- 📝 Posted:
- 🚚 Summary of:
- P0126, P0127
- ⌨ Commits:
- 💰 Funded by:
- Blue Bolt, [Anonymous]
- 🏷 Tags:
- rec98 th03 th04 th05 pc98 micro-optimization tcc tasm meta
Alright, back to continuing the
master.hpp transition started
in P0124, and repaying technical debt. The last blog post already
announced some ridiculous decompilations… and in fact, not a single
one of the functions in these two pushes was decompilable into
idiomatic C/C++ code.
As usual, that didn't keep me from trying though. The TH04 and TH05
version of the infamous 16-pixel-aligned, EGC-accelerated rectangle
blitting function from page 1 to page 0 was fairly average as far as
unreasonable decompilations are concerned.
The big blocker in TH03's
MAIN.EXE, however, turned out to be
the .MRS functions, used to render the gauge attack portraits and bomb
backgrounds. The blitting code there uses the additional FS and GS segment
registers provided by the Intel 386… which
- are not supported by Turbo C++'s inline assembler, and
- can't be turned into pointers, due to a compiler bug in Turbo C++ that
generates wrong segment prefix opcodes for the
Apparently I'm the first one to even try doing that with this compiler? I
haven't found any other mention of this bug…
Compiling via assembly (
#pragma inline) would work around
this bug and generate the correct instructions. But that would incur yet
another dependency on a 16-bit TASM, for something honestly quite
What we can always do, however, is using
__emit__() to simply
output x86 opcodes anywhere in a function. Unlike spelled-out inline
assembly, that can even be used in helper functions that are supposed to
inline… which does in fact allow us to fully abstract away this compiler
if() comparisons with pseudo-registers
wouldn't inline, but "converting" them into C++ template function
specializations does. All that's left is some C preprocessor abuse
to turn the pseudo-registers into types, and then we do retain a
poke() call in the blitting functions in the
Yeah… the result is
I may have gone too far in a few places…
One might certainly argue that all these ridiculous decompilations actually hurt the preservation angle of this project. "Clearly, ZUN couldn't have possibly written such unreasonable C++ code. So why pretend he did, and not just keep it all in its more natural ASM form?" Well, there are several reasons:
- Future port authors will merely have to translate all the pseudo-registers and inline assembly to C++. For the former, this is typically as easy as replacing them with newly declared local variables. No need to bother with function prolog and epilog code, calling conventions, or the build system.
- No duplication of constants and structures in ASM land.
- As a more expressive language, C++ can document the code much better. Meticulous documentation seems to have become the main attraction of ReC98 these days – I've seen it appreciated quite a number of times, and the continued financial support of all the backers speaks volumes. Mods, on the other hand, are still a rather rare sight.
- Having as few .ASM files in the source tree as possible looks better to casual visitors who just look at GitHub's repo language breakdown. This way, ReC98 will also turn from an "Assembly project" to its rightful state of "C++ project" much sooner.
- And finally, it's not like the ASM versions are gone – they're still part of the Git history.
Unfortunately, these pushes also demonstrated a second disadvantage in trying to decompile everything possible: Since Turbo C++ lacks TASM's fine-grained ability to enforce code alignment on certain multiples of bytes, it might actually be unfeasible to link in a C-compiled object file at its intended original position in some of the .EXE files it's used in. Which… you're only going to notice once you encounter such a case. Due to the slightly jumbled order of functions in the 📝 second, shared code segment, that might be long after you decompiled and successfully linked in the function everywhere else.
And then you'll have to throw away that decompilation after all 😕 Oh well. In this specific case (the lookup table generator for horizontally flipping images), that decompilation was a mess anyway, and probably helped nobody. I could have added a dummy .OBJ that does nothing but enforce the needed 2-byte alignment before the function if I really insisted on keeping the C version, but it really wasn't worth it.
Now that I've also described yet another meta-issue, maybe there'll really be nothing to say about the next technical debt pushes? Next up though: Back to actual progress again, with TH01. Which maybe even ends up pushing that game over the 50% RE mark?