[Bug middle-end/18424] 3.4.3 ~6x+ performance regression vs 3.3.1, constant trees not being computed.

[dmixm at marine dot febras dot ru]

------- Additional Comments From dmixm at marine dot febras dot ru  2004-11-16 
23:58 -------
In March, 2004 Richard Sandiford has offered a patch for elimination of 
this problem. See: http://gcc.gnu.org/ml/gcc/2004-03/msg01456.html  This 
patch modifies function do_jump (a file dojump.c). This change now is 
present at a branch 4.0, but does not enter in 3.4.x. I have tried to 
apply this patch to 3.4.3. It has earned only after change of a line 
    if (TREE_CODE (TREE_OPERAND (exp, 0)) == RSHIFT_EXPR 
    ... 
to 
    tree arg_nops = TREE_OPERAND (exp, 0);  /* and below the same subst. */ 
    STRIP_NOPS (arg_nops); 
    if (TREE_CODE (arg_nops) == RSHIFT_EXPR 
    ... 
and only for function foo_i. 
 
foo_i() before patch: 
        ... 
        mov r24,r25 
        ldi r25,6 
1:      lsr r24 
        dec r25 
        brne 1b 
        sbrs r24,0 
        rjmp .L2 
 
foo_i() after patch: 
        ... 
        sbrs r25,6 
        rjmp .L8 
 
But foo_ll (shift loop with count 62!) and foo_l have remained on old - 
through shift of the left argument. 
 
Source file: 
~~~~~~~~~~~ 
int foo_ll (long long x) 
{ 
    return (x & 0x4000000000000000LL) ? 1 : 3; 
} 
 
int foo_l (long x) 
{ 
    return (x & 0x40000000) ? 5 : 7; 
} 
 
int foo_i (int x) 
{ 
    return (x & 0x4000) ? 9 : 11; 
} 
 
int foo_c (char x) 
{ 
    return (x & 0x40) ? 13 : 15; 
} 
 
P.S. The code for foo_c was and remains beautiful due to work 
`gcc/combine.c' . 
 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18424