diff options
Diffstat (limited to 'runtime/arm/i64_shl.s')
| -rw-r--r-- | runtime/arm/i64_shl.s | 30 |
1 files changed, 21 insertions, 9 deletions
diff --git a/runtime/arm/i64_shl.s b/runtime/arm/i64_shl.s index 8014f88..afd92db 100644 --- a/runtime/arm/i64_shl.s +++ b/runtime/arm/i64_shl.s @@ -39,22 +39,34 @@ @@@ Shift left @ Note on ARM shifts: the shift amount is taken modulo 256. -@ Therefore, unsigned shifts by 32 bits or more produce 0. +@ If shift amount mod 256 >= 32, the shift produces 0. + +@ Algorithm: +@ RH = (XH << N) | (XL >> (32-N) | (XL << (N-32)) +@ RL = XL << N +@ If N = 0: +@ RH = XH | 0 | 0 +@ RL = XL +@ If 1 <= N <= 31: 1 <= 32-N <= 31 and 255 <= N-32 mod 256 <= 255 +@ RH = (XH << N) | (XL >> (32-N) | 0 +@ RL = XL << N +@ If N = 32: +@ RH = 0 | XL | 0 +@ RL = 0 +@ If 33 <= N <= 63: 255 <= 32-N mod 256 <= 255 and 1 <= N-32 <= 31 +@ RH = 0 | 0 | (XL << (N-32)) +@ RL = 0 .global __i64_shl __i64_shl: 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 r1, r1, lsl r2 - orr r1, r0, lsr r3 + orr r1, r1, r0, lsr r3 + sub r3, r2, #32 @ r3 = amount - 32 + orr r1, r1, r0, lsl r3 mov r0, r0, lsl r2 bx lr -1: - sub r2, r2, #32 - mov r1, r0, lsl r2 - mov r0, #0 - bx lr .type __i64_shl, %function .size __i64_shl, . - __i64_shl |