[FFmpeg-devel] [PATCH] broaden ff_set_fixed_vector

[Måns Rullgård]

"Ronald S. Bultje" <rsbultje at gmail.com> writes:

> Hi,
>
> On Jan 26, 2010, at 8:51 PM, M?ns Rullg?rd <mans at mansr.com> wrote:
>> "Ronald S. Bultje" <rsbultje at gmail.com> writes:
>>> /** Sparse representation for the algebraic codebook (fixed) vector
>>> */
>>> typedef struct {
>>>     int      n;
>>> -    int      x[10];
>>> -    float    y[10];
>>> +    struct {
>>> +        unsigned int single:1;
>>> +        unsigned int x:31;
>>> +        float        y;
>>> +    } pulses[10];
>>>     int      pitch_lag;
>>>     float    pitch_fac;
>>> } AMRFixed;
>>
>> Why the bit-fields?  GCC often generates incredibly stupid code for
>> bit-fields.
>
> I'm hoping gcc is smarter than that?

Never underestimate the stupidity of gcc.  Here's a quick test:

struct bf {
    unsigned a:1;
    unsigned b:31;
};

void foo(struct bf *p, int n, int a)
{
    int i = 0;
    do {
        if (p[i].a)
            p[i].b += a;
    } while (++i < n);
}

This is what gcc does:

00000000 <foo>:
   0:   push    {r4}            ; (str r4, [sp, #-4]!)
   4:   mov     ip, #0
   8:   mov     r4, r2
   c:   ldrb    r3, [r0]
  10:   add     ip, ip, #1
  14:   tst     r3, #1
  18:   ldrne   r3, [r0]
  1c:   addne   r2, r4, r3, lsr #1
  20:   bfine   r3, r2, #1, #31
  24:   strne   r3, [r0]
  28:   cmp     ip, r1
  2c:   add     r0, r0, #4
  30:   blt     c <foo+0xc>
  34:   pop     {r4}
  38:   bx      lr

That's twice as much code as it needs to be.

This is how it should be done:

foo:
        ldr     r3,  [r0], #4
        tst     r3,  #1
        add     r3,  r3,  r2,  lsl #1
        strne   r3,  [r0, #-4]
        subs    r1,  r1,  #1
        bne     foo
        bx      lr

And this is what armcc does:

00000000 <foo>:
   0:   push    {r4, r5, lr}
   4:   mov     r3, #0
   8:   ldr     r4, [r0, r3, lsl #2]
   c:   tst     r4, #1
  10:   addne   r4, r4, r2, lsl #1
  14:   strne   r4, [r0, r3, lsl #2]
  18:   add     r3, r3, #1
  1c:   cmp     r3, r1
  20:   blt     8 <foo+0x8>
  24:   pop     {r4, r5, pc}

-- 
M?ns Rullg?rd
mans at mansr.com