[Bug middle-end/36884] New: ifcvt poor optimization

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

If conversion is causing extraordinary bad code for AVR.
This occurs on 4.3 4.4 is no better.

Testcase:

int z;

int foo2(void)
{
  return ( ++z >= 0);
}

Conversion replaces if using store flag. However, there appears to be no
account of relative costs and rather bad interactions with mode narrowing and
widening

After conversion we have:

z++
Extract upper byte of 'z'
Sign extend
One complement
Shift right 15


RTL prior to ce1 pass:

;; Pred edge  ENTRY [100.0%]  (fallthru)
(note 3 0 2 2 [bb 2] NOTE_INSN_BASIC_BLOCK)

(note 2 3 5 2 NOTE_INSN_FUNCTION_BEG)

(insn 5 2 6 2 test1.c:15 (set (reg:HI 43 [ z ])
        (mem/c/i:HI (symbol_ref:HI ("z") <var_decl 0x7fe000b0 z>) [3 z+0 S2
A8])) 10 {*movhi} (nil))

(insn 6 5 7 2 test1.c:15 (set (reg:HI 41 [ z.5 ])
        (plus:HI (reg:HI 43 [ z ])
            (const_int 1 [0x1]))) 25 {addhi3} (expr_list:REG_DEAD (reg:HI 43 [
z ])
        (nil)))

(insn 7 6 8 2 test1.c:15 (set (mem/c/i:HI (symbol_ref:HI ("z") <var_decl
0x7fe000b0 z>) [3 z+0 S2 A8])
        (reg:HI 41 [ z.5 ])) 10 {*movhi} (nil))

(insn 8 7 9 2 test1.c:15 (set (reg:HI 44)
        (const_int 0 [0x0])) 10 {*movhi} (nil))

(insn 9 8 10 2 test1.c:15 (set (cc0)
        (reg:HI 41 [ z.5 ])) 95 {tsthi} (expr_list:REG_DEAD (reg:HI 41 [ z.5 ])
        (nil)))

(jump_insn 10 9 25 2 test1.c:15 (set (pc)
        (if_then_else (lt (cc0)
                (const_int 0 [0x0]))
            (label_ref 12)
            (pc))) 109 {branch} (expr_list:REG_BR_PROB (const_int 2100 [0x834])
        (nil)))
;; End of basic block 2 -> ( 4 3)
;; lr  out       28 [r28] 32 [__SP_L__] 34 [argL] 44
;; live  out     28 [r28] 32 [__SP_L__] 34 [argL] 44





RTL after ce1:

(note 2 3 5 2 NOTE_INSN_FUNCTION_BEG)

(insn 5 2 6 2 test1.c:15 (set (reg:HI 43 [ z ])
        (mem/c/i:HI (symbol_ref:HI ("z") <var_decl 0x7fe000b0 z>) [3 z+0 S2
A8])) 10 {*movhi} (nil))

(insn 6 5 7 2 test1.c:15 (set (reg:HI 41 [ z.5 ])
        (plus:HI (reg:HI 43 [ z ])
            (const_int 1 [0x1]))) 25 {addhi3} (expr_list:REG_DEAD (reg:HI 43 [
z ])
        (nil)))

(insn 7 6 8 2 test1.c:15 (set (mem/c/i:HI (symbol_ref:HI ("z") <var_decl
0x7fe000b0 z>) [3 z+0 S2 A8])
        (reg:HI 41 [ z.5 ])) 10 {*movhi} (nil))

(insn 8 7 9 2 test1.c:15 (set (reg:HI 44)
        (const_int 0 [0x0])) 10 {*movhi} (nil))

(insn 9 8 35 2 test1.c:15 (set (cc0)
        (reg:HI 41 [ z.5 ])) 95 {tsthi} (expr_list:REG_DEAD (reg:HI 41 [ z.5 ])
        (nil)))

(insn 35 9 36 2 test1.c:15 (set (reg:QI 49)
        (subreg:QI (reg:HI 41 [ z.5 ]) 1)) 4 {*movqi} (nil))

(insn 36 35 37 2 test1.c:15 (set (reg:HI 48)
        (sign_extend:HI (reg:QI 49))) 84 {extendqihi2} (nil))

(insn 37 36 38 2 test1.c:15 (set (reg:HI 50)
        (not:HI (reg:HI 48))) 82 {one_cmplhi2} (nil))

(insn 38 37 32 2 test1.c:15 (set (reg:HI 44)
        (lshiftrt:HI (reg:HI 50)
            (const_int 15 [0xf]))) 71 {lshrhi3} (nil))

(insn 32 38 33 2 test1.c:16 (set (reg:QI 24 r24)
        (subreg:QI (reg:HI 44) 0)) 4 {*movqi} (nil))

(insn 33 32 23 2 test1.c:16 (set (reg:QI 25 r25 [+1 ])
        (subreg:QI (reg:HI 44) 1)) 4 {*movqi} (expr_list:REG_DEAD (reg:HI 44)
        (nil)))



Final (annotated) code:


  22                    /* prologue: frame size=0 */
  23                    /* prologue end (size=0) */
  24                    .LM2:
  25 0000 8091 0000             lds r24,z
  26 0004 9091 0000             lds r25,(z)+1
  27 0008 0196                  adiw r24,1
  28 000a 9093 0000             sts (z)+1,r25
  29 000e 8093 0000             sts z,r24
  30 0012 892F                  mov r24,r25
  31 0014 9927                  clr r25  ;SIGN EXTEND
  32 0016 87FD                  sbrc r24,7 ;ditto
  33 0018 9095                  com r25 ; ditto
  34 001a 8095                  com r24 ; ONE'SCOMPLEMENT
  35 001c 9095                  com r25 ; ditto
  36                    .LM3:
  37 001e 8827                  clr r24 ;LSHIFT 15
  38 0020 990F                  lsl r25 ; ditto
  39 0022 881F                  rol r24 ; ditto
  40 0024 9927                  clr r25 ; ditto
  41                    /* epilogue: frame size=0 */
  42 0026 0895                  ret


-- 
           Summary: ifcvt poor optimization
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hutchinsonandy at gcc dot gnu dot org
GCC target triplet: avr-unknown-none


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36884