Update README

Author: dstogov

ext/opcache/jit/README-IR.md

@@ -1,20 +1,36 @@
 New JIT implementation
 ======================
 
-This branch contains a new **experimental** JIT implementation based
-on [IR - Lightweight JIT Compilation Framework](https://github.com/dstogov/ir).
+This branch provides a new JIT implementation based on [IR - Lightweight
+JIT Compilation Framework](https://github.com/dstogov/ir).
 
 Despite of the PHP 8.* JIT approach, that generates native code directly from
-PHP byte-code, this implementation generates intermediate representation (IR) and
-delegates all lower-level tasks to IR Framework.
+PHP byte-code, this implementation generates intermediate representation (IR)
+and delegates all lower-level tasks to the IR Framework. IR for JIT is like an
+AST for compiler.
 
-Both IR and PHP JIT implementation are under development. Only 50% of PHP byte-code
-instructions are covered. Windows support is missing yet. ZTS and CLANG builds are
-not tested. Few *.phpt tests fail...
+Key benefits of the new JIT implementation:
+- Usage of IR opens possibilities for better optimization and register
+  allocation (the resulting native code is more efficient)
+- PHP doesn't have to care about most low-level details (different CPUs,
+  calling conventions, TLS details, etc)
+- it's mach easier to implement support for new targets (e.g. RISCV)
+- IR framework is going to be developed separately from PHP and may accept
+  contibutions from other projects (new optimizations, improvemets, bug fixes)
 
-New IR based JIT is disabled by default. It may be enable during PHP build process,
-running configured with **--enable-opcache-jit-ir**. Otherwise, PHP is going to be
-built with old JIT implementation.
+Disadvantages:
+- JIT compilation becomes slower (this is almost invisible for tracing
+  JIT, but function JIT compilation of Wordpress becomes 4 times slower)
+
+IR Framework is included into PHP as a GIT submodule. It should be fetched by
+
+``git submodule update --init --recursive``
+
+The new JIT implementation sucessfully passes all CI workflows, but it's still
+not mature and may cause failures. To reduce risks, this patch doesn't remove
+the old JIT implementation (that is the same as PHP-8.3 JIT). It's possible
+to build PHP with the old JIT by configuring with **--disable-opcache-jit-ir**.
+In the future the old implementation should be removed.
 
 Building and Testing
 ====================
@@ -34,7 +50,7 @@ cd php-ir
 git submodule update --init --recursive
 ./buildconf --force
 mkdir install
-./configure --enable-opcache-jit-ir --with-capstone --prefix=`pwd`/install --with-config-file-path=`pwd`/install/etc
+./configure --with-capstone --prefix=`pwd`/install --with-config-file-path=`pwd`/install/etc
 make
 make install
 mkdir install/etc