From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id A91C5385840D; Mon, 30 Aug 2021 08:27:44 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org A91C5385840D From: "jankowski938 at gmail dot com" To: gcc-bugs@gcc.gnu.org Subject: [Bug c/102125] New: (ARM Cortex-M3 and newer) missed optimization. memcpy not needed operations Date: Mon, 30 Aug 2021 08:27:44 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: new X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: c X-Bugzilla-Version: 10.2.1 X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: jankowski938 at gmail dot com X-Bugzilla-Status: UNCONFIRMED X-Bugzilla-Resolution: X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status bug_severity priority component assigned_to reporter target_milestone Message-ID: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: gcc-bugs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-bugs mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 30 Aug 2021 08:27:44 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102125 Bug ID: 102125 Summary: (ARM Cortex-M3 and newer) missed optimization. memcpy not needed operations Product: gcc Version: 10.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: jankowski938 at gmail dot com Target Milestone: --- uint64_t bar64(const uint8_t *rData1) { uint64_t buffer; memcpy(&buffer, rData1, sizeof(buffer)); return buffer; } compiler options:=20 -Ox -mthumb -mcpu=3Dcortex-my where x : 2,3,s y:3,4,7 ``` bar64: sub sp, sp, #8 mov r2, r0 ldr r0, [r0] @ unaligned ldr r1, [r2, #4] @ unaligned mov r3, sp stmia r3!, {r0, r1} ldrd r0, [sp] add sp, sp, #8 bx lr ``` it is enough to: ``` mov r3, r0 ldr r0, [r0] @ unaligned ldr r1, [r3, #4] @ unaligned bx lr ``` 32 bit memcpy is optimized correctly: Full example code: ``` uint64_t foo64(const uint8_t *rData1) { uint64_t buffer; buffer =3D (((uint64_t)rData1[7]) << 56)|((uint64_t)(rData1[6]) << 48)|((uint64_t)(rData1[5]) << 40)|(((uint64_t)rData1[4]) << 32)| (((uint64_t)rData1[3]) << 24)|(((uint64_t)rData1[2]) << 16)|((uint64_t)(rData1[1]) << 8)|rData1[0]; return buffer; } uint64_t bar64(const uint8_t *rData1) { uint64_t buffer; memcpy(&buffer, rData1, sizeof(buffer)); return buffer; } uint32_t foo32(const uint8_t *rData1) { uint32_t buffer; buffer =3D (((uint32_t)rData1[3]) << 24)|(((uint32_t)rData1[2]) << 16)|((uint32_t)(rData1[1]) << 8)|rData1[0]; return buffer; } uint32_t bar32(const uint8_t *rData1) { uint32_t buffer; memcpy(&buffer, rData1, sizeof(buffer)); return buffer; } ``` compiler output: ``` foo64: mov r3, r0 ldr r0, [r0] @ unaligned ldr r1, [r3, #4] @ unaligned bx lr bar64: sub sp, sp, #8 mov r2, r0 ldr r0, [r0] @ unaligned ldr r1, [r2, #4] @ unaligned mov r3, sp stmia r3!, {r0, r1} ldrd r0, [sp] add sp, sp, #8 bx lr foo32: ldr r0, [r0] @ unaligned bx lr bar32: ldr r0, [r0] @ unaligned bx lr ``` Clang compiles without overhead: https://godbolt.org/z/P7G7Whxqz=