Fastest IO toggle

Trying my hand at assembler but I am getting an error. Trying to toggle B3.

var test = E.asm("void()",
                 "ldr  r2,gpiob_addr",
                 "movw  r3,#4",
                 "loopStart:",
                 "str  r3,[r2,#0]",
                 "str  r3,[r2,#4]",
                 "bgt  loopStart",
                 "bx  lr",
                 "gpiob_addr:",
                 ".word  0x40020418"
                 );

Error is...
Assembler failed: Invalid number "gpiob_addr" must be between 0 and 1024 and a multiple of 4