It's not used anywhere else in the VM loop, and clashes with (is shadowed
by) the n_state variable that's redeclared towards the end of the
mp_execute_bytecode function. Code size is unchanged.
Taking the address of a local variable leads to increased stack usage, so
the mp_decode_uint_skip() function is added to reduce the need for taking
addresses. The changes in this patch reduce stack usage of a Python call
by 8 bytes on ARM Thumb, by 16 bytes on non-windowing Xtensa archs, and by
16 bytes on x86-64. Code size is also slightly reduced on most archs by
around 32 bytes.
This patch fixes a regression introduced by
71a3d6ec3b
Previous to this patch the n_state variable was referring to that computed
at the very start of the mp_execute_bytecode function. This patch fixes it
so that n_state is recomputed when the code_state changes.
This patch allows the following code to run without allocating on the heap:
super().foo(...)
Before this patch such a call would allocate a super object on the heap and
then load the foo method and call it right away. The super object is only
needed to perform the lookup of the method and not needed after that. This
patch makes an optimisation to allocate the super object on the C stack and
discard it right after use.
Changes in code size due to this patch are:
bare-arm: +128
minimal: +232
unix x64: +416
unix nanbox: +364
stmhal: +184
esp8266: +340
cc3200: +128
It improves readability of code and reduces the chance to make a mistake.
This patch also fixes a bug with nan-boxing builds by rounding up the
calculation of the new NSLOTS variable, giving the correct number of slots
(being 4) even if mp_obj_t is larger than the native machine size.
The scheduler being locked general means we are running a scheduled
function, and switching to another thread violates that, so don't switch in
such a case (even though we technically could).
And if we are running a scheduled function then we want to finish it ASAP,
so we shouldn't switch to another thread.
Furthermore, ports with threading enabled will lock the scheduler during a
hard IRQ, and this patch to the VM will make sure that threads are not
switched during a hard IRQ (which would crash the VM).
Instead of caching data that is constant (code_info, const_table and
n_state), store just a pointer to the underlying function object from which
this data can be derived.
This helps reduce stack usage for the case when the mp_code_state_t
structure is stored on the stack, as well as heap usage when it's stored
on the heap.
The downside is that the VM becomes a little more complex because it now
needs to derive the data from the underlying function object. But this
doesn't impact the performance by much (if at all) because most of the
decoding of data is done outside the main opcode loop. Measurements using
pystone show that little to no performance is lost.
This patch also fixes a nasty bug whereby the bytecode can be reclaimed by
the GC during execution. With this patch there is always a pointer to the
function object held by the VM during execution, since it's stored in the
mp_code_state_t structure.
Allows to iterate over the following without allocating on the heap:
- tuple
- list
- string, bytes
- bytearray, array
- dict (not dict.keys, dict.values, dict.items)
- set, frozenset
Allows to call the following without heap memory:
- all, any, min, max, sum
TODO: still need to allocate stack memory in bytecode for iter_buf.
This improves efficiency of GIL release within the VM, by only doing the
release after a fixed number of jump-opcodes have executed in the current
thread.
When an exception is raised and is to be handled by the VM, it is stored
on the Python value stack so the bytecode can access it. CPython stores
3 objects on the stack for each exception: exc type, exc instance and
traceback. uPy followed this approach, but it turns out not to be
necessary. Instead, it is enough to store just the exception instance on
the Python value stack. The only place where the 3 values are needed
explicitly is for the __exit__ handler of a with-statement context, but
for these cases the 3 values can be extracted from the single exception
instance.
This patch removes the need to store 3 values on the stack, and instead
just stores the exception instance.
Code size is reduced by about 50-100 bytes, the compiler and VM are
slightly simpler, generate bytecode is smaller (by 2 bytes for each try
block), and the Python value stack is reduced in size for functions that
handle exceptions.
With the previous patch combining 3 emit functions into 1, it now makes
sense to also combine the corresponding VM opcodes, which is what this
patch does. This eliminates 2 opcodes which simplifies the VM and reduces
code size, in bytes: bare-arm:44, minimal:64, unix(NDEBUG,x86-64):272,
stmhal:92, esp8266:200. Profiling (with a simple script that creates many
list/dict/set comprehensions) shows no measurable change in performance.
Damien George
Ville Skyttä