[Bug middle-end/33887] [4.2/4.3 Regression] Reference to bitfield gets wrong value when optimizing

[rguenth at gcc dot gnu dot org]

------- Comment #6 from rguenth at gcc dot gnu dot org  2007-12-11 12:38 -------
In reply to comment #3:

no, 

int f2 = sv.f2;

does _not_ sign extend.  4.7/3 says "If the destination type is signed, the
value is unchanged if it can be represented in the destination type...".
Note your bitfield is unsigned.

Instead the error is with

  ++sv.f2;
  f2 = sv.f2;
  fn(f2, 0);

which should result in zero (as tested).  But we expand the above to

;; D.2041 = sv.f2 + 1
(insn 20 17 21 t.ii:27 (set (reg/f:DI 71)
        (const:DI (plus:DI (symbol_ref:DI ("sv") [flags 0x2] <var_decl
0x2b1ba3468140 sv>)
                (const_int 4 [0x4])))) -1 (nil))

(insn 21 20 22 t.ii:27 (set (reg:SI 73)
        (mem/s/c:SI (reg/f:DI 71) [0+4 S4 A32])) -1 (nil))

(insn 22 21 23 t.ii:27 (parallel [
            (set (reg:SI 72)
                (lshiftrt:SI (reg:SI 73)
                    (const_int 8 [0x8])))
            (clobber (reg:CC 17 flags))
        ]) -1 (expr_list:REG_EQUAL (lshiftrt:SI (mem/s/c:SI (reg/f:DI 71) [0+4
S4 A32])
            (const_int 8 [0x8]))
        (nil)))

(insn 23 22 0 t.ii:27 (parallel [
            (set (reg:SI 58 [ D.2041 ])
                (plus:SI (reg:SI 72)
                    (const_int 1 [0x1])))
            (clobber (reg:CC 17 flags))
        ]) -1 (nil))

;; sv.f2 = D.2041
(insn 25 23 26 t.ii:27 (set (reg/f:DI 75)
        (const:DI (plus:DI (symbol_ref:DI ("sv") [flags 0x2] <var_decl
0x2b1ba3468140 sv>)
                (const_int 4 [0x4])))) -1 (nil))

(insn 26 25 27 t.ii:27 (parallel [
            (set (reg:SI 76)
                (ashift:SI (reg:SI 58 [ D.2041 ])
                    (const_int 8 [0x8])))
            (clobber (reg:CC 17 flags))
        ]) -1 (nil))

(insn 27 26 28 t.ii:27 (set (reg:SI 77)
        (mem/s/j/c:SI (reg/f:DI 75) [0+4 S4 A32])) -1 (nil))

(insn 28 27 29 t.ii:27 (parallel [
            (set (reg:SI 78)
                (and:SI (reg:SI 77)
                    (const_int 255 [0xff])))
            (clobber (reg:CC 17 flags))
        ]) -1 (nil))

(insn 29 28 30 t.ii:27 (parallel [
            (set (reg:SI 79)
                (ior:SI (reg:SI 78)
                    (reg:SI 76)))
            (clobber (reg:CC 17 flags))
        ]) -1 (nil))

(insn 30 29 0 t.ii:27 (set (mem/s/j/c:SI (reg/f:DI 75) [0+4 S4 A32])
        (reg:SI 79)) -1 (nil))

;; fn ((int) D.2041, 0)
(insn 31 30 32 t.ii:29 (set (reg:SI 4 si)
        (const_int 0 [0x0])) -1 (nil))

(insn 32 31 33 t.ii:29 (set (reg:SI 5 di)
        (reg:SI 58 [ D.2041 ])) -1 (nil))

(call_insn 33 32 0 t.ii:29 (call (mem:QI (symbol_ref/i:DI ("_Z2fnIiiEvT_T0_")
[flags 0x1] <function_decl 0x2b1ba3461820 fn>) [0 S1 A8])
...

because reg:SI 58 is not a properly truncated intermediate result.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33887