[Bug rtl-optimization/50110] Endian reversal when adding extzv instruction

["david.meggy at icron dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50110

--- Comment #4 from David Meggy <david.meggy at icron dot com> 2011-08-17 23:15:26 UTC ---
>From rtl.def


/* Reference to a signed bit-field of specified size and position.
   Operand 0 is the memory unit (usually SImode or QImode) which
   contains the field's first bit.  Operand 1 is the width, in bits.
   Operand 2 is the number of bits in the memory unit before the
   first bit of this field.
   If BITS_BIG_ENDIAN is defined, the first bit is the msb and
   operand 2 counts from the msb of the memory unit.
   Otherwise, the first bit is the lsb and operand 2 counts from
   the lsb of the memory unit.
   This kind of expression can not appear as an lvalue in RTL.  */
DEF_RTL_EXPR(SIGN_EXTRACT, "sign_extract", "eee", RTX_BITFIELD_OPS)

/* Similar for unsigned bit-field.
   But note!  This kind of expression _can_ appear as an lvalue.  */
DEF_RTL_EXPR(ZERO_EXTRACT, "zero_extract", "eee", RTX_BITFIELD_OPS)



It appears that this behaviour is intended.  To me it seems logical that the
extract instruction is really just a "right shift", followed by an "and", where
the "and" uses a mask based on the width of the bitfield.  To count from the
MSB seems counter-intuitive to me.

If my understanding is correct this code comment should appear in
http://gcc.gnu.org/onlinedocs/gccint/Standard-Names.html#Standard-Names under
the extzv and extv headings.  I can fix up my define_insn to do the conversion
in our implementation.