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The BEAM loader will put floating point constants into the
literal pools for the module, but it will not check for duplicates.
We can do much better by having the compiler use the literal
pool for floating point constants.
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The calculation of the NewIndex field in fun entries is broken: the
sys_pre_expand and v3_kernel modules keep separate index counters
starting at zero; thus there is no guarantee that each fun within a
module will have its own unique NewIndex.
We don't really need the NewIndex any more (see below), but since
we do need the NewUniq field, we should fix NewIndex for cleanliness
sake. The simplest way is to assign NewIndex as late as possible,
namely in beam_asm, and to set it to the same value as Index.
Historical Note: Why NewIndex Was Introduced
There was once an idea that the debugger should be able to interpret
only a single function in a module (for speed). To make sure that
interpreted funs could be called from BEAM code and vice versa,
the fun identification must be visible in the abstract code.
Therefore a NewIndex field was introduced in each fun in the abstract
code.
However, it turned out that interpreting single functions does not
play well with aggressive code optimization. For example, in this
code:
f() ->
X = 1,
fun() -> X+2 end.
the variable X will seem to be free in the fun, but an aggressive
optimizer will replace X with 1 in the fun; thus the fun will no
longer have any free variables. Therefore, the debugger will always
interpret entire modules.
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Funs are identified by a triple, <Module,Uniq,Index>, where Module is
the module name, Uniq is a 27 bit hash value of some intermediate
representation of the code for the fun, and index is a small integer.
When a fun is loaded, the triple for the fun will be compared to
previously loaded funs. If all elements in the triple in the newly
loaded fun are the same, the newly loaded fun will replace the previous
fun. The idea is that if Uniq are the same, the code for the fun is also
the same.
The problem is that Uniq is only based on the intermediate representation
of the fun itself. If the fun calls local functions in the same module,
Uniq may remain the same even if the behavior of the fun has been changed.
See
http://erlang.org/pipermail/erlang-bugs/2007-June/000368.htlm
for an example.
As a long-term plan to fix this problem, the NewIndex and NewUniq
fields was added to each fun in the R8 release (where NewUniq is the
MD5 of the BEAM code for the module). Unfortunately, it turns
out that the compiler does not assign unique value to NewIndex (if it
isn't tested, it doesn't work), so we cannot use the
<Module,NewUniq,NewIndex> triple as identification.
It would be possible to use <Module,NewUniq,Index>, but that seems
ugly. Therefore, fix the problem by making Uniq more unique by
taking 27 bits from the MD5 for the BEAM code. That only requires
a change to the compiler.
Also update a test case for cover, which now fails because of the
stronger Uniq calculation. (The comment in test case about why the
Pid2 process survived is not correct.)
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Introduce the line/1 instruction in the compiler and the BEAM
virtual machine. It will not yet be generated by the compiler and
will not actually carry any information.
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Add the gc_bif's to the VM.
Add infrastructure for gc_bif's (guard bifs that can gc) with two and.
three arguments in VM (loader and VM).
Add compiler support for gc_bif with three arguments.
Add compiler (and interpreter) support for new guard BIFs.
Add testcases for new guard BIFs in compiler and emulator.
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* bg/compiler-attributes:
Remove opaque declarations from the attributes
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-opaque declarations should not be retained in the attributes
(because they will be loaded along with the code and are not
useful).
While at it, filter away those Dialyzer attributes as early
as possible - in v3_kernel.
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The original intention for the undocumented 'slim' option was
to omit non-essential parts of *.beam files to reduce the
size of the primary bootstrap. Therefore, debug information and
local function information (only used by xref, not by the loader)
are omitted, but information about the compilation time and
compiler version are still included.
Including compilation information is troublesome, however, when
committing the bootstrap into a revision control system, because
every beam file is guaranteed to be changed every time the bootstrap
is updated.
Therefore, change the 'slim' option to also omit compilation
information.
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