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* [Bug c/115019] New: wraparound bug with with -O3 and int128 @ 2024-05-10 1:31 djnz00 at gmail dot com 2024-05-10 1:37 ` [Bug c/115019] " pinskia at gcc dot gnu.org ` (2 more replies) 0 siblings, 3 replies; 4+ messages in thread From: djnz00 at gmail dot com @ 2024-05-10 1:31 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115019 Bug ID: 115019 Summary: wraparound bug with with -O3 and int128 Product: gcc Version: 14.1.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: djnz00 at gmail dot com Target Milestone: --- Created attachment 58157 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58157&action=edit reduced testcase wraparound and condition testing of 128bit values performs incorrectly under -O3: __int128_t i = ((__uint128_t)1)<<127; do { puts(itoa128(buf, i)); /* do something with i */ i += ((__uint128_t)1)<<125; } while (i); in other code, this loop runs interminably under -O3, but completes as expected without optimization; in this reduced testcase sample, it only executes one repeat of the loop (clang does it correctly); either way, there is a divergence of behavior with/without -O3 ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c/115019] wraparound bug with with -O3 and int128 2024-05-10 1:31 [Bug c/115019] New: wraparound bug with with -O3 and int128 djnz00 at gmail dot com @ 2024-05-10 1:37 ` pinskia at gcc dot gnu.org 2024-05-10 1:50 ` pinskia at gcc dot gnu.org 2024-05-10 1:54 ` pinskia at gcc dot gnu.org 2 siblings, 0 replies; 4+ messages in thread From: pinskia at gcc dot gnu.org @ 2024-05-10 1:37 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115019 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Resolution|--- |INVALID URL|https://godbolt.org/z/be37n | |7Mz3 | Host|x86_64-pc-linux-gnu | Build|x86_64-pc-linux-gnu | Status|UNCONFIRMED |RESOLVED Target|x86_64-pc-linux-gnu | --- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> --- This is an undefined behavior with signed integer overflow. Did you try with -fsanitize=undefined as requested by the bug reporting page? ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c/115019] wraparound bug with with -O3 and int128 2024-05-10 1:31 [Bug c/115019] New: wraparound bug with with -O3 and int128 djnz00 at gmail dot com 2024-05-10 1:37 ` [Bug c/115019] " pinskia at gcc dot gnu.org @ 2024-05-10 1:50 ` pinskia at gcc dot gnu.org 2024-05-10 1:54 ` pinskia at gcc dot gnu.org 2 siblings, 0 replies; 4+ messages in thread From: pinskia at gcc dot gnu.org @ 2024-05-10 1:50 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115019 --- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> --- /app/example.cpp:36:24: runtime error: negation of 0x80000000000000000000000000000000 cannot be represented in type '__int128'; cast to an unsigned type to negate this value to itself ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c/115019] wraparound bug with with -O3 and int128 2024-05-10 1:31 [Bug c/115019] New: wraparound bug with with -O3 and int128 djnz00 at gmail dot com 2024-05-10 1:37 ` [Bug c/115019] " pinskia at gcc dot gnu.org 2024-05-10 1:50 ` pinskia at gcc dot gnu.org @ 2024-05-10 1:54 ` pinskia at gcc dot gnu.org 2 siblings, 0 replies; 4+ messages in thread From: pinskia at gcc dot gnu.org @ 2024-05-10 1:54 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115019 --- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> --- (In reply to Andrew Pinski from comment #2) > /app/example.cpp:36:24: runtime error: negation of > 0x80000000000000000000000000000000 cannot be represented in type '__int128'; > cast to an unsigned type to negate this value to itself Once I fix that with: ``` static char *itoa128(char *buf, __int128_t v) { if (v < 0) { buf = utoa128(buf, -(__uint128_t)v); ``` GCC behavior is what you expect. ^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2024-05-10 1:54 UTC | newest] Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2024-05-10 1:31 [Bug c/115019] New: wraparound bug with with -O3 and int128 djnz00 at gmail dot com 2024-05-10 1:37 ` [Bug c/115019] " pinskia at gcc dot gnu.org 2024-05-10 1:50 ` pinskia at gcc dot gnu.org 2024-05-10 1:54 ` pinskia at gcc dot gnu.org
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