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From: "felix at breitweiser dot de" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/103100] New: unaligned access generated when zero-initializing large locals with SIMD-instructions and -O2 -mstrict-align Date: Fri, 05 Nov 2021 12:52:19 +0000 [thread overview] Message-ID: <bug-103100-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103100 Bug ID: 103100 Summary: unaligned access generated when zero-initializing large locals with SIMD-instructions and -O2 -mstrict-align Product: gcc Version: 11.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: felix at breitweiser dot de Target Milestone: --- Created attachment 51738 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51738&action=edit source code that generates the faulty assembly when zero-intializing large local variables, gcc 11.2 (with -O2 and -O3) uses SIMD registers to store a pair of 16-byte registers at once into memory. When doing so, gcc can generate code that does not access memory on a 16-byte aligned boundary, even though the aarch64 architecture requires memory accesses to be 16-byte aligned when using the full 16-byte SIMD registers. This happens with -mstrict-align enabled. For example: static void (*use)(unsigned char*); // to suppress optimizations extern "C" void _start() { unsigned char t2[216]={}; use(t2); } when compiled with "gcc -save-temps -O2 -mstrict-align" generates the following assembly: _start: stp x29, x30, [sp, #-240]!// assuming sp is aligned to 16-bytes here mov x1, #0x0 movi v0.4s, #0x0 add x2, sp, #0x28 // the value in x2 is 8-byte aligned, but not 16-byte aligned mov x29, sp stp xzr, xzr, [sp, #24] add x0, sp, #0x18 stp q0, q0, [x2] // x2 is not 16-byte aligned, so the store is not aligned add x2, sp, #0x48 str xzr, [sp, #232] stp q0, q0, [x2] add x2, sp, #0x68 stp q0, q0, [x2] add x2, sp, #0x88 stp q0, q0, [x2] add x2, sp, #0xa8 stp q0, q0, [x2] add x2, sp, #0xc8 stp q0, q0, [x2] blr x1 ldp x29, x30, [sp], #240 ret I have seen https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71727 and even though that is marked as fixed, this issue persists in gcc 11.2
next reply other threads:[~2021-11-05 12:52 UTC|newest] Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-11-05 12:52 felix at breitweiser dot de [this message] 2021-11-05 12:58 ` [Bug target/103100] " pinskia at gcc dot gnu.org 2021-11-05 13:17 ` pinskia at gcc dot gnu.org 2021-11-05 13:21 ` [Bug target/103100] [11/12 Regression] unaligned access generated with memset or {} " pinskia at gcc dot gnu.org 2021-11-05 13:26 ` pinskia at gcc dot gnu.org 2021-11-05 13:27 ` pinskia at gcc dot gnu.org 2021-11-05 13:35 ` rguenth at gcc dot gnu.org 2021-11-05 13:38 ` pinskia at gcc dot gnu.org 2021-11-06 5:26 ` pinskia at gcc dot gnu.org 2021-11-18 8:50 ` pinskia at gcc dot gnu.org 2021-11-19 1:55 ` pinskia at gcc dot gnu.org 2022-01-26 2:30 ` pinskia at gcc dot gnu.org 2022-04-21 7:50 ` rguenth at gcc dot gnu.org 2022-06-09 10:28 ` [Bug target/103100] [11/12/13 " pinskia at gcc dot gnu.org 2023-01-19 13:28 ` rguenth at gcc dot gnu.org 2023-01-20 13:59 ` rearnsha at gcc dot gnu.org 2023-01-23 7:29 ` felix at breitweiser dot de 2023-01-23 7:37 ` sam at gentoo dot org 2023-01-31 17:49 ` pinskia at gcc dot gnu.org 2023-01-31 17:51 ` pinskia at gcc dot gnu.org 2023-01-31 18:24 ` rearnsha at gcc dot gnu.org 2023-02-10 3:54 ` pinskia at gcc dot gnu.org 2023-03-24 12:09 ` pinskia at gcc dot gnu.org 2023-04-05 19:25 ` pinskia at gcc dot gnu.org 2023-05-29 10:06 ` [Bug target/103100] [11/12/13/14 " jakub at gcc dot gnu.org 2023-09-20 13:53 ` wilco at gcc dot gnu.org 2023-11-30 13:49 ` cvs-commit at gcc dot gnu.org
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