CC: Michal added
Paul Bolle <pebolle@xxxxxxxxxx> writes:
> On Mon, 2014-09-22 at 19:13 +0200, Martin Walch wrote:
>> expr_eliminate_dups2() in scripts/kconfig/expr.c applies two bad
>> inference rules:
>>
>> (FOO || BAR) && (!FOO && !BAR) -> n
>> (FOO && BAR) || (!FOO || !BAR) -> y
>>
>> They would be correct in propositional logic, but this is a three-valued
>> logic, and here it is wrong in that it changes semantics. It becomes
>> immediately visible when substituting FOO and BAR with m.
>
> Well, not to me. When I apply the rules under "Menu dependencies" in
> Documentation/kbuild/kconfig-language.txt, in embarrassingly small
> steps, I get:
> (m || m) && (!m && !m) ->
> (1 || 1) && ((2 - 1) && (2 - 1)) ->
> (1 || 1) && (1 && 1) ->
> max(1, 1) && min(1, 1) ->
> 1 && 1 ->
> min(1, 1) ->
> 1 -> m
>
> and
> (m && m) || (!m || !m) ->
> (1 && 1) || ((2 - 1) || (2 - 1)) ->
> (1 && 1) || (1 || 1) ->
> min(1, 1) || max(1, 1) ->
> 1 || 1 ->
> max(1, 1) ->
> 1 -> m
>
> Did I do that right? Anyway, perhaps you'd like to add a line or two how
> these two expressions actually evaluate when FOO and BAR are m.
Perhaps, it helps to use a Kconfig file that visualizes the problem:
mainmenu "Expr Test"
config expr-config
bool "Expr Config"
depends on (m || m) && (!m && !m)
# depends on (m && m) || (!m || !m)
config dummy
bool "Dummy"
config modules
bool
option modules
default y
Those two depends, according to your calculation and if I understood
correctly also to Martin's, should evaluate to 'm' and thus the menu
entry should always be visible, because both depends should evaluate to
'm'. But because of expr_eliminate_dups2() it is either invisible (the
first depends becomes 'n') or always visible (the second depends becomes
'y').
Dirk
>> Fix it by removing expr_eliminate_dups2() and the functions that have no use
>> anywhere else: expr_extract_eq_and(), expr_extract_eq_or(),
>> and expr_extract_eq() from scripts/kconfig/expr.[ch]
>>
>> Currently the bug is not triggered in mainline, so this patch does not modify
>> the configuration space there.
>>
>> As a side effect, this reduces code size in expr.c by roughly 10% and slightly
>> improves startup time for all configuration frontends.
>>
>> Signed-off-by: Martin Walch <walch.martin@xxxxxx>
>
> Does Yann still care about Kconfig? Yann has been rather quiet for a
> while now.
>
>
> Paul Bolle
>
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