GCC rychle kopirovani pameti

[Jaroslav Buchta]

Zdravim, dnes se zase nestiham divit, nevim, na cem je to jak zavisle 
ale bezna funkce memcpy vede jak v optimalizovanem tak v 
neoptimalizovanem prekladu k hloupemu kopirovani po 1B

Zkusil jsem si napsat takovouto fci

void fastcpy(void *dst, void *src, int len)
{
     while (len >= 8)
     {
         *(uint64_t*)dst = *(uint64_t*)src;
         dst += 8;
         src += 8;
         len -= 8;
     }
     while (len >= 4)
     {
         *(uint32_t*)dst = *(uint32_t*)src;
         dst += 4;
         src += 4;
         len -= 4;
     }
     if (len >= 2)
     {
         *(uint16_t*)dst = *(uint16_t*)src;
         dst += 2;
         src += 2;
         len -= 2;
     }
     if (len != 0)
     {
         *(uint8_t*)dst = *(uint8_t*)src;
     }
}

ktera vede ke kyzenemu vysledku, je to vyrazne rychlejsi, ovsem POUZE, 
KDYZ NENI NASTAVENA OPTIMALIZACE O3, tam nejak prekladac usoudi, ze je 
to kopirovani bloku a vola memcpy... Je to vubec mozne, nebo delam 
nejakou chybu???

Jak mam vynutit volani te funkce? (krome obchuzky, ze pro ni zakazu 
optimalizaci...)


////////////////////////////////

Optimalizovany kod:

100               mccState.iB = (float)(mccState.bufAdc1[2]);
080004c2:   vstr    s14, [r4, #68]  ; 0x44
080004c6:   vstr    s15, [r4, #72]  ; 0x48
102               if (!mccStateCopy.f_LockCopy)
080004ca:   bmi.n   0x8000400 <HAL_ADC_ConvCpltCallback+24>
080004cc:   movs    r2, #72 ; 0x48
080004ce:   mov     r1, r4
080004d0:   add.w   r0, r4, #76     ; 0x4c
080004d4:   bl      0x8005e00 <memcpy>
  72               *(uint32_t*)dst = *(uint32_t*)src;


Neoptimalizovany kod:

101               mccState.iC = (float)(mccState.bufAdc1[3]);
08000528:   vmov    s15, r1
0800052c:   vcvt.f32.u32    s15, s15
08000530:   vstr    s15, [r3, #72]  ; 0x48
102               if (!mccStateCopy.f_LockCopy)
08000534:   ldrb.w  r3, [r3, #76]   ; 0x4c
08000538:   tst.w   r3, #2
0800053c:   bne.n   0x8000470 <HAL_ADC_ConvCpltCallback+24>
104                   fastcpy((void *)&mccStateCopy, (void *)&mccState, 
sizeof(MCCOMSTATE));
0800053e:   ldr     r0, [pc, #16]   ; (0x8000550 
<HAL_ADC_ConvCpltCallback+248>)
08000540:   movs    r2, #76 ; 0x4c
08000542:   mov     r1, r0
08000544:   add     r0, r2
08000546:   bl      0x80003f4 <fastcpy>


           fastcpy:
080003f4:   cmp     r2, #7
080003f6:   ble.n   0x800043a <fastcpy+70>
  62       {
080003f8:   push    {r4, r5}
  65               *(uint64_t*)dst = *(uint64_t*)src;
080003fa:   ldrd    r4, r5, [r1], #8
080003fe:   strd    r4, r5, [r0], #8
  68               len -= 8;
08000402:   subs    r2, #8
  63           while (len >= 8)
08000404:   cmp     r2, #7
08000406:   bgt.n   0x80003fa <fastcpy+6>
08000408:   b.n     0x8000414 <fastcpy+32>
  72               *(uint32_t*)dst = *(uint32_t*)src;
0800040a:   ldr.w   r3, [r1], #4
0800040e:   str.w   r3, [r0], #4
  75               len -= 4;
08000412:   subs    r2, #4
  70           while (len >= 4)
08000414:   cmp     r2, #3
08000416:   bgt.n   0x800040a <fastcpy+22>
  77           if (len >= 2)
08000418:   cmp     r2, #1
0800041a:   ble.n   0x8000426 <fastcpy+50>
  79               *(uint16_t*)dst = *(uint16_t*)src;
0800041c:   ldrh.w  r3, [r1], #2
08000420:   strh.w  r3, [r0], #2
  82               len -= 2;
08000424:   subs    r2, #2
  84           if (len != 0)
08000426:   cbz     r2, 0x800042c <fastcpy+56>
  86               *(uint8_t*)dst = *(uint8_t*)src;
08000428:   ldrb    r3, [r1, #0]
0800042a:   strb    r3, [r0, #0]
  88       }
0800042c:   pop     {r4, r5}
0800042e:   bx      lr
08000430:   ldr.w   r3, [r1], #4
08000434:   str.w   r3, [r0], #4
  75               len -= 4;
08000438:   subs    r2, #4
0800043a:   cmp     r2, #3
0800043c:   bgt.n   0x8000430 <fastcpy+60>
0800043e:   cmp     r2, #1
08000440:   ble.n   0x800044c <fastcpy+88>
08000442:   ldrh.w  r3, [r1], #2
08000446:   strh.w  r3, [r0], #2
  82               len -= 2;
0800044a:   subs    r2, #2
  84           if (len != 0)
0800044c:   cmp     r2, #0
0800044e:   beq.n   0x800042e <fastcpy+58>
  86               *(uint8_t*)dst = *(uint8_t*)src;
08000450:   ldrb    r3, [r1, #0]
08000452:   strb    r3, [r0, #0]
08000454:   bx      lr