2019-10-13 18:52:53 +00:00
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## Signedness
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| `MOV al, var`<br />`MOV ah, 0`| `var` is *unsigned char* |
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## Arithmetic
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| `ADD [m8], imm8` | Only achievable through a C++ method operating on a member? |
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| `MOV AL, [m8]`<br />`ADD AL, imm8`<br />`MOV [m8], AL` | Opposite; *not* an inlined function |
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### Arithmetic on a register *after* assigning it to a variable?
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Assigment is part of the C expression. If it's a comparison, that comparison
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must be spelled out to silence the `Possibly incorrect assignment` warning.
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| `CALL somefunc`<br />`MOV ??, AX`<br />`OR AX, AX`<br />`JNZ ↑` | `while(( ?? = somefunc() ) != NULL)` |
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### `SUB ??, imm` vs. `ADD ??, -imm`
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`SUB` means that `??` is unsigned. Might require suffixing `imm` with `u` in
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case it's part of an arithmetic expression that was promoted to `int`.
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## `switch` statements
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* Sequence of the individual cases is identical in both C and ASM
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* Multiple cases with the same offset in the table, to code that doesn't
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return? Code was compiled with `-O`
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## Inlining
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Always worth a try to get rid of a potential macro. Some edge cases don't
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inline optimally though:
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* Assignments to a pointer in `SI` – that pointer is moved to `DI`,
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[clobbering that register](#clobbering-di). Try a [class method](#C++)
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instead.
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[Reverse-engineering] [th04/th05] Player sprite area invalidation
And once again, the TH05 version is un-decompilable. :/ It was pretty
close this time, though, as the entire block between PUSH DI and POP DI
kind of resembles a separate inlined function, in accordance with Turbo
C++'s automatic backup of the DI register, as researched in 7f971a0.
Except that it contains a loop, and Turbo C++ refuses to inline any
function with `do`, `while`, `for`, or `goto`. If it didn't, it would
have totally worked.
Also, yes, C++ class methods are treated identically in this regard.
Oh well. Shot type control functions next, finally!
Completes P0035, funded by zorg.
2019-09-24 18:54:32 +00:00
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## C++
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2019-10-13 19:52:02 +00:00
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* Every class method that returns `void` inlines to the ideal representation.
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* Every class method that returns `*this` inlines to the ideal representation
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*only at the first nesting level*. Example: A class method calling an
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overloaded operator returning `*this` will generate (needless) instructions
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equivalent to `MOV AX, *this`. Thus, any overloaded `=`, `+=`, `-=`, etc.
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operator should always return `void`.
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[Reverse-engineering] [th04/th05] Player sprite area invalidation
And once again, the TH05 version is un-decompilable. :/ It was pretty
close this time, though, as the entire block between PUSH DI and POP DI
kind of resembles a separate inlined function, in accordance with Turbo
C++'s automatic backup of the DI register, as researched in 7f971a0.
Except that it contains a loop, and Turbo C++ refuses to inline any
function with `do`, `while`, `for`, or `goto`. If it didn't, it would
have totally worked.
Also, yes, C++ class methods are treated identically in this regard.
Oh well. Shot type control functions next, finally!
Completes P0035, funded by zorg.
2019-09-24 18:54:32 +00:00
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**Certainty**: See the examples in `9d121c7`. This is what allows us to use
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custom types with overloaded assignment operators, with the resulting code
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generation being indistinguishable from equivalent C preprocessor macros.
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* Returning *anything else* will first store that result in `AX`, leading any
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branches at the call site to then refer to `AX`.
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**Certainty**: Maybe Borland (not Turbo) C++ has an optimization option
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against it?
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2019-09-20 17:53:52 +00:00
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## Limits of decompilability
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[Reverse-engineering] [th04/th05] Player sprite area invalidation
And once again, the TH05 version is un-decompilable. :/ It was pretty
close this time, though, as the entire block between PUSH DI and POP DI
kind of resembles a separate inlined function, in accordance with Turbo
C++'s automatic backup of the DI register, as researched in 7f971a0.
Except that it contains a loop, and Turbo C++ refuses to inline any
function with `do`, `while`, `for`, or `goto`. If it didn't, it would
have totally worked.
Also, yes, C++ class methods are treated identically in this regard.
Oh well. Shot type control functions next, finally!
Completes P0035, funded by zorg.
2019-09-24 18:54:32 +00:00
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### `MOV BX, SP`-style functions, or others with no standard stack frame
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2019-09-20 17:53:52 +00:00
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These almost certainly weren't compiled from C. By disabling stack frames
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using `#pragma option -k-`, it *might* be possible to still get the exact same
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code out of Turbo C++ – even though it will most certainly look horrible, and
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barely more readable than assembly (or even less so), with tons of inline ASM
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and register pseudovariables. However, it's futile to even try if the function
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contains one of the following:
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* A reference to the `DI` register. In that case, Turbo C++ always inserts a
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`PUSH DI` at the beginning (before the `MOV BX, SP`), and a `POP DI` before
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returning.
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**Certainty:** Confirmed through reverse-engineering `TCC.EXE`, no way
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around it.
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