On 9/28/19 8:14 PM, Oleg Endo wrote:
> Hi,
>
> I've been dragging this patch along with me for a while.
> At the moment, I don't have the resources to fully test it as requested
> by Ian in the PR discussion.
>
> So I would like to ask for general comments on this one and hope that
> folks with bigger automated test setups can run the patch through their
> machinery for little endian targets.
>
>
> Summary of the story:
>
> I've noticed this issue on the RX on GCC 6, but it seems it's been
> there forever.
>
> On RX, fp-bit is used for software floating point emulation. The RX
> target also uses "MS bit-field" layout by default. This means that
> code like
>
> struct
> {
> fractype fraction:FRACBITS __attribute__ ((packed));
> unsigned int exp:EXPBITS __attribute__ ((packed));
> unsigned int sign:1 __attribute__ ((packed));
> } bits;
>
> will result in sizeof (bits) != 8
>
> For some reason, this bit-field style declaration is used only for
> FLOAT_BIT_ORDER_MISMATCH, which generally seems to be set for little
> endian targets. In other cases (i.e. big endian) open coded bit field
> extraction and packing is used on the base integer type, like
>
> fraction = src->value_raw & ((((fractype)1) << FRACBITS) - 1);
> exp = ((int)(src->value_raw >> FRACBITS)) & ((1 << EXPBITS) - 1);
> sign = ((int)(src->value_raw >> (FRACBITS + EXPBITS))) & 1;
>
> This works of course regardless of the bit-field packing layout of the
> target.
>
> Joseph suggested to pack the struct bit, which would fix the issue.
> https://gcc.gnu.org/ml/gcc-bugs/2017-08/msg01651.html
>
> However, I would like to propose to remove the special case of
> FLOAT_BIT_ORDER_MISMATCH altogether as in the attached patch.
>
> Any comments?
So probably the most interesting target for this test is v850-elf as
it's got a reasonably well functioning simulator, hard and soft FP
targets, little endian, and I'm familiar with its current set of failures.
I can confirm that your patch makes no difference in the test results
(which includes execution results).
In fact, there haven't been any problems on any target in my tester that
I can tie back to this change.
At this point I'd say let's go for it.
jeff
>
> Cheers,
> Oleg
>
>
>
> libgcc/ChangeLog
>
> PR libgcc/77804
> * fp-bit.h: Remove FLOAT_BIT_ORDER_MISMATCH.
> * fp-bit.c (pack_d, unpack_d): Remove special cases for
> FLOAT_BIT_ORDER_MISMATCH.
> * config/arc/t-arc: Remove FLOAT_BIT_ORDER_MISMATCH.
>
