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ReC98/Research/Borland C++ decompilation.md

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[Decompilation] [th05] Yuuka's shot control functions Ooh, shot position being determined by RNG at lower shot levels? That's some RNG manipulation TAS potential right there! Maybe. Part of P0037, funded by zorg.
2019-10-12 14:57:14 +00:00
## Local variables
| | |
|-|-|
[Decompilation] [th03] Shot update and render functions Meh, can't overload arithmetic operators that take a Subpixel without generating a needless load and store, even with -Z. But heck, slightly uglifying subpixel/subpixel arithmetic is exactly the right trade-off. Completes P0061, funded by Touhou Patch Center.
2019-12-03 21:45:08 +00:00
| `DX` | First 8-bit variable declared *if no other function is called*<br />Second 16-bit variable declared *if no other function is called* |
[Decompilation] [th05] Yuuka's shot control functions Ooh, shot position being determined by RNG at lower shot levels? That's some RNG manipulation TAS potential right there! Maybe. Part of P0037, funded by zorg.
2019-10-12 14:57:14 +00:00
| `[bp-1]` | First 8-bit variable declared *otherwise* |
| `SI` | First 16-bit variable declared |
[Decompilation] [th03] Shot update and render functions Meh, can't overload arithmetic operators that take a Subpixel without generating a needless load and store, even with -Z. But heck, slightly uglifying subpixel/subpixel arithmetic is exactly the right trade-off. Completes P0061, funded by Touhou Patch Center.
2019-12-03 21:45:08 +00:00
| `DI` | Second 16-bit variable declared *if other functions are called* |
[Decompilation] [th05] Yuuka's shot control functions Ooh, shot position being determined by RNG at lower shot levels? That's some RNG manipulation TAS potential right there! Maybe. Part of P0037, funded by zorg.
2019-10-12 14:57:14 +00:00
Example:
| ASM | Declaration sequence in C |
|----------|---------------------------|
| `SI` | `int near *var_1;` |
| `[bp-1]` | `char var_2;` |
| `[bp-2]` | `char var_3;` |
[Decompilation] [th01] Inter-page rectangle moves Semi-unused, that is, the one use of this function doesn't actually move the rectangle to a different position. Ironically, the non-moving back-to-front function immediately above *is* unused… Also, too bad that stack order is the only reason we can't use structs to combine all plane variables into a single object. Part of P0067, funded by Splashman.
2020-01-10 20:25:29 +00:00
### 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.
[Reverse-engineering] [th05] Player shot cycle bitflags A TH05 innovation that actually makes the game code easier to read?! Although it was quite hard to actually reverse-engineer it, with the confusing flag ordering pointing to some deeper meaning behind the flags, which really doesn't exist. Completes P0036, funded by zorg.
2019-10-13 18:52:53 +00:00
## Signedness
| | |
|-|-|
| `MOV al, var`<br />`MOV ah, 0`| `var` is *unsigned char* |
[Decompilation] [th01] master.lib resident palette function reimplementations Which store colors as GRB, as suggested by the structure's ID string. Even master.lib's own functions add an additional XCHG AH, AL instruction to get colors into and out of this format. MASTER.MAN suggests that it's some sort of standard on PC-98. It does match the order of ths hardware's palette register ports, after all. (0AAh = green, 0ACh = red, 0AEh = blue) Now we also know why __seg* wasn't used more commonly, as lamented in c8e8e98. Turbo C++ simply doesn't support a lot of arithmetic on segment pointers. And then that undecompilable far call to a function within the same segment, but inside a different translation unit… Also, thanks again to Egor for the SCOPY@ hack that debuted in 0460072. Would have probably struggled with this a lot more without that. And *then* you realize that TH01 effectively doesn't even use the resident palette. 😐 And yes, we're procrastinating the whole issue of potentially using a single translation unit for all three binaries by using a common segment name, because it *really* isn't that easy. Completes P0066, funded by Keyblade Wiedling Neko and Splashman.
2020-01-05 15:10:00 +00:00
| `MOV al, var`<br />`CBW` | `var` is *char*, `AX` is *int* |
[Reverse-engineering] [th05] Player shot cycle bitflags A TH05 innovation that actually makes the game code easier to read?! Although it was quite hard to actually reverse-engineer it, with the confusing flag ordering pointing to some deeper meaning behind the flags, which really doesn't exist. Completes P0036, funded by zorg.
2019-10-13 18:52:53 +00:00
## Arithmetic
| | |
|-|-|
| `ADD [m8], imm8` | Only achievable through a C++ method operating on a member? |
| `MOV AL, [m8]`<br />`ADD AL, imm8`<br />`MOV [m8], AL` | Opposite; *not* an inlined function |
### 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`<br />`MOV ??, AX`<br />`OR AX, AX`<br />`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`
[Decompilation] [th01] master.lib resident palette function reimplementations Which store colors as GRB, as suggested by the structure's ID string. Even master.lib's own functions add an additional XCHG AH, AL instruction to get colors into and out of this format. MASTER.MAN suggests that it's some sort of standard on PC-98. It does match the order of ths hardware's palette register ports, after all. (0AAh = green, 0ACh = red, 0AEh = blue) Now we also know why __seg* wasn't used more commonly, as lamented in c8e8e98. Turbo C++ simply doesn't support a lot of arithmetic on segment pointers. And then that undecompilable far call to a function within the same segment, but inside a different translation unit… Also, thanks again to Egor for the SCOPY@ hack that debuted in 0460072. Would have probably struggled with this a lot more without that. And *then* you realize that TH01 effectively doesn't even use the resident palette. 😐 And yes, we're procrastinating the whole issue of potentially using a single translation unit for all three binaries by using a common segment name, because it *really* isn't that easy. Completes P0066, funded by Keyblade Wiedling Neko and Splashman.
2020-01-05 15:10:00 +00:00
## 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
[Decompilation] [th05] Yuuka's shot control functions Ooh, shot position being determined by RNG at lower shot levels? That's some RNG manipulation TAS potential right there! Maybe. Part of P0037, funded by zorg.
2019-10-12 14:57:14 +00:00
Borland C++ just pushes the entire word. Will cause IDA to mis-identify
certain local variables as `word`s when they aren't.
[Reverse-engineering] [th05] Player shot cycle bitflags A TH05 innovation that actually makes the game code easier to read?! Although it was quite hard to actually reverse-engineer it, with the confusing flag ordering pointing to some deeper meaning behind the flags, which really doesn't exist. Completes P0036, funded by zorg.
2019-10-13 18:52:53 +00:00
## 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](#clobbering-di). Try a [class method](#C++)
instead.
[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
## C++
[Decompilation] [th03] Shot update and render functions Meh, can't overload arithmetic operators that take a Subpixel without generating a needless load and store, even with -Z. But heck, slightly uglifying subpixel/subpixel arithmetic is exactly the right trade-off. Completes P0061, funded by Touhou Patch Center.
2019-12-03 21:45:08 +00:00
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
[Decompilation] Prepare the C side for the shot type control functions That should make this convoluted copypasta a bit easier to read. And sure, I could have done something about the loop as well, but SHOT_FUNC_INIT already hides enough control flow behind a macro… Part of P0037, funded by zorg.
2019-10-13 19:52:02 +00:00
equivalent to `MOV AX, *this`. Thus, any overloaded `=`, `+=`, `-=`, etc.
operator should always return `void`.
[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
**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.
[Decompilation] [th03] Shot update and render functions Meh, can't overload arithmetic operators that take a Subpixel without generating a needless load and store, even with -Z. But heck, slightly uglifying subpixel/subpixel arithmetic is exactly the right trade-off. Completes P0061, funded by Touhou Patch Center.
2019-12-03 21:45:08 +00:00
* 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`.
[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
**Certainty**: Maybe Borland (not Turbo) C++ has an optimization option
against it?
[Research] Find out why we can't decompile TH05's hud_bar_put() Part of P0033, funded by zorg.
2019-09-20 17:53:52 +00:00
## Limits of decompilability
[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
### `MOV BX, SP`-style functions, or others with no standard stack frame
[Research] Find out why we can't decompile TH05's hud_bar_put() Part of P0033, funded by zorg.
2019-09-20 17:53:52 +00:00
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:
[Decompilation] [th05] Yuuka's shot control functions Ooh, shot position being determined by RNG at lower shot levels? That's some RNG manipulation TAS potential right there! Maybe. Part of P0037, funded by zorg.
2019-10-12 14:57:14 +00:00
<a id="clobbering-di"></a>
[Research] Find out why we can't decompile TH05's hud_bar_put() Part of P0033, funded by zorg.
2019-09-20 17:53:52 +00:00
* 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.