ws2812 with stm32f0

You should try replacing the branch with a series of nops. On an ARM (and many others), the branch instruction is used because it takes more cycles to execute than many others.

Of course, you could also consider scrapping the code and doing it properly. The SPI can easily be used to drive one of these LEDs and the code would be far easier to maintain and port to different processors, speeds etc.

OK this is my mistake (partly), partly it is a Keil error (I think).
in first instruction in _asm is

lsl curbyte,#24

used variable curbyte defined and declares in C code, as local variable in function ws2812_sendarray(), this code do not works and compiler return

    45:         __asm__ volatile(
0x080010BE E006      B        0x080010CE
    46:    "                            lsl curbyte,#24              \n\t"
0x080010C0 2508      MOVS     r5,#0x08
0x080010C2 601C      STR      r4,[r3,#0x00]
0x080010C4 BF00      NOP
0x080010C6 D200      BCS      0x080010CA
0x080010C8 6002      STR      r2,[r0,#0x00]
0x080010CA 6002      STR      r2,[r0,#0x00]
0x080010CC D1F9      BNE      0x080010C2

notice lsl curbyte,#24 not include in asm code

Code will be works only if curbyte will be global variable!!! in other words all c variable in by code used in _asm() must be declared as global. Compiler return

    46:                         "        lsl curbyte,#24           \n\t"
0x080010C8 EA4F6707  LSL      r7,r7,#24
0x080010CC 6057      STR      r7,[r2,#0x04]
0x080010CE 2708      MOVS     r7,#0x08
0x080010D0 6017      STR      r7,[r2,#0x00]
0x080010D2 6857      LDR      r7,[r2,#0x04]
0x080010D4 EA4F0747  LSL      r7,r7,#1
0x080010D8 6057      STR      r7,[r2,#0x04]
0x080010DA 602E      STR      r6,[r5,#0x00]
0x080010DC BF00      NOP

lsl curbyte,#24 is in assembler code as LSL r7,r7,#24

It's interesting, that if in _asm(); use for example
lsl datlen,#24 (variable datlen is from void ws2812_sendarray(uint8_t *data,int datlen)
all vorsk OK and compiler Translate correctly LSL r0,r0,#24

conclusion
Why the compiler does not say that it does not know what the local variable c is?
Why he knows datlen?
I do not understand much

NO.

It is totally your mistake. You don't have sufficient understanding of the tools.

yes I have not
However on the other hand
I would expect at least a warning from the tool, warning in __asm is used undeclared variable etc.