When the divisor is a constant value, we can emit more efficient code.
For powers of two, we can use bit shifts. For other values, we can
instead use a multiplication by magic constant method.
- Example 1 - Division by 16 (power of two)
Before:
mov w24, #0x10 ; =16
udiv w27, w25, w24
After:
lsr w27, w25, #4
- Example 2 - Division by 10 (fast)
Before:
mov w25, #0xa ; =10
udiv w27, w26, w25
After:
mov w27, #0xcccd ; =52429
movk w27, #0xcccc, lsl #16
umull x27, w26, w27
lsr x27, x27, #35
- Example 3 - Division by 127 (slow)
Before:
mov w26, #0x7f ; =127
udiv w27, w27, w26
After:
mov w26, #0x408 ; =1032
movk w26, #0x8102, lsl #16
umaddl x27, w27, w26, x26
lsr x27, x27, #38
A (partial) port of #9481 to ARM64. This commit adds special cases for
immediate values equal to 0 or 0xFFFFFFFF, allowing for more efficient
or no code to be generated.
When a guest register is an immediate, it may be necessary to move this
value into a register. This is handled by gpr.R(), which lacks context
on how the register will be used. This leads to cases where the
immediate is written to a register, only for it to be overwritten. Take
for example this code generated by srwx:
0x5280031b mov w27, #0x18
0x53187edb lsr w27, w22, #24
gpr.BindToRegister() does have this context through the do_load
parameter, but didn't handle immediates. By adding this logic, we can
intelligently skip the write when do_load is false.
Bram Speeckaert