Possible bug: Unaligned bit-field address when predictive commoning.

[Richard Biener]

On Thu, May 12, 2016 at 12:10 PM, Claudiu Zissulescu
<Claudiu.Zissulescu@synopsys.com> wrote:
> Hi,
>
> I've been trying the following simple test case on latest gcc, and it seems to produce unwanted unaligned accesses for bit-fields.
>
> Test cases:
>
> struct lock_chain {
>   unsigned int irq_context: 2,
>     depth: 6,
>     base: 24;
> };
>
> struct lock_chain * foo (struct lock_chain *chain)
> {
>   int i;
>   for (i = 0; i < 100; i++)
>     {
>       chain[i+1].base = chain[i].base;
>     }
>   return chain;
> }
>
> GCC options -O3 (we need predictive commoning to kick in).
>
> The result for ARM:
>
>         .cpu arm926ej-s
>         .eabi_attribute 20, 1
>         .eabi_attribute 21, 1
>         .eabi_attribute 23, 3
>         .eabi_attribute 24, 1
>         .eabi_attribute 25, 1
>         .eabi_attribute 26, 1
>         .eabi_attribute 30, 2
>         .eabi_attribute 34, 0
>         .eabi_attribute 18, 4
>         .file   "t01.c"
>         .text
>         .align  2
>         .global foo
>         .syntax unified
>         .arm
>         .fpu softvfp
>         .type   foo, %function
> foo:
>         @ args = 0, pretend = 0, frame = 0
>         @ frame_needed = 0, uses_anonymous_args = 0
>         @ link register save eliminated.
>         ldr     r1, [r0, #1]                    <<<<<<<<<<<< Unaligned access
>         add     r3, r0, #4
>         lsl     r1, r1, #8
>         add     ip, r0, #404
> .L2:
>         ldrb    r2, [r3]        @ zero_extendqisi2
>         orr     r2, r1, r2
>         str     r2, [r3], #4
>         cmp     r3, ip
>         bne     .L2
>         bx      lr
>         .size   foo, .-foo
>         .ident  "GCC: (GNU) 7.0.0 20160502 (experimental)"
>
> Please observe the ldr instruction will access an unaligned address.
>
> I observed this issue for ARC processor, but because the ARM is more popular, I've use it for this case.
>
> Is this expected behavior or do I miss something?

Looks like a bug - please file a bugreport.

Richard.

> Thank you,
> Claudiu