http://www.sbcl.org/all-news.html?2.0.2#2.0.2
New in version 2.0.2
new feature: CMUCL-style block compilation support has been added, allowing whole program optimization. It has been documented in a new section of the manual entitled "Advanced Compiler Use and Efficiency Hints". In particular, users of block compilation will find a large speedup for numerical code, as functions which call or return floating point values will keep everything unboxed.
http://www.sbcl.org/manual/index.html#Advanced-Compiler-Use-and-Efficiency-Hints
For more advanced usages of the compiler, please see the chapter of the same name in the CMUCL manual. Many aspects of the compiler have stayed exactly the same, and there is a much more detailed explanation of the compiler’s behavior and how to maximally optimize code in their manual. In particular, while SBCL no longer supports byte-code compilation, it does support CMUCL’s block compilation facility allowing whole program optimization and increased use of the local call convention. We do not currently support the start-block and end-block declarations, although we do support the same arguments to compile-file regarding block compilation.
What is "block compilation" anyway?
https://mstmetent.blogspot.com/2020/02/block-compilation-fresh-in-sbcl-202.html