Unexpected unaligned access on arm

[Lars Poeschel]

Hi!

I am experiencing a problem on ARM with unaligned access where I think
it should not happen. I could track this down to the attached example. I
compile the code using the following command:

arm-linux-gnueabihf-gcc -Wall -Os -g -Wa,-mthumb -mthumb-interwork -mno-unaligned-access -mhard-float -march=armv7-a -static -o gcc_example gcc_example.c

The problem is in net_set_ip_header function. The relevant produced
assembler output is this:

   1044c:	b513      	push	{r0, r1, r4, lr}
   1044e:	4604      	mov	r4, r0
   10450:	9200      	str	r2, [sp, #0]
   10452:	300c      	adds	r0, #12
   10454:	4a11      	ldr	r2, [pc, #68]	; (1049c <net_set_ip_header+0x50>)
   10456:	4b10      	ldr	r3, [pc, #64]	; (10498 <net_set_ip_header+0x4c>)
   10458:	447a      	add	r2, pc
   1045a:	9101      	str	r1, [sp, #4]
   1045c:	f840 3c0c 	str.w	r3, [r0, #-12]
   ...
   10498:	14000045 	.word	0x14000045
   1049c:	00060a48 	.word	0x00060a48

The compiler is smart and optimizes setting the constants to the struct
ip_udp_hdr fields ip_hl_v ip_tos and ip_len to one single 32bit (word)
access. It stores the three constants pc relative in a word. This gets
loaded to r3 and then stored to the structure in memory (str.w
instruction).
Since I compile with -mno-unaligned-access and the relevant structure
members are 8 / 16bit and even the function argument uint8_t *pkt pointer
tell the compiler that these are not neccessarily 32bit aligned, I think
it should not do this.
The relevant code crashes on my cpu depending on the value of the pkt
pointer when calling the function of course. The
pkt += ETH_HDR_SIZE;
line can be used to modify the pointer if needed.
If I compile with -O0 or -O1 it does not happen. It does also not
happen when commenting out the asm("nop"); as a optimization barrier.
I tested this with gcc 8.3.1, 9.3.0 and 10.1.0. They all behave more or
less the same in this regard.

What am I missing ? What am I doing wrong ?

Thank you,
Lars
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