302stack_allocate.subx

   1 # A function which pushes n zeros on the stack.
   2 # Really only intended to be called from code generated by mu.subx (for array
   3 # vars on the stack).
   4
   5 == code
   6
   7 #? Entry:
   8 #?     # . prologue
   9 #?     89/<- %ebp 4/r32/esp
  10 #?     #
  11 #?     68/push 0xfcfdfeff/imm32
  12 #?     b8/copy-to-eax 0x34353637/imm32
  13 #? $dump-stack0:
  14 #?     (push-n-zero-bytes 4)
  15 #?     68/push 0x20/imm32
  16 #? $dump-stack9:
  17 #?     b8/copy-to-eax 1/imm32/exit
  18 #?     cd/syscall 0x80/imm8
  19
  20 # This is not a regular function, so it won't be idiomatic.
  21 # Registers must be properly restored.
  22 # Registers can be spilled, but that modifies the stack and needs to be
  23 # cleaned up.
  24
  25 # Overhead:
  26 #   62 + n*6 instructions to push n bytes.
  27 # If we just emitted code to push n zeroes, it would be:
  28 #   5 bytes for 4 zero bytes, so 1.25 bytes per zero. And that's not even
  29 #   instructions.
  30 # But on the other hand it would destroy the instruction cache, where this
  31 # approach requires 15 instructions, fixed.
  32
  33 # n must be positive
  34 push-n-zero-bytes:  # n: int
  35 $push-n-zero-bytes:prologue:
  36     89/<- *Push-n-zero-bytes-ebp 5/r32/ebp  # spill ebp without affecting stack
  37     89/<- %ebp 4/r32/esp
  38 $push-n-zero-bytes:copy-ra:
  39     # -- esp = ebp
  40     89/<- *Push-n-zero-bytes-eax 0/r32/eax
  41     8b/-> *esp 0/r32/eax
  42     2b/subtract *(ebp+4) 4/r32/esp
  43     # -- esp+n = ebp
  44     89/<- *esp 0/r32/eax
  45     8b/-> *Push-n-zero-bytes-eax 0/r32/eax
  46 $push-n-zero-bytes:bulk-cleaning:
  47     89/<- *Push-n-zero-bytes-esp 4/r32/esp
  48     81 0/subop/add *Push-n-zero-bytes-esp 4/imm32
  49     81 0/subop/add *(ebp+4) 4/imm32
  50     (zero-out *Push-n-zero-bytes-esp *(ebp+4))  # n+4
  51 $push-n-zero-bytes:epilogue:
  52     8b/-> *Push-n-zero-bytes-ebp 5/r32/ebp  # restore spill
  53     c3/return
  54
  55 == data
  56 Push-n-zero-bytes-ebp:  # (addr int)
  57   0/imm32
  58 Push-n-zero-bytes-esp:  # (addr int)
  59   0/imm32
  60 Push-n-zero-bytes-eax:
  61   0/imm32