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* [Bug middle-end/102829] New: Redundant null check after atomic load from that address @ 2021-10-19 8:17 laurynas.biveinis at gmail dot com 2021-10-19 8:31 ` [Bug tree-optimization/102829] " rguenth at gcc dot gnu.org ` (2 more replies) 0 siblings, 3 replies; 4+ messages in thread From: laurynas.biveinis at gmail dot com @ 2021-10-19 8:17 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102829 Bug ID: 102829 Summary: Redundant null check after atomic load from that address Product: gcc Version: 12.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: middle-end Assignee: unassigned at gcc dot gnu.org Reporter: laurynas.biveinis at gmail dot com Target Milestone: --- The following source code ---- struct d { long b; d *e() { __atomic_load_n(&b, 0); return this; } }; d *j; void i(); void k() { auto l = j->e(); if (l) i(); } ---- produces the following bit of assembly with -O3 on x86_64 (https://godbolt.org/z/G1ThvnMWP): ... mov rdx, QWORD PTR [rax] test rax, rax je .L1 ... It first dereferences the address at RAX, and later checks whether RAX == 0. Since we already tried accessing memory at that address, the nullptr check seems redundant. Moreover, since the address happens to be of 'this' of variable j, it being equal to nullptr is UB anyway and may be assumed to be != nullptr? Clang tests show the redundant test being present up to version 11 inclusive (https://godbolt.org/z/rT3ha4enb) and absent from version 12 onwards (https://godbolt.org/z/Gfb9EWMfq) ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/102829] Redundant null check after atomic load from that address 2021-10-19 8:17 [Bug middle-end/102829] New: Redundant null check after atomic load from that address laurynas.biveinis at gmail dot com @ 2021-10-19 8:31 ` rguenth at gcc dot gnu.org 2021-10-19 14:03 ` laurynas.biveinis at gmail dot com 2021-10-22 21:46 ` pinskia at gcc dot gnu.org 2 siblings, 0 replies; 4+ messages in thread From: rguenth at gcc dot gnu.org @ 2021-10-19 8:31 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102829 Richard Biener <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Component|middle-end |tree-optimization Ever confirmed|0 |1 Blocks| |85316 Status|UNCONFIRMED |NEW Keywords| |missed-optimization Last reconfirmed| |2021-10-19 --- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> --- We don't derive non-nullness for the pointer dereferenced by the atomic builtins yet. Referenced Bugs: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85316 [Bug 85316] [meta-bug] VRP range propagation missed cases ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/102829] Redundant null check after atomic load from that address 2021-10-19 8:17 [Bug middle-end/102829] New: Redundant null check after atomic load from that address laurynas.biveinis at gmail dot com 2021-10-19 8:31 ` [Bug tree-optimization/102829] " rguenth at gcc dot gnu.org @ 2021-10-19 14:03 ` laurynas.biveinis at gmail dot com 2021-10-22 21:46 ` pinskia at gcc dot gnu.org 2 siblings, 0 replies; 4+ messages in thread From: laurynas.biveinis at gmail dot com @ 2021-10-19 14:03 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102829 --- Comment #2 from Laurynas Biveinis <laurynas.biveinis at gmail dot com> --- FWIW adding "if (this == nullptr) __builtin_unreachable();" between __atomic_load_n and return fails to workaround this issue ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/102829] Redundant null check after atomic load from that address 2021-10-19 8:17 [Bug middle-end/102829] New: Redundant null check after atomic load from that address laurynas.biveinis at gmail dot com 2021-10-19 8:31 ` [Bug tree-optimization/102829] " rguenth at gcc dot gnu.org 2021-10-19 14:03 ` laurynas.biveinis at gmail dot com @ 2021-10-22 21:46 ` pinskia at gcc dot gnu.org 2 siblings, 0 replies; 4+ messages in thread From: pinskia at gcc dot gnu.org @ 2021-10-22 21:46 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102829 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Severity|normal |enhancement ^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2021-10-22 21:46 UTC | newest] Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2021-10-19 8:17 [Bug middle-end/102829] New: Redundant null check after atomic load from that address laurynas.biveinis at gmail dot com 2021-10-19 8:31 ` [Bug tree-optimization/102829] " rguenth at gcc dot gnu.org 2021-10-19 14:03 ` laurynas.biveinis at gmail dot com 2021-10-22 21:46 ` pinskia at gcc dot gnu.org
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