ref: d8cd5d197de6756443c4a5aa109ff78656ebc200
dir: /sys/src/libc/alpha/tas.s/
TEXT _tas(SB), $-8 MOVQ R0, R1 /* l */ tas1: MOVLL (R1), R0 /* l->key */ BNE R0, tas2 MOVQ $1, R2 MOVLC R2, (R1) /* l->key = 1 */ BEQ R2, tas1 /* write failed, try again? */ tas2: RET