From mboxrd@z Thu Jan 1 00:00:00 1970 From: catalin.marinas@arm.com (Catalin Marinas) Date: Wed, 25 May 2011 12:14:08 +0100 Subject: [PATCH] ARM: Do not allow unaligned accesses when CONFIG_ALIGNMENT_TRAP In-Reply-To: <20110524171331.GA2941@arm.com> References: <20110523111648.10474.78396.stgit@e102109-lin.cambridge.arm.com> <20110523132124.GI17672@n2100.arm.linux.org.uk> <1306229953.19557.14.camel@e102109-lin.cambridge.arm.com> <20110524171331.GA2941@arm.com> Message-ID: <20110525111405.GA12010@e102109-lin.cambridge.arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Tue, May 24, 2011 at 06:13:31PM +0100, Dave Martin wrote: > On Tue, May 24, 2011 at 04:26:35PM +0100, Catalin Marinas wrote: > > BTW, are we sure that the code generated with unaligned accesses is > > better? AFAIK, while processors support unaligned accesses, they may > > not always be optimal. > > The code gcc generates to synthesise an unaligned access using aligned > accesses is pretty simplistic: ... > For code which natively needs to read unaligned fields from data structures, > I sincerely doubt that the CPU will not beat the above code for efficiency... > > So if there's code doing unaligned access to data structures for a good > reason, building with unaligned access support turned on in the compiler > seems a good idea, if that code might performance-critical for anything. gcc generates unaligned accesses in the the pcpu_dump_alloc_info() function. We have a local variable like below (9 bytes): char empty_str[] = "--------"; and it looks like other stack accesses are unaligned: c0082ba0 : c0082ba0: e92d4ff0 push {r4, r5, r6, r7, r8, r9, sl, fp, lr} c0082ba4: e3074118 movw r4, #28952 ; 0x7118 c0082ba8: e24dd04c sub sp, sp, #76 ; 0x4c c0082bac: e34c402a movt r4, #49194 ; 0xc02a c0082bb0: e58d1014 str r1, [sp, #20] c0082bb4: e58d0020 str r0, [sp, #32] c0082bb8: e8b40003 ldm r4!, {r0, r1} c0082bbc: e58d003f str r0, [sp, #63] <----------- !!!!! c0082bc0: e59d0014 ldr r0, [sp, #20] c0082bc4: e5d43000 ldrb r3, [r4] I haven't tried with -mno-unaligned-access, I suspect the variables on the stack would be aligned. -- Catalin