[Bug target/15184] [4.8/4.9/5 Regression] Direct access to byte inside word not working with -march=pentiumpro

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=15184

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |thomas.preudhomme at arm dot com

--- Comment #28 from Richard Biener <rguenth at gcc dot gnu.org> ---
c.f. comment#24 and using bswap for this kind of value composition

gu (unsigned char c)
{
  short unsigned int y.7_2;
  signed short y.8_3;
  signed short _4;
  int _6;
  int _7;
  signed short _8;
  signed short _9;
  short unsigned int y.9_10;

  <bb 2>:
  y.7_2 = y;
  y.8_3 = (signed short) y.7_2;
  _4 = y.8_3 & 255;
  _6 = (int) c_5(D);
  _7 = _6 << 8;
  _8 = (signed short) _7;
  _9 = _4 | _8;
  y.9_10 = (short unsigned int) _9;
  y = y.9_10;
  return;

this could be

  BIT_FIELD_REF <y, ....> = c_5;

or if not both stores and loads are seen at least

  y.7_2 = y;
  y.9_10 = BIT_FIELD_EXPR <y.7_2, ..., c_5(D)>;
  y = y.9_10;

with the new BIT_FIELD_EXPR code I proposed some time ago for bitfield
insertion.

Technically not limited to byte offsets of course, so maybe not a 100%
fit for the bswap pass.  And of course the BIT_FIELD_EXPR would still
need to be expanded properly (extv/insv support would fit naturally).

Without BIT_FIELD_EXPR the bswap pass could see if the final value
has a single use in a store and then still use a BIT_FIELD_REF on that.