[Bug tree-optimization/31136] [4.2 Regression] FRE ignores bit-field truncation (C and C++ front-end don't produce bit-field truncation

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

------- Comment #13 from rguenth at gcc dot gnu dot org  2007-04-21 16:37 -------
The interesting thing is that we

Created value VH.0 for (<unnamed-unsigned:4>) 31

The bug (compared to the trunk) is, that tree-ssa-pre.c:try_look_through_load
on the 4.2 branch manages to propagate the 31 while trunk does not
(surprisingly).

On 4.2 we have for the def_stmt

#   SFT.0D.1539_2 = V_MUST_DEF <SFT.0D.1539_1>;
sD.1526.b6D.1525 = 31

while on the trunk

# SFT.0_10 = VDEF <SFT.0_9(D)> { SFT.0 }
s.b6 = 31

and the predicate !ZERO_SSA_OPERANDS (def_stmt, SSA_OP_VIRTUAL_USES) evaluates
differently on them.  *sigh*

This causes us to have the unfolded expression created from
create_value_expr_from which we then fold incorrectly by folding
of double conversion code.

One fix is to fold the expression we generate with like

Index: tree-ssa-pre.c
===================================================================
--- tree-ssa-pre.c      (revision 124018)
+++ tree-ssa-pre.c      (working copy)
@@ -2973,6 +2973,9 @@ create_value_expr_from (tree expr, basic
       TREE_OPERAND (vexpr, i) = val;
     }

+  if (UNARY_CLASS_P (vexpr))
+    vexpr = fold (vexpr);
+
   return vexpr;
 }

which then results in the correct

main ()
{
  short unsigned int D.1536;
  short unsigned int D.1535;
  int D.1534;
  <unnamed-unsigned:6> D.1533;
  <unnamed-unsigned:4> D.1532;
  <unnamed-unsigned:4> D.1531;
  <unnamed-unsigned:6> D.1530;

<bb 2>:
  s.b6 = 31;
  D.1530_3 = 31;
  D.1531_4 = 15;
  s.b4 = D.1531_4;
  D.1532_7 = 15;
  D.1533_8 = 15;
  s.b6 = D.1533_8;
  D.1535_10 = BIT_FIELD_REF <s, 16, 0>;
  D.1536_11 = D.1535_10 & 1008;
  D.1534_12 = D.1536_11 != 240;
  return D.1534_12;

}

now another question is, why we "regressed" here on the mainline.  Danny?
(I guess we might get more unfolded trees by constants propagated by
the look from load code - like an addition)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31136