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* [Bug c/102909] New: Missing -Wunused-but-set-variable warning @ 2021-10-23 15:27 mytbk920423 at gmail dot com 2021-10-23 15:51 ` [Bug c/102909] " jakub at gcc dot gnu.org ` (3 more replies) 0 siblings, 4 replies; 5+ messages in thread From: mytbk920423 at gmail dot com @ 2021-10-23 15:27 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102909 Bug ID: 102909 Summary: Missing -Wunused-but-set-variable warning Product: gcc Version: 11.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: mytbk920423 at gmail dot com Target Milestone: --- GCC misses the -Wunused-but-set-variable in the some code (GCC and Clang output can be seen in https://godbolt.org/z/7658M4qra). I first found a bug caused by the following code, in which no compilers warn: void ioapic_set_max_vectors(void *ioapic_base, int mre_count) { u32 reg; u8 count; reg = io_apic_read(ioapic_base, 0x01); count = reg >> 16; if (mre_count > 0) count = mre_count - 1; // reg is defined but not used after this reg &= ~(0xff << 16); reg |= count << 16; // ``count`` should be ``reg`` in the following line io_apic_write(ioapic_base, 0x01, count); } I think that's because the variable ``reg`` is only unused in part of the control flow, so I change the code as following. However, GCC doesn't warn on this either. void ioapic_set_max_vectors(void *ioapic_base, int mre_count) { u32 reg; u32 new_reg; u8 count; reg = io_apic_read(ioapic_base, 0x01); count = reg >> 16; if (mre_count > 0) count = mre_count - 1; // new_reg is defined but not used new_reg = reg & (~(0xff << 16)); new_reg |= count << 16; // ``count`` should be ``new_reg`` in the following io_apic_write(ioapic_base, 0x01, count); } ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/102909] Missing -Wunused-but-set-variable warning 2021-10-23 15:27 [Bug c/102909] New: Missing -Wunused-but-set-variable warning mytbk920423 at gmail dot com @ 2021-10-23 15:51 ` jakub at gcc dot gnu.org 2021-10-24 1:39 ` mytbk920423 at gmail dot com ` (2 subsequent siblings) 3 siblings, 0 replies; 5+ messages in thread From: jakub at gcc dot gnu.org @ 2021-10-23 15:51 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102909 Jakub Jelinek <jakub at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |jakub at gcc dot gnu.org --- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> --- The warning works as documented. It warns about variables that are only stored to and never read. new_reg in the second testcase or reg is not such a variable, it is also read. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/102909] Missing -Wunused-but-set-variable warning 2021-10-23 15:27 [Bug c/102909] New: Missing -Wunused-but-set-variable warning mytbk920423 at gmail dot com 2021-10-23 15:51 ` [Bug c/102909] " jakub at gcc dot gnu.org @ 2021-10-24 1:39 ` mytbk920423 at gmail dot com 2021-10-24 1:47 ` mytbk920423 at gmail dot com 2021-10-24 6:09 ` pinskia at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: mytbk920423 at gmail dot com @ 2021-10-24 1:39 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102909 --- Comment #2 from Iru Cai <mytbk920423 at gmail dot com> --- So it looks something like https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44677 GCC thinks ``a`` is set but not used in ``a = 1 + b;``, but is used in ``a = 1; a += b;``. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/102909] Missing -Wunused-but-set-variable warning 2021-10-23 15:27 [Bug c/102909] New: Missing -Wunused-but-set-variable warning mytbk920423 at gmail dot com 2021-10-23 15:51 ` [Bug c/102909] " jakub at gcc dot gnu.org 2021-10-24 1:39 ` mytbk920423 at gmail dot com @ 2021-10-24 1:47 ` mytbk920423 at gmail dot com 2021-10-24 6:09 ` pinskia at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: mytbk920423 at gmail dot com @ 2021-10-24 1:47 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102909 --- Comment #3 from Iru Cai <mytbk920423 at gmail dot com> --- Looks like this kind of things are detected in the front-end. The GNAT front-end can warn on the similar things: procedure Main is A : Integer; B : constant Integer := 1; begin A := 0; A := A + B; end Main; $ gcc -O2 -gnatwa -gnatwe -c main.adb main.adb:6:09: warning: possibly useless assignment to "A", value might not be referenced ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/102909] Missing -Wunused-but-set-variable warning 2021-10-23 15:27 [Bug c/102909] New: Missing -Wunused-but-set-variable warning mytbk920423 at gmail dot com ` (2 preceding siblings ...) 2021-10-24 1:47 ` mytbk920423 at gmail dot com @ 2021-10-24 6:09 ` pinskia at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2021-10-24 6:09 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102909 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |RESOLVED Resolution|--- |DUPLICATE --- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> --- Dup of bug 44677. *** This bug has been marked as a duplicate of bug 44677 *** ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2021-10-24 6:09 UTC | newest] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2021-10-23 15:27 [Bug c/102909] New: Missing -Wunused-but-set-variable warning mytbk920423 at gmail dot com 2021-10-23 15:51 ` [Bug c/102909] " jakub at gcc dot gnu.org 2021-10-24 1:39 ` mytbk920423 at gmail dot com 2021-10-24 1:47 ` mytbk920423 at gmail dot com 2021-10-24 6:09 ` pinskia at gcc dot gnu.org
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