[RFH][libgcc] fp-bit bit ordering (PR 78804)

[Oleg Endo]

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?

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.
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