undefined reference to `__divmoddi4'

[tclin914]

Hi,

I got a problem when I run avr-lit --llvm-sysroot /work/llvm-build.

"/usr/bin/avr-ld" /tmp/statistics-04f55b.o -o /tmp/.tmp5oeKTE --gc-sections -L/usr/lib/avr/lib/avr5 -L/usr/lib/avr/lib/avr5 -L/usr/lib/gcc/avr/4.9.2/avr5 -l:crtatmega328p.o -lgcc -lm -lc -latmega328p -mavr5
/tmp/statistics-04f55b.o: In function `LBB67_7':
statistics.cpp:(.progmem.data+0x276a):` undefined reference to `__divmoddi4'

It seems that libgcc doesn't have the function __divmoddi4. But LLVM will lower i64 sdivrem to libcall __divmoddi4. Are you use libgcc or compiler-rt?

Thank you.

[dylanmckay]

I am using libgcc, which as far as I remember, does have the __divmoddi4 symbol.

AVR-GCC/avr-libc has special cases to the calling convention for certain arithmetic routines that require special lowering to match the custom calling convention.

See https://gcc.gnu.org/wiki/avr-gcc heading "Exceptions to the Calling Convention".

The AVR LLVM backend will always emit these special calls to __divmod..., making it incompatible with compiler-rt. This affects runtime library calls to some division and multiplication routines.

Ideally, compiler-rt proper would be extended to support AVR, and then the AVR backend could be taught to ignore the ABI special cases when compiler-rt is linked. It's been a while since I last looked at it, but I seem to recall compiler-rt missing some AVR libgcc routines that are used by avr-gcc and correspondingly, AVR-LLVM. I'm pretty sure the __divmod* family of functions for small integer types were the problem, as compiler-rt didn't seem to cater for esoteric targets like AVR that have limited or a complete lack of multiplication/division hardware support.

[aykevl]

I believe this is a bug in LLVM that is fixed in the master branch: https://reviews.llvm.org/D78437

Are you using LLVM 10 or the nightly build (LLVM 11)?