About two years ago, I did a comparison of C compilers for the STM8. It can now be found at http://www.colecovision.eu/stm8/compilers-2016.shtml.
I also redid the comparison and placed it at http://www.colecovision.eu/stm8/compilers.shtml.
For Cosmic, SDCC, IAR the current versions of the compilers were used. Due to difficulties with Raisonance evaluation licenses, an older version of Raisonance was used.
Quick summary of what happened since 2016: Cosmic and IAR improved a bit in various aspects. SDCC made huge progress. In mid-2016, code speed and size were weaknesses of SDCC. Now, for Dhrystone and Coremark, SDCC generates faster code than any other compiler (but floating-point performance and thus Whetstone scores are still bad). In terms of code size, it is now about on par with Cosmic, which means better than IAR and worse than Raisonance.