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86fbd6d76d strengthened type optimization in lets. As a result of
the stronger optimizations, special care had to be taken to
suppress false warnings.
It turns out that false warnings can still slip through. Slapping
on a 'compiler_generated' annotation at the top-level of a
complex term such as #c_tuple{} may not suppress all warnings.
We will need to go deeper into the term to eliminate all warnings.
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According to EEP-43 for maps, a 'badmap' exception should be
generated when an attempt is made to update non-map term such as:
<<>>#{a=>42}
That was not implemented in the OTP 17.
José Valim suggested that we should take the opportunity to
improve the errors coming from map operations:
http://erlang.org/pipermail/erlang-questions/2015-February/083588.html
This commit implement better errors from map operations similar
to his suggestion.
When a map update operation (Map#{...}) or a BIF that expects a map
is given a non-map term, the exception will be:
{badmap,Term}
This kind of exception is similar to the {badfun,Term} exception
from operations that expect a fun.
When a map operation requires a key that is not present in a map,
the following exception will be raised:
{badkey,Key}
José Valim suggested that the exception should be
{badkey,Key,Map}. We decided not to do that because the map
could potentially be huge and cause problems if the error
propagated through links to other processes.
For BIFs, it could be argued that the exceptions could be simply
'badmap' and 'badkey', because the bad map and bad key can be found in
the argument list for the BIF in the stack backtrace. However, for the
map update operation (Map#{...}), the bad map or bad key will not be
included in the stack backtrace, so that information must be included
in the exception reason itself. For consistency, the BIFs should raise
the same exceptions as update operation.
If more than one key is missing, it is undefined which of
keys that will be reported in the {badkey,Key} exception.
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Make sure that we take extract all possible type information when
optimizing a 'let' construct.
Since the stronger optimization may generate false warnings, we also
need to take special care to suppress false warnings.
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If we have a sequence of put_map_* instructions operating on the
same map, it will be more efficient if we can have one is_map/2
instruction before put_map_* instructions, so that each put_map_*
does not need to test whether the argument is a map.
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I originally decided that in 'value' context, rewriting a let statement
where the variables were not in the body to a sequence was not worth
it, because the variables would be unused in only one let in a
thousand lets (roughly).
I have reconsidered.
The main reason is that if we do the rewrite, core_lib:is_var_used/2
will be used much more frequently, which will help us to find bugs
in it sooner.
Another reason is that the way letify/2 is currently implemented
with its own calls to core_lib:is_var_used/2 is only safe as long
as all the bindings are independent of each other. We could make
letify/2 smarter, but if we introduce this new optimization there
is no need.
Measuring compilation speed, I have not seen any significant slowdown.
It seems that although core_lib:is_var_used/2 is called much more
frequently, most calls will be fast because is_var_used/2 will quickly
find a use of the variable.
Also add a test case to cover a line opt_guard_try/1 that was
no longer covered.
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In c34ad2d5, the compiler learned to silence some warnings for
expressions that were explicitly assigned to the '_' variable,
as in this example:
_ = list_to_integer(S),
ok
That commit intentionally only made it possible to silence warnings
for BIFs that could cause an exception. Warnings would still be
produced for:
_ = date(),
ok
because date/0 can never fail and thus making the call completely
useless. The reasoning was that such warnings can always be
eliminated by eliminating the offending code.
While that is true, there is the question about rules and their
consistency. It is surprising that '_' can be used to silence
some warnings, but has no effect on other warnings.
Therefore, we will teach the compiler to silence warnings for
the following constructs:
* Calls to safe BIFs such as date/0
* Expressions that will cause an exception such as 'X/0'
* Terms that are built but not used, such as '{x,X}'
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The fallback to latin-1 encoding would not work if the invalid
UTF-8 characters occurred in a skipped branch in an -ifdef/-ifndef.
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Even if a binary key is written as a literal the compiler may
choose to make an expression. Emit a warning in those cases
and saying the case will not match.
This is a limitation in current implementation.
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Increases coverage.
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The default encoding for Erlang modules is now UTF-8, and the
compilation would fail if a module contained byte sequences that
are not valid UTF-8 sequences.
In a large project with say many hundreds of Erlang modules
with names of developers such as "Björn" or "Håkan" encoded in
latin-1, that could mean that many hundreds of files would need
to be modified just to get started testing OTP 17.
As a temporary measure to ease the transition, automatically
fall back to the latin-1 encoding with a warning for any module
that contains invalid byte sequences and for which no encoding
has been specified.
The intention is to remove this workaround in OTP 18 or 19.
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Boolean case expressions with redundant clauses could make the compiler
crash:
case X == 0 of
false -> no;
false -> no;
true -> yes
end.
Reported-by: Ulf Norell
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Without this, sys_core_fold could crash on non-matching clauses using maps
patterns.
Reported-by: Ulf Norell
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Case expressions such as:
case {Expr1,Expr} of
{V1,V2} -> ...
end
are already optimized to not actually build the tuple. Generalize
the optimization to avoid building any kind of composite term,
such as:
case {ok,[A,B]} of
{ok,[X,Y]} -> ...
end
We don't expect programmers to write such code directly, but
inlining can produce such code.
We need to be careful about the warnings we produce. If the case
expression is a literal, it is expected that no warnings should be
produced for clauses that don't match. We must make sure that we
continue to suppress those warnings.
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When compiling comprehensions with generators which are foldable to
'true', a misleading warning is emitted by sys_core_fold because a
clause resulting from the compilation of the comprehension to Core
Erlang is not marked as generated by the compiler.
An example of such a comprehension is [ true || true ].
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Run testcases in parallel will make the test suite run slightly
faster. Another reason for this change is that we want more testing
of parallel testcase support in common_test.
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In 3d0f4a3085f11389e5b22d10f96f0cbf08c9337f (an update to conform
with common_test), in all test_lib:recompile(?MODULE) calls, ?MODULE
was changed to the actual name of the module. That would cause
test_lib:recompile/1 to compile the module with the incorrect
compiler options in cloned modules such as record_no_opt_SUITE,
causing worse coverage.
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The compiler (sys_core_fold) tries to avoid constructing tuples
in case expressions. The following code:
c(A, B) ->
case {A,B} of
{ok,X} -> X;
{_,_} -> error
end.
will be rewritten so that no tuple is built. If a clause
requires a tuple to be built as in this code:
c(A, B) ->
case {A,B} of
{ok,X} -> X;
V -> V %The tuple will be built here
end.
the tuple will be built in the clause(s) in which it is needed.
If the value returned from the case is not used as in this code:
c(A, B) ->
case {A,B} of
V -> V %Warning: a term is constructed, but never used
end,
ok.
there will be an incorrect warning. Basically, what happens is
that the code is reduced to:
c(A, B) ->
{A,B}, %Warning: a term is constructed, but never used
ok.
and the optimizer sees that the {A,B} tuple can't possibly be used.
Eliminate the warning by adding a 'compiler_generated' annotation
to the tuple.
Reported-by: Kostis Sagonas
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* bg/compiler-suppress-result-ignored:
compiler tests: Eliminate "result of expression is ignored" warnings
Silence warnings for expressions that are assigned to "_"
OTP-8602 bg/compiler-suppress-result-ignored
It is now possible to suppress the warning in code such as
"list_to_integer(S), ok" by assigning the ignored value "_" like this: "_ =
list_to_integer(S), ok".
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There is currently no zero-cost way to silence the warning
"the result of the expression is ignored", which is issued
for code such as:
list_to_integer(S),
ok
Such code can be useful for assertions or input validation.
Teach the compiler to silence the warning for expressions
that are explicitly assigned to to the "_" variable,
such as:
_ = list_to_integer(S),
ok
Implement it by having the v3_core pass annotate calls in
Core Erlang like this:
let <_> = ( call 'erlang':'list_to_integer'(S) -| ['result_not_wanted'] )
in 'ok'
and modifiy sys_core_fold to suppress the warning for any call
having the annotation.
We deliberately do not make it possible to silence the warnings
for expressions like:
{build,an,unnecessary,term}, ok
or
is_list(L), ok
because we don't know of any real-world scenarios in which that would
be useful.
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