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In code such as:
case {a,Map} of
{a,#{}}=T ->
T
end
we must NOT rewrite a map pattern to a map expression like this:
case Map of
#{} ->
{a,#{}}
end
because the pattern '#{}' will match any map, but the expression
'#{}' will construct an empty map.
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The optimization of a 'case' statement could lead to incorrect
code that would cause an exception at run-time.
Here is an example to show how the optimization went wrong. Start
with the following code:
f({r,#{key:=Val},X}=S) ->
case S of
{r,_,_} ->
setelement(3, Val, S)
end.
(The record operations have already been translated to the
corresponding tuple operations.) The first step in case_opt/3 is
to substitute S to obtain:
f({r,#{key:=Val},X}=S) ->
case {r,#{key:=Val},X} of
{r,_,_} ->
setelement(3, Val, S)
end.
After that substitution the 'case' can be simplified to:
f({r,#{key:=Val},_}=S) ->
case #{key:=Val} of
NewVar ->
setelement(3, Val, S)
end.
That is the result from case_opt/3. Now eval_case/2 notices
that since there is only one clause left in the 'case', the
'case' can eliminated:
f({r,#{key:=Val},_}=S) ->
NewVar = #{key:=Val},
setelement(3, Val, S).
Since the map construction may have a side effect, it was not
eliminated, but assigned to a variable that is never used.
The problem is that '#{key:=Val}' is fine as a pattern, but in a
construction of a new map, the '=>' operator must be used. So the
map construction will fail, generating an exception.
As a conservative correction for a maintenance release, we will
abort the 'case' optimization if the substitution into the 'case'
expression is anything but data items (tuples, conses, or
literals) or variables.
Reported-by: Dmitry Aleksandrov
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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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* 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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