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From: "eggert at cs dot ucla.edu" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c/97370] New: comedy of boolean errors for '!a & (b|c)' Date: Sun, 11 Oct 2020 16:35:15 +0000 [thread overview] Message-ID: <bug-97370-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97370 Bug ID: 97370 Summary: comedy of boolean errors for '!a & (b|c)' Product: gcc Version: 10.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: eggert at cs dot ucla.edu Target Milestone: --- I ran into this problem while compiling a proposed patch for GNU grep. For the following program a.c: _Bool f (_Bool a, _Bool b, _Bool c) { return !a & (b|c); } _Bool g (_Bool a, _Bool b, _Bool c) { return !(a) & (b|c); } _Bool h (_Bool a, _Bool b, _Bool c) { return ~a & (b|c); } _Bool i (_Bool a, _Bool b, _Bool c) { return (b|c) & !a; } The command 'gcc -Wall -S a.c' generates bogus diagnostics for 'f', 'g', and 'h' (see the diagnostics at the end of this comment). * 'f' is incorrectly diagnosed even though it's the same thing as 'i' after commuting the operands of '&'. ('i' is correctly allowed.) * The diagnostic for 'f' suggests 'g', but 'g' produces the same diagnostic. * The diagnostic for 'f' sugggests 'h', but 'h' produces a different diagnostic. I understand why 'bool = ~bool' should be diagnosed (bug#77490), but 'h' should not be diagnosed since 'bool & ~bool' always has the usual boolean interpretation. I finally ended up using the equivalent of 'i' in GNU grep, but I should have been able to use any of 'f', 'g', or 'h' without worrying about generating a bogus warning. Here are the bogus diagnostics in question: ---- a.c: In function 'f': a.c:1:46: warning: suggest parentheses around operand of '!' or change '&' to '&&' or '!' to '~' [-Wparentheses] 1 | _Bool f (_Bool a, _Bool b, _Bool c) { return !a & (b|c); } | ^~ a.c: In function 'g': a.c:2:46: warning: suggest parentheses around operand of '!' or change '&' to '&&' or '!' to '~' [-Wparentheses] 2 | _Bool g (_Bool a, _Bool b, _Bool c) { return !(a) & (b|c); } | ^~~~ a.c: In function 'h': a.c:3:46: warning: '~' on a boolean expression [-Wbool-operation] 3 | _Bool h (_Bool a, _Bool b, _Bool c) { return ~a & (b|c); } | ^ a.c:3:46: note: did you mean to use logical not? 3 | _Bool h (_Bool a, _Bool b, _Bool c) { return ~a & (b|c); } | ^ | !
next reply other threads:[~2020-10-11 16:35 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-10-11 16:35 eggert at cs dot ucla.edu [this message] 2020-10-11 17:01 ` [Bug c/97370] " harald at gigawatt dot nl 2020-10-12 7:22 ` eggert at cs dot ucla.edu 2020-10-12 18:16 ` harald at gigawatt dot nl 2020-10-13 3:36 ` eggert at gnu dot org
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