[Bug target/86968] Unaligned big-endian (scalar_storage_order) access on armv7-a yields 4 ldrb instructions rather than ldr+rev

[thopre01 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86968

--- Comment #9 from Thomas Preud'homme <thopre01 at gcc dot gnu.org> ---
(In reply to Thomas Preud'homme from comment #8)
> (In reply to Thomas Preud'homme from comment #7)
> > (In reply to Thomas Preud'homme from comment #6)
> > > Happens at expand time. Diving in.
> > 
> > There's a giant if in expand_expr_real_1 with the following comment:
> > 
> > /* In cases where an aligned union has an unaligned object
> >    as a field, we might be extracting a BLKmode value from
> >    an integer-mode (e.g., SImode) object.  Handle this case
> >    by doing the extract into an object as wide as the field
> >    (which we know to be the width of a basic mode), then
> >    storing into memory, and changing the mode to BLKmode.  */
> > 
> > The "if" is entered in the big endian unaligned case but not in the other
> > case. In the aligned case, it continues after the if until the call to
> > flip_storage_order which will generate the bswap.
> 
> In the aligned case, the if is not taken because alignment of the memory Vs
> access is sufficient. There is provision to call flip_storage_order but only
> if the access is a RECORD and here the mode class is INT.
> 
> Therefore unaligned access are handled by extract_bit_field. This in turns
> call extract_bit_field_1 and later extract_integral_bit_field where things
> are different between little endian and big endian. For little endian, it
> goes in the following if block:
> 
>   /* If OP0 is a memory, try copying it to a register and seeing if a
>      cheap register alternative is available.  */
>   if (MEM_P (op0) & !reverse)
> 
> For big endian it continues and calls extract_fixed_bit_field. I'm wondering
> if an extra call to flip_storage_order when reverse is true would solve the
> issue. Need to understand better whe is it safe to call flip_storage_order.

It gives me the expected assembly but I need to convince myself that this is
always safe.