Re: [PATCH] Sanitizing the middle-end interface to the back-end for strict alignment

[Jeff Law <law at redhat dot com>]

On 8/17/19 1:44 AM, Bernd Edlinger wrote:
> On 8/15/19 9:47 PM, Bernd Edlinger wrote:
>> Hi,
>>
>> this is the split out part from the "Fix not 8-byte aligned ldrd/strd on ARMv5 (PR 89544)"
>> which is sanitizing the middle-end interface to the back-end for strict alignment,
>> and a couple of bug-fixes that are necessary to survive boot-strap.
>> It is intended to be applied after the PR 89544 fix.
>>
>> I think it would be possible to change the default implementation of STACK_SLOT_ALIGNMENT
>> to make all stack variables always naturally aligned instead of doing that only
>> in assign_parm_setup_stack, but would still like to avoid changing too many things
>> that do not seem to have a problem.  Since this would affect many targets, and more
>> kinds of variables that may probably not have a strict alignment problem.
>> But I am ready to take your advice though.
>>
>>
>> Boot-strapped and reg-tested on x86_64-pc-linux-gnu and arm-linux-gnueabihf
>> Is it OK for trunk?
>>
>>
> 
> Hmm, actually the hunk in assign_parm_setup_stack is not only failing
> an assertion, but rather a wrong code bug:
> 
> I found now a test case that generates silently wrong code and is fixed
> by this patch.
> 
> $ cat unaligned-argument-3.c 
> /* { dg-do compile } */
> /* { dg-require-effective-target arm_arm_ok } */
> /* { dg-options "-marm -mno-unaligned-access -O3" } */
> 
> typedef int __attribute__((aligned(1))) s;
> 
> void x(char*, s*);
> void f(char a, s f)
> {
>   x(&a, &f);
> }
> 
> /* { dg-final { scan-assembler-times "str\t\[^\\n\]*\\\[sp\\\]" 1 } } */
> /* { dg-final { scan-assembler-times "str\t\[^\\n\]*\\\[sp, #3\\\]" 0 } } */
> 
> currently with -marm -mno-unaligned-access -O3 we generate:
> 
> f:
> 	@ args = 0, pretend = 0, frame = 8
> 	@ frame_needed = 0, uses_anonymous_args = 0
> 	str	lr, [sp, #-4]!
> 	sub	sp, sp, #12
> 	mov	r3, r0
> 	str	r1, [sp, #3]  <- may trap
> 	add	r0, sp, #7
> 	add	r1, sp, #3
> 	strb	r3, [sp, #7]
> 	bl	x
> 	add	sp, sp, #12
> 	@ sp needed
> 	ldr	pc, [sp], #4
> 
> 
> So I would like to add a test case to the patch as attached.
> 
> Tested with a cross, that both dg-final fail currently and are fixed
> with the other patches applied.
> 
> Is it OK for trunk?
OK when the patch that fixes this is ACK'd.

jeff