有符号乘法运算
Signed Multiply Long multiplies two 32-bit register values, and writes the result to the 64-bit destination register.
将两个32位寄存器值相乘，并将结果写入64位目标寄存器。
64-bit variant
SMULL <Xd>, <Wn>, <Wm>
is equivalent to
SMADDL <Xd>, <Wn>, <Wm>, XZR
403f34: 528cccfc mov w28, #0x6667 // #26215
403f38: 529999b7 mov w23, #0xcccd // #52429
403f3c: 72acccdc movk w28, #0x6666, lsl #16
403f40: 72b99997 movk w23, #0xcccc, lsl #16
403f44: d503201f nop
403f48: 1adb0f42 sdiv w2, w26, w27
403f4c: 9b3c7c40 smull x0, w2, w28