diff options
Diffstat (limited to 'runtime/arm/i64_shr.s')
| -rw-r--r-- | runtime/arm/i64_shr.s | 31 |
1 files changed, 23 insertions, 8 deletions
diff --git a/runtime/arm/i64_shr.s b/runtime/arm/i64_shr.s index f10b770..9d60441 100644 --- a/runtime/arm/i64_shr.s +++ b/runtime/arm/i64_shr.s @@ -38,20 +38,35 @@ @@@ Shift right unsigned +@ Note on ARM shifts: the shift amount is taken modulo 256. +@ If shift amount mod 256 >= 32, the shift produces 0. + +@ Algorithm: +@ RL = (XL >> N) | (XH << (32-N) | (XH >> (N-32)) +@ RH = XH >> N +@ If N = 0: +@ RL = XL | 0 | 0 +@ RH = XH +@ If 1 <= N <= 31: 1 <= 32-N <= 31 and 255 <= N-32 mod 256 <= 255 +@ RL = (XL >> N) | (XH >> (32-N) | 0 +@ RH = XH >> N +@ If N = 32: +@ RL = 0 | XH | 0 +@ RH = 0 +@ If 33 <= N <= 63: 255 <= 32-N mod 256 <= 255 and 1 <= N-32 <= 31 +@ RL = 0 | 0 | (XH >> (N-32)) +@ RH = 0 + .global __i64_shr __i64_shr: and r2, r2, #63 @ normalize amount to 0...63 - rsbs r3, r2, #32 @ r3 = 32 - amount - ble 1f @ branch if <= 0, namely if amount >= 32 + rsb r3, r2, #32 @ r3 = 32 - amount mov r0, r0, lsr r2 - orr r0, r1, lsl r3 + orr r0, r0, r1, lsl r3 + sub r3, r2, #32 @ r3 = amount - 32 + orr r0, r0, r1, lsr r3 mov r1, r1, lsr r2 bx lr -1: - sub r2, r2, #32 - mov r0, r1, lsr r2 - mov r1, #0 - bx lr .type __i64_shr, %function .size __i64_shr, . - __i64_shr |