From 3b5eede3fc9308b7c7c065e01c81f36fb6814fc1 Mon Sep 17 00:00:00 2001
From: Jonathan G Rennison <j.g.rennison@gmail.com>
Date: Wed, 25 May 2022 20:04:36 +0100
Subject: [PATCH] VarAction2: Detect and replace operations which always
 produce 0

---
 src/newgrf.cpp           | 21 +++++++++++++++++++++
 src/newgrf_spritegroup.h | 26 ++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/src/newgrf.cpp b/src/newgrf.cpp
index 0d569e0acf..b442b9ab95 100644
--- a/src/newgrf.cpp
+++ b/src/newgrf.cpp
@@ -5677,6 +5677,27 @@ static void NewSpriteGroup(ByteReader *buf)
 							/* Delete useless zero operations */
 							group->adjusts.pop_back();
 							inference = prev_inference;
+						} else if (adjust.and_mask == 0 && IsEvalAdjustWithZeroAlwaysZero(adjust.operation)) {
+							/* Operation always returns 0, replace it and any useless prior operations */
+							group->adjusts.pop_back();
+							while (!group->adjusts.empty()) {
+								const DeterministicSpriteGroupAdjust &prev = group->adjusts.back();
+								if (prev.variable != 0x7E && !IsEvalAdjustWithSideEffects(prev.operation)) {
+									/* Delete useless operation */
+									group->adjusts.pop_back();
+								} else {
+									break;
+								}
+							}
+							DeterministicSpriteGroupAdjust &replacement = group->adjusts.emplace_back();
+							replacement.operation = group->adjusts.size() == 1 ? DSGA_OP_ADD : DSGA_OP_RST;
+							replacement.variable = 0x1A;
+							replacement.shift_num = 0;
+							replacement.type = DSGA_TYPE_NONE;
+							replacement.and_mask = 0;
+							replacement.add_val = 0;
+							replacement.divmod_val = 0;
+							inference = VA2AIF_SIGNED_NON_NEGATIVE | VA2AIF_ONE_OR_ZERO | VA2AIF_PREV_MASK_ADJUST;
 						} else {
 							switch (adjust.operation) {
 								case DSGA_OP_SMIN:
diff --git a/src/newgrf_spritegroup.h b/src/newgrf_spritegroup.h
index 2bfbf29b42..316157af7c 100644
--- a/src/newgrf_spritegroup.h
+++ b/src/newgrf_spritegroup.h
@@ -186,6 +186,32 @@ inline bool IsEvalAdjustWithZeroRemovable(DeterministicSpriteGroupAdjustOperatio
 	}
 }
 
+inline bool IsEvalAdjustWithZeroAlwaysZero(DeterministicSpriteGroupAdjustOperation op)
+{
+	switch (op) {
+		case DSGA_OP_UMIN:
+		case DSGA_OP_MUL:
+		case DSGA_OP_AND:
+		case DSGA_OP_RST:
+			return true;
+
+		default:
+			return false;
+	}
+}
+
+inline bool IsEvalAdjustWithSideEffects(DeterministicSpriteGroupAdjustOperation op)
+{
+	switch (op) {
+		case DSGA_OP_STO:
+		case DSGA_OP_STOP:
+			return true;
+
+		default:
+			return false;
+	}
+}
+
 inline bool IsEvalAdjustUsableForConstantPropagation(DeterministicSpriteGroupAdjustOperation op)
 {
 	switch (op) {