RArayiner·Oct 28, 2013
Macros unfortunately make themselves most useful in cases that don't lend themselves to short snippets.
My favorite example involves looking at some real code.
Compare how to add a new intrinsic to LLVM: http://llvm.org/docs/ExtendingLLVM.html versus how to do it for SBCL: http://pvk.ca/Blog/Lisp/hacking_SSE_intrinsics-part_1.html.
Look at how SBCL's x86-64 backend defines the x86-64 register set and specifies what kind of values can go in what registers: https://github.com/sbcl/sbcl/blob/master/src/compiler/x86-64....
Compare that to the equivalent for LLVM: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X8....
The register descriptions are written in a very similar, declarative, way, except the SBCL code uses macros built into the language while the LLVM code uses a separate tool called TableGen that generates C++ code from t
Read the rest
he declarative descriptions. Using a separate tool means complicating the build process, having an ad-hoc parser/expander that can be a source of bugs, losing access to compiler facilities that allow you to step through the expansions and look at the expanded code, etc.
If you can't get through the prefix syntax, look at how the OpenDylan compiler uses a macro to define the x86 instruction set: https://github.com/dylan-lang/opendylan/blob/master/sources/...
Also see the x86-64 assembler I wrote in Common Lisp: https://github.com/rayiner/amd64-asm/blob/master/encoders.li.... Specifically, lines 374-504, 621+.
Each instruction description basically specifies a list of alternative patterns that are matched against the assembly source. When a pattern matches, it is handed off to a compiled function that generates the actual encoded bytes. That function is automatically generated at compile-time from the declarative list of patterns. Not exactly brilliant code like you'll find in SBCL, but in a couple of weekends I was getting bit-for-bit identical assembly to YASM over a substantial subset of the instruction set. Part of the testsuite uses the same instruction descriptions that are used to generate the encoder: https://github.com/rayiner/amd64-asm/blob/master/testsuite.l... (Line 128).