topper:
> > If you add GCCBuiltin to the intrinsic definition, clang will
> automatically
> > do the conversion from C to IR. EmitTargetArchBuiltin will only be called
> > if no GCCBuiltin can be found.
> >
> > On Sun, Oct 24, 2021 at 6:48 AM Mara Sophie Grosch via cfe-dev <
> > cfe-dev at lists.llvm.org> wrote:
> >
> > > Hi lists,
> > >
> > > I'm trying to add a new compiler builtin for AVR architecture,
> > > `void __builtin_avr_delay_cycles(int32)`.
> > >
> > > I already made clang recognize this as a builtin with the correct type.
> > > Now there are some assumptions on what I have to do:
> > >
> > > * I think have to map this to an arch-specific intrinsic in LLVM
> > > * in clang/lib/CodeGen/CGBuiltin I'd have to add a function for AVR
> > >    builtins and call it from EmitTargetArchBuiltinExpr
> > > * in this new function, I would generate the actual instructions -
> > >    building the "generate code from builtin call" part
> > >
> > > I added an intrinsic for this[1] and added the function in CGBuiltin
> and
> > > call it for Arch == llvm::Triple::avr - but cannot get my new code to
> be
> > > called. I even added an `assert(false)` in EmitTargetArchBuiltinExpr(),
> > > which should cause the compiler to crash on that assert whenever it is
> > > supposed to handle a builtin for AVR targets - but that assert never
> > > fires.
> > >
> > > Instead, I get the error given below [2], which I cannot make a lot out
> > > of it.
> > >
> > > With my not-yet-very-good knowledge how everything is combined together
> > > in clang and llvm, I assume this might be because it tries to pass a
> i32
> > > to the builtin (which is expected), but cannot do that because the arch
> > > cannot pass i32s around in arguments? But that shouldn't be needed for
> a
> > > builtin - that call to it shouldn't be in the resulting machine code
> > > anyway.
> > >
> > > Any clues where to find more information on how to achieve what I'm
> > > trying to do would be appreciated. I poked around at a lot of different
> > > places in the code base by now, trying to find more info by myself, but
> > > am stuck.
> > >
> > > The builtin in question is supposed to generate a loop taking the given
> > > amount of clock cycles to complete - something commonly used on
> > > microcontrollers to have timing. Frequency of the CPU is known at
> > > compile time via preprocessor definition for that and the builtin
> > > already exists on gcc, I just want to also have it in clang.
> > >
> > > Thanks for any help and comments
> > > Mara
> > > (I subscribed to cfe-dev in digest mode only - answering me directly
> too
> > > would be appreciated)
> > >
> > >
> > > [1] I added this into llvm/include/llvm/IR/IntrinsicsAVR.td, which I
> > > include from llvm/include/llvm/IR/Intrinsics.td:
> > >
> > > let TargetPrefix = "avr" in {  // All intrinsics start with "llvm.avr."
> > >    def int_avr_delay_cycles : GCCBuiltin<"__builtin_avr_delay_cycles">,
> > >                Intrinsic<[], [llvm_i32_ty], [IntrInaccessibleMemOnly,
> > > IntrCold, IntrNoDuplicate, IntrNoMerge, IntrHasSideEffects]>;
> > > }
> > >
> > >
> > > [2] Trying to compile code using this builtin results in this error:
> > > ExpandIntegerOperand Op #2: t8: ch = llvm.avr.delay.cycles t5,
> > > TargetConstant:i16<2445>, Constant:i32<8000000>
> > > _______________________________________________
> > > cfe-dev mailing list
> > > cfe-dev at lists.llvm.org
> > > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> > >
> > --
> > ~Craig
>
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-- 
~Craig
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