On 2/28/19 1:10 PM, Richard Earnshaw (lists) wrote:
> On 27/01/2019 11:20, Bernd Edlinger wrote:
>>
>> $ arm-linux-gnueabihf-gcc -march=armv5te -O3 -S test.c
>> $ cat test.s
>> f:
>> @ args = 12, pretend = 0, frame = 0
>> @ frame_needed = 0, uses_anonymous_args = 0
>> @ link register save eliminated.
>> push {r4, r5}
>> mov r0, #8
>> ldrd r4, [sp, #12]
>
> So this is wrong; before the compiler can use 'f' it has to copy it to a
> suitably aligned location; it can't directly reuse the value on the
> stack as that is not sufficiently aligned for the type.
>
Yes, meanwhile I found out that the value would be copied in a new place
if an address was taken, but there is an issue with
output_move_double not checking the value's MEM_ALIGN.
>>
>> So isn't this wrong code, returning 8 for alignof when it is really 4,
>> and wouldn't it crash on armv5 and armv6 with SCTLR.U=0 ?
>
> Returning 8 is correct; since that is the alignment of the type; but GCC
> does need to copy underaligned types to suitably aligned memory before
> it uses them; it must not use the *value* that is passed directly,
> unless it can prove that doing so is safe (and as you point out, on
> armv5 it is not).
>
> I think technically, this is separate bug from the PCS one that was
> fixed, so needs a new PR.
>
Could you have a look at the patch I sent to fix the wrong code issue:
https://gcc.gnu.org/ml/gcc-patches/2019-02/msg00248.html
Is there a chance that this can still go into gcc-9?
Or do I have to to open a PR for it first?
Thanks
Bernd.
