mirror of https://github.com/nmlgc/ReC98.git
[Decompilation] [th02] Get zun_res1.c right
4½ years after aa56a7c, it turns out that the correct decompilation
involves… no-ops generated by assigning variables that just happen to
be in registers to themselves?! Which does get optimized out, but only
after TCC folded identical tail code in all branches of a function,
thus effectively functioning as an optimization barrier.
The initial attempt used register pseudovariables, but this definitely
is the best possible way this could work – portable, and doesn't
unnecessarily shred the code into tiny inlined functions pieces. The
mindblowing thing here is that ZUN could have actually written this to
have additional, albeit unnecessary, lines to place breakpoints on. But
that means he must have chosen those two local variables in SI and DI
completely by chance… 🤯
The best thing though? ~#pragma inline is gone~
Part of P0076, funded by [Anonymous] and -Tom-.
This commit is contained in:
nmlgc
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b8ca607c38
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@ -81,24 +81,29 @@ certain local variables as `word`s when they aren't.
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### `-O` (Optimize jumps)
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Inhibited by identical variable declarations within more than one scope – the
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optimizer will only merge the code *after* the last ASM reference to that
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declared variable. Yes, even though the emitted ASM would be identical:
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```c
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if(a) {
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int v = set_v();
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do_something_else();
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use(v);
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} else if(underline) {
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// Second declaration of [v]. Even though it's assigned to the same stack
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// offset, the second `PUSH w` call will still be emitted separately.
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// Thus, jump optimization only reuses the `CALL use` instruction.
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// Move the `int v;` declaraion to the beginning of the function to avoid
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// this.
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int v = set_v();
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use(v);
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}
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```
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Inhibited by:
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* identical variable declarations within more than one scope – the
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optimizer will only merge the code *after* the last ASM reference to that
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declared variable. Yes, even though the emitted ASM would be identical:
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```c
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if(a) {
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int v = set_v();
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do_something_else();
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use(v);
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} else if(underline) {
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// Second declaration of [v]. Even though it's assigned to the same stack
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// offset, the second `PUSH w` call will still be emitted separately.
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// Thus, jump optimization only reuses the `CALL use` instruction.
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// Move the `int v;` declaraion to the beginning of the function to avoid
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// this.
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int v = set_v();
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use(v);
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}
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```
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* distinct instances of assignments of local variables in registers to itself
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## Inlining
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@ -4,8 +4,6 @@
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* configuration file required in order to run TH02.
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*/
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#pragma inline
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#include <stddef.h>
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#include "th02/th02.h"
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#include "th02/snd/snd.h"
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@ -35,43 +33,24 @@ void cfg_write(seg_t resident_sgm)
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dos_close(handle);
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}
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#define LOGO \
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"東方封魔録用 常駐プログラム ZUN_RES.com Version1.01 (c)zun 1997"
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int main(int argc, const char **argv)
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{
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int pascal scoredat_verify(void);
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static const char MIKOConfig[] = RES_ID;
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static const char LOGO[] =
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"\n"
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"\n"
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"東方封魔録用 常駐プログラム ZUN_RES.com Version1.01 (c)zun 1997\n";
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static const char ERROR_SCOREDAT[] =
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"ハイスコアファイルがおかしいの、もう一度実行してね。\n";
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static const char ERROR_NOT_RESIDENT[] =
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"わたし、まだいませんよぉ\n\n";
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static const char REMOVED[] =
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"さよなら、また会えたらいいな\n\n";
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static const char ERROR_UNKNOWN_OPTION[] =
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"そんなオプション付けられても、困るんですけど\n\n";
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static const char ERROR_ALREADY_RESIDENT[] =
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"わたし、すでにいますよぉ\n\n";
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static const char ERROR_OUT_OF_MEMORY[] =
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"作れません、わたしの居場所がないの！\n\n";
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static const char INITIALIZED[] =
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"それでは、よろしくお願いします\n\n";
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seg_t sgm;
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const char *res_id = MIKOConfig;
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const char *res_id = RES_ID;
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int i;
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char far *resident;
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sgm = resdata_exist(res_id, RES_ID_STRLEN, RES_PARASIZE);
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dos_puts2(LOGO);
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dos_puts2("\n\n" LOGO "\n");
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graph_clear();
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// No, I tried all permutations of command-line switches,
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// gotos and returns, and no pure C solution seems to work!
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if(scoredat_verify() == 1) __asm {
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push offset ds:ERROR_SCOREDAT
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jmp error_puts
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if(scoredat_verify() == 1) {
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dos_puts2("ハイスコアファイルがおかしいの、もう一度実行してね。\n");
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return 1;
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}
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if(argc == 2) {
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#define arg1_is(capital, small) \
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@ -79,33 +58,32 @@ int main(int argc, const char **argv)
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&& (argv[1][1] == (capital) || argv[1][1] == (small))
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if(arg1_is('R', 'r')) {
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if(!sgm) {
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dos_puts2(ERROR_NOT_RESIDENT);
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asm jmp error_ret
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dos_puts2("わたし、まだいませんよぉ\n\n");
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return 1;
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}
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dos_free(sgm);
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dos_puts2(REMOVED);
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dos_puts2("さよなら、また会えたらいいな\n\n");
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return 0;
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} else if(arg1_is('D', 'd')) {
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debug = 1;
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} else {
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dos_puts2(ERROR_UNKNOWN_OPTION);
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dos_puts2("そんなオプション付けられても、困るんですけど\n\n");
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sgm = sgm; /* optimization barrier #1 */
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return 1;
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}
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}
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if(sgm) {
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dos_puts2(ERROR_ALREADY_RESIDENT);
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dos_puts2("わたし、すでにいますよぉ\n\n");
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argv = argv; /* optimization barrier #2 */
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return 1;
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}
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sgm = resdata_create(res_id, RES_ID_STRLEN, RES_PARASIZE);
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if(!sgm) {
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asm push offset ds:ERROR_OUT_OF_MEMORY
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error_puts:
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asm call near ptr dos_puts2
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error_ret:
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dos_puts2("作れません、わたしの居場所がないの！\n\n");
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return 1;
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}
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resident = MK_FP(sgm, 0);
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dos_puts2(INITIALIZED);
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dos_puts2("それでは、よろしくお願いします\n\n");
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for(i = offsetof(resident_t, stage); i < sizeof(resident_t); i++) {
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resident[i] = 0;
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}
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