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6529 :: asm issues

• Shucks (orginal) [2020-07-09T22:16:35+02:00] view original

  I know that a newcomer and hobbycoder should probably not mess with asm statements.. but I want to =) I'm already sorry that I am mixing at&t and intel syntax in the code below.

  
  gcc --version
  gcc (x86_64-win32-seh-rev1, Built by MinGW-W64 project) 6.3.0
  
  nim -v
  Nim Compiler Version 1.2.4 [Windows: amd64]
  Compiled at 2020-06-26
  Run

  • asmNoStackFrame

  I would expect that this function below would be naked and just does a add and jmp call

  proc test {.asmnoStackFrame.} =
    asm """
      add $0x1, %eax
      jmp 0x004118ED
    """
  Run

  but debugging this function shows:

  
  push ebp
  mov ebp, esp
  add eax, 01
  jmp 0x004118ED
  Run

  Why does it modify the stack?

  • Nim identifiers in asm

  How to use nim identifiers? In visual cpp you can simply use variables in inline asm but:

  
  const myaddr = 0x123456
  
  asm """
    jmp `myaddr`
  """
  Run

  doesn't compiles at all. Im wondering how I could copy a register address into a nim identifier aswell.

  • intel syntax doesn't works with nim identifiers

  I really would like to use intel syntax but code compiled with --passC:"-masm=intel" doesn't recognizes any nim identifiers. Is there simply no support for it?

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• mratsim (orginal) [2020-07-10T00:03:21+02:00] view original

  I would expect that this function below would be naked and just does a add and jmp call

  Unless you compile with optimizations on, all C function push the stack base pointer on the stack on function entry (ebp), then save the stack pointer (esp) in ebp then do the function. Compile with optimizations, in particular -fomit-stack-pointer if you want to avoid the extra save/restore.

  Nim identifiers in asm

  The ASM statement doesn't support string interpolation, you need to build the string via a macro or use GCC syntax

  Macro

  import macros, strutils
  
  macro myASMStmt(myAddr: static int): untyped =
    result = nnkAsmStmt.newTree(
      newEmptyNode(),
      newLit "jmp 0x" & toHex(myAddr)
    )
    echo result.toStrLit
  
  const myaddr = 0x123456
  
  proc foo() =
    myASMStmt(myaddr)
  Run

  GCC, use the immediate syntax

  proc bar() =
    asm """
      jmp %[offset]
      :
      : [offset] "i" (`myaddr`)
      :
    """
  Run

  If you want concrete standalone example of Nim ASM:

  Experiments on bigint addition:

  • https://github.com/mratsim/finite-fields/blob/ab033bb5/add_carry.nim#L73-L113

  A macro assembler to ease usage of assembly in Nim (living in a branch for now):

  • https://github.com/mratsim/constantine/blob/5b538d4e/constantine/primitives/macro_assembler_x86.nim

  Concrete usages of the macro assembler (note like Intel syntax it's (dst, operands) order:

  • https://github.com/mratsim/constantine/blob/5b538d4e/constantine/arithmetic/finite_fields_asm_x86.nim

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