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

[Mara Sophie Grosch via llvm-dev]

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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