ZJIT: Improve documentation and make it easy to generate the types graph

Merged: https://github.com/ruby/ruby/pull/15330

doc/jit/zjit.md

@@ -16,6 +16,35 @@ To build ZJIT on macOS:
 make -j miniruby
 ```
 
+To build ZJIT on Linux:
+
+```bash
+./autogen.sh
+
+./configure \
+    --enable-zjit=dev \
+    --prefix="$HOME"/.rubies/ruby-zjit \
+    --disable-install-doc
+
+make -j miniruby
+```
+
+Note that `--enable-zjit=dev` does a lot of IR validation, which will help to catch errors early but mean compilation and warmup are significantly slower.
+
+The valid values for `--enable-zjit` are, from fastest to slowest:
+* `--enable-zjit`: enable ZJIT in release mode for maximum performance
+* `--enable-zjit=stats`: enable ZJIT in extended-stats mode
+* `--enable-zjit=dev_nodebug`: enable ZJIT in development mode but without slow runtime checks
+* `--enable-zjit=dev`: enable ZJIT in debug mode for development, also enables `RUBY_DEBUG`
+
+### Regenerate bindings
+
+When modifying `zjit/bindgen/src/main.rs` you need to regenerate bindings in `zjit/src/cruby_bindings.inc.rs` with:
+
+```bash
+make zjit-bindgen
+```
+
 ## Documentation
 
 You can generate and open the source level documentation in your browser using:
@@ -24,6 +53,16 @@ You can generate and open the source level documentation in your browser using:
 cargo doc --document-private-items -p zjit --open
 ```
 
+### Graph of the Type System
+
+You can generate a graph of the ZJIT type hierarchy using:
+
+```bash
+ruby zjit/src/hir_type/gen_hir_type.rb > zjit/src/hir_type/hir_type.inc.rs
+dot -O -Tpdf zjit_types.dot
+open zjit_types.dot.pdf
+```
+
 ## Testing
 
 Note that tests link against CRuby, so directly calling `cargo test`, or `cargo nextest` should not build. All tests are instead accessed through `make`.
@@ -139,14 +178,8 @@ end
 
 ### Performance Ratio
 
-The `ratio_in_zjit` stat shows the percentage of Ruby instructions executed in JIT code vs interpreter. This metric only appears when ZJIT is built with `--enable-zjit=stats` (which enables `rb_vm_insn_count` tracking) and represents a key performance indicator for ZJIT effectiveness.
-
-To build with stats support:
-
-```bash
-./configure --enable-zjit=stats
-make -j
-```
+The `ratio_in_zjit` stat shows the percentage of Ruby instructions executed in JIT code vs interpreter.
+This metric only appears when ZJIT is built with `--enable-zjit=stats` [or more](#build-instructions) (which enables `rb_vm_insn_count` tracking) and represents a key performance indicator for ZJIT effectiveness.
 
 ### Tracing side exits
 

zjit/src/hir_type/gen_hir_type.rb

@@ -25,20 +25,20 @@ class Type
 end
 
 # Helper to generate graphviz.
-def to_graphviz_rec type
+def to_graphviz_rec type, f
   type.subtypes.each {|subtype|
-    puts type.name + "->" + subtype.name + ";"
+    f.puts type.name + "->" + subtype.name + ";"
   }
   type.subtypes.each {|subtype|
-    to_graphviz_rec subtype
+    to_graphviz_rec subtype, f
   }
 end
 
 # Generate graphviz.
-def to_graphviz type
-  puts "digraph G {"
-  to_graphviz_rec type
-  puts "}"
+def to_graphviz type, f
+  f.puts "digraph G {"
+  to_graphviz_rec type, f
+  f.puts "}"
 end
 
 # ===== Start generating the type DAG =====
@@ -218,3 +218,7 @@ $inexact_c_names.each {|type_name, c_name|
 }
 puts "  ];"
 puts "}"
+
+File.open("zjit_types.dot", "w") do |f|
+  to_graphviz(any, f)
+end