Previously, mark operation weren't logged at all, while it's quite useful
to see cascade of marks in case of over-marking (and in other cases too).
Previously, sweep was logged for each block of object in memory, but that
doesn't make much sense and just lead to longer output, harder to parse
by a human. Instead, log sweep only once per object. This is similar to
other memory manager operations, e.g. an object is allocated, then freed.
Or object is allocated, then marked, otherwise swept (one log entry per
operation, with the same memory address in each case).
Map indicies are most commonly a qstr, and adding a fast-path for hashing
of a qstr increases overall performance of the runtime.
On pyboard there is a 4% improvement in the pystone benchmark for a cost
of 20 bytes of code size. It's about a 2% improvement on unix.
When looking up and extracting an attribute of an instance, some
attributes must bind self as the first argument to make a working method
call. Previously to this patch, any attribute that was callable had self
bound as the first argument. But Python specs require the check to be
more restrictive, and only functions, closures and generators should have
self bound as the first argument
Addresses issue #1675.
POSIX doesn't guarantee something like that to work, but it works on any
system with careful signal implementation. Roughly, the requirement is
that signal handler is executed in the context of the process, its main
thread, etc. This is true for Linux. Also tested to work without issues
on MacOSX.
This makes all tests pass again for 64bit windows builds which would
previously fail for anything printing ranges (builtin_range/unpack1)
because they were printed as range( ld, ld ).
This is done by reusing the mp_vprintf implementation for MICROPY_OBJ_REPR_D
for 64bit windows builds (both msvc and mingw-w64) since the format specifier
used for 64bit integers is also %lld, or %llu for the unsigned version.
Note these specifiers used to be fetched from inttypes.h, which is the
C99 way of working with printf/scanf in a portable way, but mingw-w64
wants to be backwards compatible with older MS C runtimes and uses
the non-portable %I64i instead of %lld in inttypes.h, so remove the use
of said header again in mpconfig.h and define the specifiers manually.
Ideally we'd use %zu for size_t args, but that's unlikely to be supported
by all runtimes, and we would then need to implement it in mp_printf.
So simplest and most portable option is to use %u and cast the argument
to uint(=unsigned int).
Note: reason for the change is that UINT_FMT can be %llu (size suitable
for mp_uint_t) which is wider than size_t and prints incorrect results.
MICROPY_ENABLE_COMPILER can be used to enable/disable the entire compiler,
which is useful when only loading of pre-compiled bytecode is supported.
It is enabled by default.
MICROPY_PY_BUILTINS_EVAL_EXEC controls support of eval and exec builtin
functions. By default they are only included if MICROPY_ENABLE_COMPILER
is enabled.
Disabling both options saves about 40k of code size on 32-bit x86.
To let unix port implement "machine" functionality on Python level, and
keep consistent naming in other ports (baremetal ports will use magic
module "symlinking" to still load it on "import machine").
Fixes#1701.
For builds where mp_uint_t is larger than size_t, it doesn't make
sense to use such a wide type for qstrs. There can only be as many
qstrs as there is address space on the machine, so size_t is the correct
type to use.
Saves about 3000 bytes of code size when building unix/ port with
MICROPY_OBJ_REPR_D.
size_t is the correct type to use to count things related to the size of
the address space. Using size_t (instead of mp_uint_t) is important for
the efficiency of ports that configure mp_uint_t to larger than the
machine word size.
This allows to have single itertaor type for various internal iterator
types (save rodata space by not having repeating almost-empty type
structures). It works by looking "iternext" method stored in particular
object instance (should be first object field after "base").
Fixes#1684 and makes "not" match Python semantics. The code is also
simplified (the separate MP_BC_NOT opcode is removed) and the patch saves
68 bytes for bare-arm/ and 52 bytes for minimal/.
Previously "not x" was implemented as !mp_unary_op(x, MP_UNARY_OP_BOOL),
so any given object only needs to implement MP_UNARY_OP_BOOL (and the VM
had a special opcode to do the ! bit).
With this patch "not x" is implemented as mp_unary_op(x, MP_UNARY_OP_NOT),
but this operation is caught at the start of mp_unary_op and dispatched as
!mp_obj_is_true(x). mp_obj_is_true has special logic to test for
truthness, and is the correct way to handle the not operation.
Oftentimes, libc, libm, etc. don't come compiled with CPU compressed code
option (Thumb, MIPS16, etc.), but we may still want to use such compressed
code for MicroPython itself.
To use, put the following in mpconfigport.h:
#define MICROPY_OBJ_REPR (MICROPY_OBJ_REPR_D)
#define MICROPY_FLOAT_IMPL (MICROPY_FLOAT_IMPL_DOUBLE)
typedef int64_t mp_int_t;
typedef uint64_t mp_uint_t;
#define UINT_FMT "%llu"
#define INT_FMT "%lld"
Currently does not work with native emitter enabled.
This allows the mp_obj_t type to be configured to something other than a
pointer-sized primitive type.
This patch also includes additional changes to allow the code to compile
when sizeof(mp_uint_t) != sizeof(void*), such as using size_t instead of
mp_uint_t, and various casts.
- add mp_int_t/mp_uint_t typedefs in mpconfigport.h
- fix integer suffixes/formatting in mpconfig.h and mpz.h
- use MICROPY_NLR_SETJMP=1 in Makefile since the current nlrx64.S
implementation causes segfaults in gc_free()
- update README
This takes previous IEEE-754 single precision float implementation, and
converts it to fully portable parametrizable implementation using C99
functions like signbit(), isnan(), isinf(). As long as those functions
are available (they can be defined in adhoc manner of course), and
compiler can perform standard arithmetic and comparison operations on a
float type, this implementation will work with any underlying float type
(including types whose mantissa is larger than available intergral integer
type).
This change makes the code behave how it was supposed to work when first
written. The avail_slot variable is set to the first free slot when
looking for a key (which would come from deleting an entry). So it's
more efficient (for subsequent lookups) to insert a new key into such a
slot, rather than the very last slot that was searched.
MICROPY_PERSISTENT_CODE must be enabled, and then enabling
MICROPY_PERSISTENT_CODE_LOAD/SAVE (either or both) will allow loading
and/or saving of code (at the moment just bytecode) from/to a .mpy file.
Main changes when MICROPY_PERSISTENT_CODE is enabled are:
- qstrs are encoded as 2-byte fixed width in the bytecode
- all pointers are removed from bytecode and put in const_table (this
includes const objects and raw code pointers)
Ultimately this option will enable persistence for not just bytecode but
also native code.
Currently, the only place that clears the bit is in gc_collect.
So if a block with a finalizer is allocated, and subsequently
freed, and then the block is reallocated with no finalizer then
the bit remains set.
This could also be fixed by having gc_alloc clear the bit, but
I'm pretty sure that free is called way less than alloc, so doing
it in free is more efficient.
This patch adds/subtracts a constant from the 30-bit float representation
so that str/qstr representations are favoured: they now have all the high
bits set to zero. This makes encoding/decoding qstr strings more
efficient (and they are used more often than floats, which are now
slightly less efficient to encode/decode).
Saves about 300 bytes of code space on Thumb 2 arch.
py/mphal.h contains declarations for generic mp_hal_XXX functions, such
as stdio and delay/ticks, which ports should provide definitions for. A
port will also provide mphalport.h with further HAL declarations.
This makes format specifiers ~ fully compatible with CPython.
Adds 24 bytes for stmhal port (because previosuly we had to catch and report
it's unsupported to user).
Scenario: module1 depends on some common file from lib/, so specifies it
in its SRC_MOD, and the same situation with module2, then common file
from lib/ eventually ends up listed twice in $(OBJ), which leads to link
errors.
Make is equipped to deal with such situation easily, quoting the manual:
"The value of $^ omits duplicate prerequisites, while $+ retains them and
preserves their order." So, just use $^ consistently in all link targets.
This saves around 1000 bytes (Thumb2 arch) because in repr "C" it is
costly to check and extract a qstr. So making such check/extract a
function instead of a macro saves lots of code space.
This new object representation puts floats into the object word instead
of on the heap, at the expense of reducing their precision to 30 bits.
It only makes sense when the word size is 32-bits.
Cortex-M0, M0+ and M1 only have ARMv6-M Thumb/Thumb2 instructions. M3,
M4 and M7 have a superset of these, named ARMv7-M. This patch adds a
config option to enable support of the superset of instructions.
It makes much more sense to do constant folding in the parser while the
parse tree is being built. This eliminates the need to create parse
nodes that will just be folded away. The code is slightly simpler and a
bit smaller as well.
Constant folding now has a configuration option,
MICROPY_COMP_CONST_FOLDING, which is enabled by default.
With this patch parse nodes are allocated sequentially in chunks. This
reduces fragmentation of the heap and prevents waste at the end of
individually allocated parse nodes.
Saves roughly 20% of RAM during parse stage.
This patch adds more fine grained error message control for errors when
parsing integers (now has terse, normal and detailed). When detailed is
enabled, the error now escapes bytes when printing them so they can be
more easily seen.
When creating constant mpz's, the length of the mpz must be exactly how
many digits are used (not allocated) otherwise these numbers are not
compatible with dynamically allocated numbers.
Addresses issue #1448.
4 spaces are added at start of line to match previous indent, and if
previous line ended in colon.
Backspace deletes 4 space if only spaces begin a line.
Configurable via MICROPY_REPL_AUTO_INDENT. Disabled by default.
This optimises (in speed and code size) for the common case where the
binary op for the bool object is supported. Unsupported binary ops
still behave the same.
Function annotations are only needed when the native emitter is enabled
and when the current scope is emitted in viper mode. All other times
the annotations can be skipped completely.
Fetch the current usb mode and return a string representation when
pyb.usb_mode() is called with no args. The possible string values are interned
as qstr's. None will be returned if an incorrect mode is set.
Indeed, this flag efectively selects architecture target, and must
consistently apply to all compiles and links, including 3rd-party
libraries, unlike CFLAGS, which have MicroPython-specific setting.
unix-cpy was originally written to get semantic equivalent with CPython
without writing functional tests. When writing the initial
implementation of uPy it was a long way between lexer and functional
tests, so the half-way test was to make sure that the bytecode was
correct. The idea was that if the uPy bytecode matched CPython 1-1 then
uPy would be proper Python if the bytecodes acted correctly. And having
matching bytecode meant that it was less likely to miss some deep
subtlety in the Python semantics that would require an architectural
change later on.
But that is all history and it no longer makes sense to retain the
ability to output CPython bytecode, because:
1. It outputs CPython 3.3 compatible bytecode. CPython's bytecode
changes from version to version, and seems to have changed quite a bit
in 3.5. There's no point in changing the bytecode output to match
CPython anymore.
2. uPy and CPy do different optimisations to the bytecode which makes it
harder to match.
3. The bytecode tests are not run. They were never part of Travis and
are not run locally anymore.
4. The EMIT_CPYTHON option needs a lot of extra source code which adds
heaps of noise, especially in compile.c.
5. Now that there is an extensive test suite (which tests functionality)
there is no need to match the bytecode. Some very subtle behaviour is
tested with the test suite and passing these tests is a much better
way to stay Python-language compliant, rather than trying to match
CPy bytecode.
Previous to this patch there were some cases where line numbers for
errors were 0 (unknown). Now the compiler attempts to give a better
line number where possible, in some cases giving the line number of the
closest statement, and other cases the line number of the inner-most
scope of the error (eg the line number of the start of the function).
This helps to give good (and sometimes exact) line numbers for
ViperTypeError exceptions.
This patch also makes sure that the first compile error (eg SyntaxError)
that is encountered is reported (previously it was the last one that was
reported).
When looking to see if the REPL input needs to be continued on the next
line, don't look inside strings for unmatched ()[]{} ''' or """.
Addresses issue #1387.
ViperTypeError now includes filename and function name where the error
occurred. The line number is the line number of the start of the
function definition, which is the best that can be done without a lot
more work.
Partially addresses issue #1381.
This patch makes configurable, via MICROPY_QSTR_BYTES_IN_HASH, the
number of bytes used for a qstr hash. It was originally fixed at 2
bytes, and now defaults to 2 bytes. Setting it to 1 byte will save
ROM and RAM at a small expense of hash collisions.
Previous to this patch all interned strings lived in their own malloc'd
chunk. On average this wastes N/2 bytes per interned string, where N is
the number-of-bytes for a quanta of the memory allocator (16 bytes on 32
bit archs).
With this patch interned strings are concatenated into the same malloc'd
chunk when possible. Such chunks are enlarged inplace when possible,
and shrunk to fit when a new chunk is needed.
RAM savings with this patch are highly varied, but should always show an
improvement (unless only 3 or 4 strings are interned). New version
typically uses about 70% of previous memory for the qstr data, and can
lead to savings of around 10% of total memory footprint of a running
script.
Costs about 120 bytes code size on Thumb2 archs (depends on how many
calls to gc_realloc are made).
I checked the entire codebase, and every place that vstr_init_len
was called, there was a call to mp_obj_new_str_from_vstr after it.
mp_obj_new_str_from_vstr always tries to reallocate a new buffer
1 byte larger than the original to store the terminating null
character.
In many cases, if we allocated the initial buffer to be 1 byte
longer, we can prevent this extra allocation, and just reuse
the originally allocated buffer.
Asking to read 256 bytes and only getting 100 will still cause
the extra allocation, but if you ask to read 256 and get 256
then the extra allocation will be optimized away.
Yes - the reallocation is optimized in the heap to try and reuse
the buffer if it can, but it takes quite a few cycles to figure
this out.
Note by Damien: vstr_init_len should now be considered as a
string-init convenience function and used only when creating
null-terminated objects.
Previous to this patch, if "abcd" and "ab" were possible completions
to tab-completing "a", then tab would expand to "abcd" straight away
if this identifier appeared first in the dict.
The TimeoutError is useful for some modules, specially the the
socket module. TimeoutError can then be alised to socket.timeout
and then Python code can differentiate between socket.error and
socket.timeout.
When "micropython -m pkg.mod" command was used, relative imports in pkg.mod
didn't work, because pkg.mod.__name__ was set to __main__, and the fact that
it's a package submodule was missed. This is an original workaround to this
issue. TODO: investigate and compare how CPython deals with this issue.
Previous to this patch each time a bytes object was referenced a new
instance (with the same data) was created. With this patch a single
bytes object is created in the compiler and is loaded directly at execute
time as a true constant (similar to loading bignum and float objects).
This saves on allocating RAM and means that bytes objects can now be
used when the memory manager is locked (eg in interrupts).
The MP_BC_LOAD_CONST_BYTES bytecode was removed as part of this.
Generated bytecode is slightly larger due to storing a pointer to the
bytes object instead of the qstr identifier.
Code size is reduced by about 60 bytes on Thumb2 architectures.
Previous to this patch a call such as list.append(1, 2) would lead to a
seg fault. This is because list.append is a builtin method and the first
argument to such methods is always assumed to have the correct type.
Now, when a builtin method is extracted like this it is wrapped in a
checker object which checks the the type of the first argument before
calling the builtin function.
This feature is contrelled by MICROPY_BUILTIN_METHOD_CHECK_SELF_ARG and
is enabled by default.
See issue #1216.
mpconfigport.mk contains configuration options which affect the way
MicroPython is linked. In this regard, it's "stronger" configuration
dependency than even mpconfigport.h, so if we rebuild everything on
mpconfigport.h change, we certianly should of that on mpconfigport.mk
change too.
If heap allocation for the Python-stack of a function fails then we may
as well allocate the Python-stack on the C stack. This will allow to
run more code without using the heap.
This allows to do "ar[i]" and "ar[i] = val" in viper when ar is a Python
object and i and/or val are native viper types (eg ints).
Patch also includes tests for this feature.
This patch converts Q(abc) to "Q(abc)" to protect the abc from the
C preprocessor, then converts back after the preprocessor is finished.
So now we can safely put includes in mpconfig(port).h, and also
preprocess qstrdefsport.h (latter is now done also in this patch).
Addresses issue #1252.
C's printf will pad nan/inf differently to CPython. Our implementation
originally conformed to C, now it conforms to CPython's way.
Tests for this are also added in this patch.
This drops the size of unicode_isxdigit from 0x1e + 0x02 filler to
0x14 bytes (so net code reduction of 12 bytes) and will make
unicode_is_xdigit perform slightly faster.
This allows using (almost) the same code for printing floats everywhere,
removes the dependency on sprintf and uses just snprintf and
applies an msvc-specific fix for snprintf in a single place so
nan/inf are now printed correctly.
mp_obj_get_int_truncated will raise a TypeError if the argument is not
an integral type. Use mp_obj_int_get_truncated only when you know the
argument is a small or big int.
Hashing is now done using mp_unary_op function with MP_UNARY_OP_HASH as
the operator argument. Hashing for int, str and bytes still go via
fast-path in mp_unary_op since they are the most common objects which
need to be hashed.
This lead to quite a bit of code cleanup, and should be more efficient
if anything. It saves 176 bytes code space on Thumb2, and 360 bytes on
x86.
The only loss is that the error message "unhashable type" is now the
more generic "unsupported type for __hash__".
Unfortunately, MP_OBJ_STOP_ITERATION doesn't have means to pass an associated
value, so we can't optimize StopIteration exception with (non-None) argument
to MP_OBJ_STOP_ITERATION.
When generator raises exception, it is automatically terminated (by setting
its code_state.ip to 0), which interferes with this check.
Triggered in particular by CPython's test_pep380.py.
Exceptions in .close() should be ignored (dumped to sys.stderr, not
propagated), but in uPy, they are propagated. Fix would require
nlr-wrapping .close() call, which is expensive. Bu on the other hand,
.close() is not called often, so maybe that's not too bad (depends,
if it's finally called and that causes stack overflow, there's nothing
good in that). And yet on another hand, .close() can be implemented to
catch exceptions on its side, and that should be the right choice.
The code was apparently broken after 9988618e0e
"py: Implement full func arg passing for native emitter.". This attempts to
propagate those changes to ARM emitter.
User instances are hashable by default (using __hash__ inherited from
"object"). But if __eq__ is defined and __hash__ not defined in particular
class, instance is not hashable.
Having NotImplemented as MP_OBJ_SENTINEL turned out to be problematic
(it needs to be checked for in a lot of places, otherwise it'll crash
as would pass MP_OBJ_IS_OBJ()), so made a proper singleton value like
Ellipsis, both of them sharing the same type.
From https://docs.python.org/3/library/constants.html#NotImplemented :
"Special value which should be returned by the binary special methods
(e.g. __eq__(), __lt__(), __add__(), __rsub__(), etc.) to indicate
that the operation is not implemented with respect to the other type;
may be returned by the in-place binary special methods (e.g. __imul__(),
__iand__(), etc.) for the same purpose. Its truth value is true."
Some people however appear to abuse it to mean "no value" when None is
a legitimate value (don't do that).
Can complete names in the global namespace, as well as a chain of
attributes, eg pyb.Pin.board.<tab> will give a list of all board pins.
Costs 700 bytes ROM on Thumb2 arch, but greatly increases usability of
REPL prompt.
This doesn't handle case fo enclosed except blocks, but once again,
sys.exc_info() support is a workaround for software which uses it
instead of properly catching exceptions via variable in except clause.
The implementation is very basic and non-compliant and provided solely for
CPython compatibility. The function itself is bad Python2 heritage, its
usage is discouraged.
Before this patch a "with" block needed to create a bound method object
on the heap for the __exit__ call. Now it doesn't because we use
load_method instead of load_attr, and save the method+self on the stack.
This fixes a long standing problem that viper code generation gave
terrible error messages, and actually no errors on pyboard where
assertions are disabled.
Now all compile-time errors are raised as proper Python exceptions, and
are of type ViperTypeError.
Addresses issue #940.
Adds support for the following Thumb2 VFP instructions, via the option
MICROPY_EMIT_INLINE_THUMB_FLOAT:
vcmp
vsqrt
vneg
vcvt_f32_to_s32
vcvt_s32_to_f32
vmrs
vmov
vldr
vstr
vadd
vsub
vmul
vdiv
Previous to this patch the printing mechanism was a bit of a tangled
mess. This patch attempts to consolidate printing into one interface.
All (non-debug) printing now uses the mp_print* family of functions,
mainly mp_printf. All these functions take an mp_print_t structure as
their first argument, and this structure defines the printing backend
through the "print_strn" function of said structure.
Printing from the uPy core can reach the platform-defined print code via
two paths: either through mp_sys_stdout_obj (defined pert port) in
conjunction with mp_stream_write; or through the mp_plat_print structure
which uses the MP_PLAT_PRINT_STRN macro to define how string are printed
on the platform. The former is only used when MICROPY_PY_IO is defined.
With this new scheme printing is generally more efficient (less layers
to go through, less arguments to pass), and, given an mp_print_t*
structure, one can call mp_print_str for efficiency instead of
mp_printf("%s", ...). Code size is also reduced by around 200 bytes on
Thumb2 archs.
In particular, numbers which are less than 1.0 but which
round up to 1.0.
This also makes those numbers which round up to 1.0 to
print with e+00 rather than e-00 for those formats which
print exponents.
Addresses issue #1178.
This simplifies the API for objects and reduces code size (by around 400
bytes on Thumb2, and around 2k on x86). Performance impact was measured
with Pystone score, but change was barely noticeable.
Fixes msvc linker warnings about mismatching sizes between the mp_obj_fdfile_t
struct defined in file.c and the mp_uint_t declarations found in modsys.c and modbuiltins.c
This patch gets full function argument passing working with native
emitter. Includes named args, keyword args, default args, var args
and var keyword args. Fully Python compliant.
It reuses the bytecode mp_setup_code_state function to do all the hard
work. This function is slightly adjusted to accommodate native calls,
and the native emitter is forced a bit to emit similar prelude and
code-info as bytecode.
splitlines() occurs ~179 times in CPython3 standard library, so was
deemed worthy to implement. The method has subtle semantic differences
from just .split("\n"). It is also defined as working for any end-of-line
combination, but this is currently not implemented - it works only with
LF line-endings (which should be OK for text strings on any platforms,
but not OK for bytes).
I.e. in this mode, C stack will never be used to call a Python function,
but if there's no free heap for a call, it will be reported as
RuntimeError (as expected), not MemoryError.
When just the bytecode emitter is needed there is no need to have a
dynamic method table for the emitter back-end, and we can instead
directly call the mp_emit_bc_XXX functions. This gives a significant
reduction in code size and a very slight performance boost for the
compiler.
This patch saves 1160 bytes code on Thumb2 and 972 bytes on x86, when
native emitters are disabled.
Overall savings in code over the last 3 commits are:
bare-arm: 1664 bytes.
minimal: 2136 bytes.
stmhal: 584 bytes (it has native emitter enabled).
cc3200: 1736 bytes.
First pass for the compiler is computing the scope (eg if an identifier
is local or not) and originally had an entire table of methods dedicated
to this, most of which did nothing. With changes from previous commit,
this set of methods can be removed and the methods from the bytecode
emitter used instead, with very little modification -- this is what is
done in this commit.
This factoring has little to no impact on the speed of the compiler
(tested by compiling 3763 Python scripts and timing it).
This factoring reduces code size by about 270-300 bytes on Thumb2 archs,
and 400 bytes on x86.
mp_obj_t internal representation doesn't have to be a pointer to object,
it can be anything.
There's also a support for back-conversion in the form of MP_OBJ_UNCAST.
This is kind of optimization/status quo preserver to minimize patching the
existing code and avoid doing potentially expensive MP_OBJ_CAST over and
over. But then one may imagine implementations where MP_OBJ_UNCAST is very
expensive. But such implementations are unlikely interesting in practice.
Despite initial guess, this code factoring does not hamper performance.
In fact it seems to improve speed by a little: running pystone(1.2) on
pyboard (which gives a very stable result) this patch takes pystones
from 1729.51 up to 1742.16. Also, pystones on x64 increase by around
the same proportion (but it's much noisier).
Taking a look at the generated machine code, stack usage with this patch
is unchanged, and call is tail-optimised with all arguments in
registers. Code size decreases by about 50 bytes on Thumb2 archs.
"Base" should rather refer to "base type"."Base object for attribute
lookup" should rather be just "object".
Also, a case of common subexpression elimination.
Given that there's already support for "fixed table" maps, which are
essentially ordered maps, the implementation of OrderedDict just extends
"fixed table" maps by adding an "is ordered" flag and add/remove
operations, and reuses 95% of objdict code, just making methods tolerant
to both dict and OrderedDict.
Some things are missing so far, like CPython-compatible repr and comparison.
OrderedDict is Disabled by default; enabled on unix and stmhal ports.
These allow to fine-tune the compiler to select whether it optimises
tuple assignments of the form a, b = c, d and a, b, c = d, e, f.
Sensible defaults are provided.
This is rarely used feature which takes enough code to implement, so is
controlled by MICROPY_PY_ARRAY_SLICE_ASSIGN config setting, default off.
But otherwise it may be useful, as allows to update arbitrary-sized data
buffers in-place.
Slice is yet to implement, and actually, slice assignment implemented in
such a way that RHS of assignment should be array of the exact same item
typecode as LHS. CPython has it more relaxed, where RHS can be any sequence
of compatible types (e.g. it's possible to assign list of int's to a
bytearray slice).
Overall, when all "slice write" features are implemented, it may cost ~1KB
of code.
This makes exception traceback info self contained (ie doesn't rely on
list object, which was a bit of a hack), reduces code size, and reduces
RAM footprint of exception by eliminating the list object.
Addresses part of issue #1126.
