LLVM properly generates ADC code when you use __addcarry_u64 contrary to GCC[1] which generates really dumb code with setc/mov in and out of carry. However __ad

LLVM properly generates ADC code when you use __addcarry_u64 contrary to GCC[1] which generates really dumb code with setc/mov in and out of carry. However __addcarryx_u64 which is supposed to generate ADCX and ADOX is not properly handled and only generates ADC. For the iXYZ themselves, the carries are properly handled and it can generate mulx at least: https://github.com/herumi/mcl/blob/master/src/asm/x86-64.bmi2.s#L104 https://github.com/herumi/mcl/blob/master/src/asm/x86-64.bmi... (from straight LLVM IR). I don't think ADCX/ADOX are possible though even in LLVM IR. I think you are mixing with one comment on the GCC mailing list about GCC not having the adequate representation of carry and even less a representation that can separate a carry and overflow chains[2] [1] https://gcc.godbolt.org/z/2h768y https://gcc.godbolt.org/z/2h768y [2] https://gcc.gnu.org/legacy-ml/gcc-help/2017-08/msg00100.html https://gcc.gnu.org/legacy-ml/gcc-help/2017-08/msg00100.html