The code for Tier 2 is now only compiled when configured
with `--enable-experimental-jit[=yes|interpreter]`.
We drop support for `PYTHON_UOPS` and -`Xuops`,
but you can disable the interpreter or JIT
at runtime by setting `PYTHON_JIT=0`.
You can also build it without enabling it by default
using `--enable-experimental-jit=yes-off`;
enable with `PYTHON_JIT=1`.
On Windows, the `build.bat` script supports
`--experimental-jit`, `--experimental-jit-off`,
`--experimental-interpreter`.
In the C code, `_Py_JIT` is defined as before
when the JIT is enabled; the new variable
`_Py_TIER2` is defined when the JIT *or* the
interpreter is enabled. It is actually a bitmask:
1: JIT; 2: default-off; 4: interpreter.
Add an option (--enable-experimental-jit for configure-based builds
or --experimental-jit for PCbuild-based ones) to build an
*experimental* just-in-time compiler, based on copy-and-patch (https://fredrikbk.com/publications/copy-and-patch.pdf).
See Tools/jit/README.md for more information on how to install the required build-time tooling.
Note that the support is not actually enabled yet, and so we won't be publishing these packages. However, for those who want to build it themselves (even by reusing the Azure Pipelines definition), it's now relatively easy to enable.
Using a response file will eliminate the headache associated with batch argument/quote processing.
For example I unsucessfully compiled python with visualcpptools when specifying VSInstallDir in the batch file (cannot find vcruntime.h)
```batch
build.bat -p x64 -e -M --no-tkinter "/p:VCInstallDir=%VCInstallDir%"
```
but it build successfully when specifying it in a response file
msbuild.rsp:
```
/p:VCInstallDir=%VCInstallDir%
```
This modifies the behavior of the '-e' flag to PCbuild\build.bat: when '-e'
is not supplied, no attempt will be made to build extension modules that
require external libraries, even if the external libraries are present.
Also adds '--no-<module>' flags to PCbuild\build.bat, where '<module>' is
one of 'ssl', 'tkinter', or 'bsddb', to allow skipping just those modules
(if '-e' is given).
Rather than using Burn "Persisted" variables we now add registry keys for each added feature. These can be detected by the installer regardless of which version installed them, and we use this for Modify and Upgrade. In particular, Upgrades can't access the Persisted variables, but can find well-known registry keys.
There are also some changes to the bootstrap app to properly handle upgrades.
Finally, a few minor improvements to the Windows build to keep things tidier.
Double quotes in expanded variables can match literal double quotes,
which makes for a big mess when passing in a quoted argument (like
"/p:externalsDir=..\externals", for example).
The user-facing scripts in PCbuild have been updated to be easier to use
and the buildbot scripts in Tools\buildbot have been updated to use the
user-facing scripts in PCbuild wherever possible.
