From c96952f47b36a9369746fdc9f180409ea3afbd1f Mon Sep 17 00:00:00 2001
From: nmlgc <nmlgc@nmlgc.net>
Date: Thu, 4 Apr 2024 18:47:58 +0200
Subject: [PATCH] [Research] Discover how pointer constness affects pointer
 parameter codegen

In most cases, this discovery will mean that we have to bite the bullet
and remove `const` from pointer parameters if the generated ASM
wouldn't match otherwise. That by itself doesn't really simplify the
code, but at least we get to remove a single bloated `reinterpret_cast`
from one function call already.

Part of P0278, funded by Yanga.
---
 Research/Borland C++ decompilation.md | 19 ++++++++++++++-----
 th01/hiscore/regist.cpp               |  2 +-
 th01/hiscore/regist.hpp               |  2 +-
 th01/main_01.cpp                      |  3 +--
 4 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/Research/Borland C++ decompilation.md b/Research/Borland C++ decompilation.md
index 6fc9e1b4..d94e3d44 100644
--- a/Research/Borland C++ decompilation.md	
+++ b/Research/Borland C++ decompilation.md	
@@ -362,8 +362,14 @@ certain local variables as `word`s when they aren't.
 
 ### Pushing pointers
 
-When passing a `near` pointer to a function that takes a `far` one, the
-segment argument is sometimes `PUSH`ed immediately, before evaluating the
+Passing `far` pointers to subscripted array elements requires code to calculate
+the offset. Turbo C++ emits this calculation (and not the `PUSH` itself) either
+before or after the segment is `PUSH`ed. If either
+
+1. the pointer is `near`, or
+2. the parameter is a near or far `const *`,
+
+the segment argument is always pushed immediately, before evaluating the
 offset:
 
 ```c++
@@ -375,11 +381,14 @@ struct s100 {
 
 extern s100 structs[5];
 
-void __cdecl process(s100 *element);
+void __cdecl process_mut(s100 *element);
+void __cdecl process_const(const s100 *element);
 
 void foo(int i) {
-  process((s100 near *)(&structs[i])); // PUSH DS; (AX = offset); PUSH AX;
-  process((s100 far *)(&structs[i]));  // (AX = offset); PUSH DS; PUSH AX;
+  process_mut((s100 near *)(&structs[i]));   // PUSH DS; (AX = offset); PUSH AX;
+  process_mut((s100 far *)(&structs[i]));    // (AX = offset); PUSH DS; PUSH AX;
+  process_const((s100 near *)(&structs[i])); // PUSH DS; (AX = offset); PUSH AX;
+  process_const((s100 far *)(&structs[i]));  // PUSH DS; (AX = offset); PUSH AX;
 }
 ```
 
diff --git a/th01/hiscore/regist.cpp b/th01/hiscore/regist.cpp
index fcc3d2ff..751f32c8 100644
--- a/th01/hiscore/regist.cpp
+++ b/th01/hiscore/regist.cpp
@@ -603,7 +603,7 @@ static const int SCOREDAT_NOT_CLEARED = (SCOREDAT_CLEARED - 10);
 void regist_menu(
 	score_t score,
 	int16_t stage_num_or_scoredat_constant,
-	const sshiftjis_t route[SCOREDAT_ROUTE_LEN + 1]
+	sshiftjis_t route[SCOREDAT_ROUTE_LEN + 1]
 )
 {
 	scoredat_name_z_t names[SCOREDAT_PLACES];
diff --git a/th01/hiscore/regist.hpp b/th01/hiscore/regist.hpp
index 2034e605..d252a680 100644
--- a/th01/hiscore/regist.hpp
+++ b/th01/hiscore/regist.hpp
@@ -18,5 +18,5 @@
 void regist_menu(
 	score_t score,
 	int16_t stage_num_or_scoredat_constant,
-	const sshiftjis_t route[SCOREDAT_ROUTE_LEN + 1]
+	sshiftjis_t route[SCOREDAT_ROUTE_LEN + 1]
 );
diff --git a/th01/main_01.cpp b/th01/main_01.cpp
index 9f4f7b4d..99497491 100644
--- a/th01/main_01.cpp
+++ b/th01/main_01.cpp
@@ -1006,8 +1006,7 @@ int main(void)
 	}
 	resident->score = score;
 
-	// ZUN bloat: Unnecessary cast.
-	regist_menu(score, (stage_id + 1), reinterpret_cast<const sshiftjis_t *>(
+	regist_menu(score, (stage_id + 1), (
 		!stage_on_route(stage_id) ? SCOREDAT_ROUTE_SHRINE :
 		(route == ROUTE_MAKAI) ? SCOREDAT_ROUTE_MAKAI : SCOREDAT_ROUTE_JIGOKU
 	));