ReC98/Research/Borland C++ decompilation.md

Local variables

DX	First 8-bit variable declared if no other function is called Second 16-bit variable declared if no other function is called
[bp-1]	First 8-bit variable declared otherwise
SI	First 16-bit variable declared
DI	Second 16-bit variable declared if other functions are called

Example:

ASM	Declaration sequence in C
SI	int near *var_1;
[bp-1]	char var_2;
[bp-2]	char var_3;

Grouping

Any structures or classes that contain more than a single scalar-type member are grouped according to their declaration order, and placed after (that is, further away from BP) than all scalar-type variables. This means that it's not possible to bundle a set of variables with the same meaning into a structure (e.g. pointers to all 4 VRAM planes) if a scalar-type variable is placed inbetween two of these structure instances on the stack: Those structure instances would be grouped and always placed next to each other, no matter where the scalar-type variable is declared in relation to them.

Signedness

MOV al, var MOV ah, 0	var is unsigned char
MOV al, var CBW	var is char, AX is int

Arithmetic

ADD [m8], imm8	Only achievable through a C++ method operating on a member?
MOV AL, [m8] ADD AL, imm8 MOV [m8], AL	Opposite; not an inlined function
CWD SUB AX, DX SAR AX, 1	AX / 2, AX is int

Arithmetic on a register after assigning it to a variable?

Assigment is part of the C expression. If it's a comparison, that comparison must be spelled out to silence the Possibly incorrect assignment warning.

CALL somefunc MOV ??, AX OR AX, AX JNZ ↑	while(( ?? = somefunc() ) != NULL)

SUB ??, imm vs. ADD ??, -imm

SUB means that ?? is unsigned. Might require suffixing imm with u in case it's part of an arithmetic expression that was promoted to int.

switch statements

• Sequence of the individual cases is identical in both C and ASM
• Multiple cases with the same offset in the table, to code that doesn't return? Code was compiled with -O

Function calls

NOP insertion

Happens for every far call to outside of the current translation unit, even if both the caller and callee end up being linked into the same code segment.

Certainty: Seems like there might be a way around that, apart from temporarily spelling out these calls in ASM until both functions are compiled as part of the same translation unit. Found nothing so far, though.

Pushing byte arguments to functions

Borland C++ just pushes the entire word. Will cause IDA to mis-identify certain local variables as words when they aren't.

Flags

-O (Optimize jumps)

Also merges multiple ADD SP, imm8 stack-clearing instructions after __cdecl function calls into a single one with their combined parameter size.

Inhibited by:

• identical variable declarations within more than one scope – the optimizer will only merge the code after the last ASM reference to that declared variable. Yes, even though the emitted ASM would be identical:

  if(a) {
      int v = set_v();
      do_something_else();
      use(v);
  } else if(underline) {
      // Second declaration of [v]. Even though it's assigned to the same stack
      // offset, the second `PUSH w` call will still be emitted separately.
      // Thus, jump optimization only reuses the `CALL use` instruction.
      // Move the `int v;` declaraion to the beginning of the function to avoid
      // this.
      int v = set_v();
      use(v);
  }
  

• distinct instances of assignments of local variables in registers to itself

• inlined calls to empty functions

Inlining

Always worth a try to get rid of a potential macro. Some edge cases don't inline optimally though:

• Assignments to a pointer in SI – that pointer is moved to DI, clobbering that register. Try a class method instead.

Initialization

Any initialization of a variable with static storage duration (even a const one) that involves function calls (even those that would regularly inline) will emit a #pragma startup function to perform that initialization at runtime. This extends to C++ constructors, making macros the only way to initialize such variables with arithmetic expressions at compile time.

#define FOO(x) (x << 1)

inline char foo(const char x)  {
    return FOO(x);
}

const char static_storage[3] = { FOO(1), foo(2), FOO(3) };

Resulting ASM (abbreviated):

  .data
static_storage  db  2, 0, 6

  .code
@_STCON_$qv	proc	near
	push bp
	mov  bp, sp
	mov  static_storage[1], 4
	pop	 bp
	ret
@_STCON_$qv	endp

C++

Class methods inline to their ideal representation if all of these are true:

• returns void || (returns *this && is at the first nesting level of inlining)
• takes no parameters || takes only built-in, scalar-type parameters

Examples:

• A class method (first nesting level) calling an overloaded operator (second nesting level) returning *this will generate (needless) instructions equivalent to MOV AX, *this. Thus, any overloaded =, +=, -=, etc. operator should always return void.

  Certainty: See the examples in 9d121c7. This is what allows us to use custom types with overloaded assignment operators, with the resulting code generation being indistinguishable from equivalent C preprocessor macros.

• Returning anything else but void or *this will first store that result in AX, leading any branches at the call site to then refer to AX.

  Certainty: Maybe Borland (not Turbo) C++ has an optimization option against it?

Limits of decompilability

MOV BX, SP-style functions, or others with no standard stack frame

These almost certainly weren't compiled from C. By disabling stack frames using #pragma option -k-, it might be possible to still get the exact same code out of Turbo C++ – even though it will most certainly look horrible, and barely more readable than assembly (or even less so), with tons of inline ASM and register pseudovariables. However, it's futile to even try if the function contains one of the following:

• A reference to the DI register. In that case, Turbo C++ always inserts a PUSH DI at the beginning (before the MOV BX, SP), and a POP DI before returning.

  Certainty: Confirmed through reverse-engineering TCC.EXE, no way around it.