prepare_context.s

.global PrepareContextInBuffer
.global SwitchBufferContext

PrepareContextInBuffer:
    # rdi, rsi, rdx, rcx, r8, r9
    # 1. %rdi - buffer
    # 2. %rsi - CallRunner
    # 3. %rdx - runner

    # Switch to stack from buffer
    movq %rsp, %r11 # save rsp to stack
    movq %rdi, %rsp # load buffer to rsp

    subq $64, %rsp # Just to be safe

    # buffer stack must be 16-byte aligned
    andq $-16, %rsp
    addq $8, %rsp

    # 3-thd arg (runner)
    # mov to 7-th argument in buffer
    movq %rdx, 8(%rsp)

    # Prepare for context switching

    pushq %rsi # push CallRunner

    # Callee-saved registers
    pushq $0 # r15
    pushq $0 # r14
    pushq $0 # r13
    pushq $0 # r12
    pushq $0 # rbp
    pushq $0 # rbx

    # Return value for PrepareContextInBuffer
    # rsp to buffer
    movq %rsp, %rax

    # Restore callstack
    movq %r11, %rsp # rsp := tmp
    retq

SwitchBufferContext:
    # rdi, rsi, rdx, rcx, r8, r9
    # 1. %rdi - source
    # 2. %rsi - destination

    pushq %r15
    pushq %r14
    pushq %r13
    pushq %r12

    pushq %rbx
    pushq %rbp


    movq %rsp, (%rdi)
    movq (%rsi), %rsp

    pop %rbp
    pop %rbx


    pop %r12
    pop %r13
    pop %r14
    pop %r15

    retq