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

Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Sun Oct 24 09:23:34 PDT 2021

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>
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