path: root/linux/302stack_allocate.subx

# A function which pushes n zeros on the stack.
# Really only intended to be called from code generated by mu.subx (for array
# vars on the stack).

== code

#? Entry:
#?     # . prologue
#?     89/<- %ebp 4/r32/esp
#?     #
#?     68/push 0xfcfdfeff/imm32
#?     b8/copy-to-eax 0x34353637/imm32
#? $dump-stack0:
#?     (push-n-zero-bytes 4)
#?     68/push 0x20/imm32
#? $dump-stack9:
#?     b8/copy-to-eax 1/imm32/exit
#?     cd/syscall 0x80/imm8

# This is not a regular function, so it won't be idiomatic.
# Registers must be properly restored.
# Registers can be spilled, but that modifies the stack and needs to be
# cleaned up.

# Overhead:
#   62 + n*6 instructions to push n bytes.
# If we just emitted code to push n zeroes, it would be:
#   5 bytes for 4 zero bytes, so 1.25 bytes per zero. And that's not even
#   instructions.
# But on the other hand it would destroy the instruction cache, where this
# approach requires 15 instructions, fixed.

# n must be positive
push-n-zero-bytes:  # n: int
$push-n-zero-bytes:prologue:
    89/<- *Push-n-zero-bytes-ebp 5/r32/ebp  # spill ebp without affecting stack
    89/<- %ebp 4/r32/esp
$push-n-zero-bytes:copy-ra:
    # -- esp = ebp
    89/<- *Push-n-zero-bytes-eax 0/r32/eax
    8b/-> *esp 0/r32/eax
    2b/subtract *(ebp+4) 4/r32/esp
    # -- esp+n = ebp
    89/<- *esp 0/r32/eax
    8b/-> *Push-n-zero-bytes-eax 0/r32/eax
$push-n-zero-bytes:bulk-cleaning:
    89/<- *Push-n-zero-bytes-esp 4/r32/esp
    81 0/subop/add *Push-n-zero-bytes-esp 4/imm32
    81 0/subop/add *(ebp+4) 4/imm32
    (zero-out *Push-n-zero-bytes-esp *(ebp+4))  # n+4
$push-n-zero-bytes:epilogue:
    8b/-> *Push-n-zero-bytes-ebp 5/r32/ebp  # restore spill
    c3/return

== data
Push-n-zero-bytes-ebp:  # (addr int)
  0/imm32
Push-n-zero-bytes-esp:  # (addr int)
  0/imm32
Push-n-zero-bytes-eax:
  0/imm32