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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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For a long time, there has been an optimization for:
{V1,V2,...} = case Expr of Pat -> ... {Val1,Val2,...}; ... end
that avoids building the tuples. The construct looks like this
in Core Erlang:
let <V> = case X of
Pattern -> {Y,Z}
end
in case V of
{A,B} -> A+B
end
The current optimization will try to replace the second 'case'
with a 'let':
let <A,B> = case X of
Pattern -> <Y,Z>
end
in A+B
Simple variations of the construct would prevent the optimizations;
for example this one:
let <V> = case X of
Pattern -> {'ok',Val}
end
in case V of
{ok,Val} -> Val
end
The problem is that the optimization tries to do too much. By
making the optimization do less and have it depend on other
optimizations to finish the job, it will become more powerful.
Thus we can rewrite the code like this:
let <V1,V2> = case X of
Pattern -> <'ok',Val>
end
in let <V> = {V1,V2}
in case V of
{ok,Val} -> Val
end
Note that the second case is unchanged. The other optimizations in the
sys_core_fold module will optimize the second 'case' and eliminate the
building of the tuple.
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Optimize away 'not' in sys_core_fold instead of in beam_block
and beam_dead, as we can do a better job in sys_core_fold.
I modified the test suite temporarily to never turn off Core Erlang
modifications and looked at the coverage. With the new optimizations
active in sys_core_fold, the code in beam_block and beam_dead did not
find a single 'not' that it could optimize. That proves that the new
optimization is at least as good as the old one. Manually, I could
also verify that the new optimization would optimize some variations
of 'not' that the old one would not handle.
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More aggressive optimizations that we plan to introduce could cause
spurious compiler warnings.
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The 'try' ... 'catch' is problematic. Firstly, if no optimization
is possible, an exception will always be thrown. Secondly, bugs
in the code will go unnoticed.
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'=:=' is a cheaper operation than '==', so we should always
use '=:=' if the result will be the same as if '==' were used.
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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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649d6e73 simplified opt_simple_let_2/6 a little bit too much,
so that some list comprehensions in effect context were not
properly tail-recursive.
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In cd1eaf0116190, opt_simple_let_2/6 was updated to do the same
optimizations in 'value' and 'effect' context.
Coalesce the clauses for 'value' and 'effect' context to one to
make it clear that they do the same thing.
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The code for inlining high-order functions from the lists module
is quite annoying when you try to navigate the sys_core_fold
module. Break out the code into its own module.
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Those functions allow us to clean up some more code.
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Introduce access functions to hide the low-level details of how
type information is implemented.
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Essentially, core_lib:literal_value/1 became useless when literals
were introduced in R12. Since we always create #c_literal{} records
whenever possible, literal_value/1 would *only* succeed when it was
passed a #c_literal{} argument.
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We are about to deprecate core_lib:get_anno/1 and core_lib:set_anno/2,
so we should stop using them in the compiler.
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Rename add_scope/2 to sub_add_scope/2 to be similar in naming as
the other functions that operates on #sub{} (in particular,
sub_subst_scope/1). Also, move the definition to be near to the
other sub_* functions.
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* bjorn/compiler/dup-bug-fix/OTP-12453:
Teach case_opt/3 to avoid unnecessary building
sys_core_fold: Optimize let statements more aggressively
Suppress warnings for expressions that are assigned to '_'
trace_bif_SUITE: Ensure that a call to time/0 is not removed
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* maint:
Update primary bootstrap
Correct unsafe optimization of '==' and '/='
Conflicts:
bootstrap/lib/compiler/ebin/sys_core_fold.beam
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Since '=:=' is cheaper than '==', the compiler tries to replace
'==' with '=:=' if the result of comparison will be the same.
As an example:
V == {a,b}
can be rewritten to:
V =:= {a,b}
since the literal on the right side contains no numeric values
that '==' would compare differently to '=:='.
With the introduction of maps, we will need to take them into
account. Since the comparison of maps is planned to change in 18.0,
we will be very conservative and only do the optimization if
both keys and values are non-numeric.
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Given this code:
f(S) ->
F0 = F1 = {S,S},
[F0,F1].
case_opt/3 would "optimize" it like this:
f(S) ->
F1 = {S,S},
F0 = {S,S},
[F0,F1].
Similarly, this code:
g({a,_,_}=T) ->
{b,
[_,_] = [T,none],
x}.
would be rewritten to:
g({a,Tmp1,Tmp2}=T) ->
Tmp3 = {a,Tmp1,Tmp2},
{b,
[Tmp3,none],
x}.
where the tuple is rebuilt instead of using the T variable.
Rewrite case_opt/3 to be more careful while optimizing.
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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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* maint:
Update primary bootstrap
Be more careful about map patterns when evalutating element/2
Do not convert map patterns to map expressions
Conflicts:
bootstrap/lib/compiler/ebin/sys_core_fold.beam
lib/compiler/test/match_SUITE.erl
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We must not convert map patterns to map expressions.
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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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* bjorn/compiler/map-fixes:
cerl: Remove a clause in fold_map_pairs/3 that will never be reached
Move grouping of map constructions from v3_core to v3_kernel
core_pp: Correct printing of map literals
Strengthen and modernize compile_SUITE
core_parse: Always fold literal conses
cerl: Make sure that we preserve the invariants for maps
cerl_clauses: Fix indentation
sys_core_fold: Strengthen optimization of letrecs in effect context
Fix handling of binary map keys in comprehensions
core_lib: Teach is_var_used/2 to handle keys in map patterns
warnings_SUITE: Eliminate compiler warning for a shadowed variable
lc_SUITE: Add shadow/1
Modernize lc_SUITE
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sys_core_fold:eval_element/3 attempts to evaluate calls to element/2
at compile time or to warn when the call will obviously fail. For
example:
element(1, [a])
will obviously fail and eval_element/3 will produce a warning.
eval_element/3 uses the helper functions is_not_integer/1 and
is_not_tuple/1 to test whether the arguments are known to be
incorrect. The clauses that attempt to match #c_map{} in those
helper function will never be executed, because #c_map{} will
never occur directly in an argument for a function call.
For example, code such as:
element(1, #{a=>Val})
will be translated to:
let <NewVar> = #{a=>Val}
in element(1, NewVar)
since maps are not considered safe (some map operations may
cause an exception at run time).
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v3_core is careful to always create literals whenever possible.
Correct core_parse so it, too, always creates literals out
of literal conses. With that correction, we can remove the
workaround in sys_core_fold (introduced in 26a5dea3cb5e101)
that handles non-literal flags in a binary.
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We used to evaluate the body of a 'letrec' in value context, even
if the 'letrec' was being evaluated in effect context. In most
cases, the context does not matter because the body is usually
just an 'apply' which will never be optimized away.
However, in the case of incorrect code described in the previous
commit, it does matter. We can find such bad code by evaluating
the body in effect context. For example, if we have the following
incorrect code:
letrec
f/1 = fun(A) -> ... <use of Var> ...
in let Var = <<2:301>>
in apply(Arg)
If the letrec is evaluated in effect context, the code will be
reduced to:
letrec
f/1 = fun(A) -> ... <use of Var> ...
in seq Var = <<2:301>> do apply(Arg)
Now Var will be unbound and a later compiler pass will crash to
ensure that the bad Core Erlang code is noticed.
Also add a test case to ensure that the compiler crashes if the
bug fixed in the previous commit re-surfaces.
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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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The scope is supposed to contain all variables that are currently
live. We need this information for certain optimizations to
avoid capturing a name (a name that is in the scope must be renamed;
for an example, see move_let_into_expr/2 or any function that calls
sub_subst_scope/1). We also use the scope to optimize sub_del_var/2
and sub_is_val/2.
When optimizing case expressions, the scope could be reset to an
empty list (because sub_new/0 was called instead of sub_new/1).
That could cause name capture if inlining was turned on.
As simple way to force this bug is to uncomment the
"-define(DEBUG, 1)." near the beginning of the file. Without this
correction, most files in the test suite fail to compile.
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The pass sys_core_fold did not correctly handle non-matching patterns in code
such as:
0 = case <<>> of
<<>> -> 0;
a -> 1
end.
Function case_opt_lit/3 is rewritten in two passes to first remove any
non-matching clause and only then potentially remove the related patterns
in each clause.
Reported-by: Ulf Norell
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Not only variables are allowed as arguments, the name should reflect that.
Change cerl Map argument interface
* cerl:map_arg/1 is more suitable then cerl:map_val/1 in this case.
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For updates of Map literals which may cause an error will be
determined in runtime, i.e. instructions are emitted for those
updates.
The changes in cerl now requires compiler-5.0 to compile because of
is_map/1 guard.
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Reject all expressions that are known to fail.
Emit 'badarg' for those expressions.
Ex.
[]#{ a => 1}
Is not a valid map update expression.
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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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* nox/compiler/sys_core_fold-erlang-is_function-2:
Do not mark all calls to erlang:is_function/2 as safe
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Calls to erlang:is_function/2 where the second is not a literal nonnegative
integer can crash at runtime and thus can't be marked as safe.
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Calls to erlang:is_record/3 where the second and third arguments are not
respectively a literal atom and a literal integer can't be transformed to guards
and thus are not safe.
Reported-by: Ulf Norell
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* egil/compiler/maps-get_map_elements:
compiler: Strengthen Maps compile tests
compiler: Remove dead warning
erts: Fix erts_debug:disassemble/1
compiler: Transform list of Args to exact literal type
compiler: Test Maps aliasing
compiler: Use aliasing in map pair patterns
compiler: Check literal order in beam_validator
erts: Introduce new instructions for combined key fetches
compiler: Change map instructions for fetching values
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* nox/compiler/v3_core-case-arg-opt:
Optimise case arguments in sys_core_fold
Run sys_core_fold twice if any inliner is used
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Starting in e12b7d5331c58b41db06cadfa4af75b78b62a2b1,
sys_core_fold:eval_case/2 will crash on handwritten but legal
Core Erlang programs such as:
case let <Var> = Arg in {'x',Var} of
{x,X} -> X
end
The problem is that the only clause *is* guaranteed to match, but
cerl_clauses:match_list/2 does not understand that; all it can say is
that the clause *may* match. In those circumstances, we will need to
keep the case.
Also make sure that we keep the case if the guard is something else
than 'true'. That is not strictly necessary, because in a legal Core
Erlang program the guard in the last clause in a case must always
evaluate to 'true', so removing the guard test would still leave the
program correct. Keeping the guard, however, will make it somewhat
easier to debug an incorrect Core Erlang program. (The unsafe_case
test case has guard test in the only clause in a case, so we don't
need to write a new test case to test that.)
Reported-by: Anthony Ramine
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If the argument of a case expression is a let, a seq or a case with two clauses
where the second one is a failing clause; it can be moved outside the case and
further optimisations can be performed.
module 'foo' ['t'/4]
attributes []
't'/4 =
fun (_cor14,Sub0,_cor15,_cor16) ->
let Ssa = call 'erlang':'get' ('foo') in
case let Ssa = {'ssa',_cor14,Sub0,_cor15,_cor16} in
let _rec11 = call 'erlang':'+' (_cor14, 1) in
let _cor4 = call 'erlang':'setelement' (2, Ssa, _rec11) in
{{'tmp',_cor14},_cor4} of
{NewReg,Foo} when 'true' ->
{NewReg,Foo,Ssa}
_cor20 when 'true' ->
primop 'match_failure' ({'case_clause',_cor20})
end
end
==>
module 'foo' ['t'/4]
attributes []
't'/4 =
fun (_cor14,Sub0,_cor15,_cor16) ->
let Ssa = call 'erlang':'get' ('foo') in
let _fol0 = {'ssa',_cor14,Sub0,_cor15,_cor16} in
let _rec11 = call 'erlang':'+' (_cor14, 1) in
let _cor4 = call 'erlang':'setelement' (2, _fol0, _rec11) in
let NewReg = {'tmp',_cor14} in
{NewReg,_cor4,Ssa}
end
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* bjorn/compiler/optimizations/OTP-11584:
sys_core_fold: Prevent case expressions from being evaluated twice
sys_core_fold_SUITE: For cleanliness, move id/1 to the end
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* bjorn/eep37/OTP-11537:
Issue a warning when a named fun is constructed but not used
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