🚀 go-pugleaf

Version: GCC 2.2.2 (m68k, sun3)

There is a bug in the optimizer for bitfield operations.


bug.c:
--------------------
struct term
{
  unsigned state: 1;
  unsigned info: 7;
  unsigned content: 24;
};

f (struct term t)
{
  struct term t1[1], t2[1]; /* force into memory */
  t1[0] = t;
  g ((char) t1->content);
  t2[0] = t;
  g ((short) t2->content);
}
--------------------

Compile with: gcc -O -S bug.c

bug.s:
--------------------
#NO_APP
gcc2_compiled.:
.text
	.even
.globl _f
_f:
	link a6,#-8
	movel a2,sp@-
	movel d2,sp@-
	movel a6@(8),d2
	movel d2,a6@(-4)
	moveb a6@(-2),d1	| <--- wrong
	extbl d1
	movel d1,sp@-
	lea _g,a2
	jbsr a2@
	addqw #4,sp
	movel d2,a6@(-8)
	bfexts a6@(-6){#0:#16},d1
	movel d1,sp@-
	jbsr a2@
	movel a6@(-16),d2
	movel a6@(-12),a2
	unlk a6
	rts
--------------------

The bug occurs in the combine pass, where these insns

-------------------- (bug.c.flow)
(insn 13 11 14 (set (reg:SI 31)
       (zero_extract:SI (mem/s:QI (plus:SI (reg:SI 14)
                   (const_int -3)))
           (const_int 24)
           (const_int 0))) 233 {extzv} (nil)
   (nil))

(insn 14 13 16 (set (reg:SI 32)
       (sign_extend:SI (subreg:QI (reg:SI 31) 0))) 49 {extendqisi2} (insn_list 13 (nil))
   (expr_list:REG_DEAD (reg:SI 31)
       (nil)))

(insn 16 14 18 (set (mem:SI (pre_dec:SI (reg:SI 15)))
       (reg:SI 32)) 29 {movsi+1} (insn_list 14 (nil))
   (expr_list:REG_DEAD (reg:SI 32)
       (nil)))
--------------------

are combined into ((const_int -2) is wrong)

-------------------- (bug.c.combine)
(insn 16 14 18 (set (mem:SI (pre_dec:SI (reg:SI 15)))
       (sign_extend:SI (mem/s:QI (plus:SI (reg:SI 14)
                   (const_int -2))))) 49 {extendqisi2} (nil)
   (nil))
--------------------
--
Andreas Schwab
schwab@ls5.informatik.uni-dortmund.de