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backends/plugins/elf/shorts-segment-manager.h

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+/* ScummVM - Graphic Adventure Engine
+ *
+ * ScummVM is the legal property of its developers, whose names
+ * are too numerous to list here. Please refer to the COPYRIGHT
+ * file distributed with this source distribution.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#ifndef SHORTS_SEGMENT_MANAGER_H
+#define SHORTS_SEGMENT_MANAGER_H
+
+#include "common/scummsys.h"
+
+#if defined(DYNAMIC_MODULES) && defined(USE_ELF_LOADER) && defined(MIPS_TARGET)
+
+#include "backends/plugins/elf/elf32.h"
+
+#include "common/singleton.h"
+#include "common/list.h"
+
+#define ShortsMan ShortSegmentManager::instance()
+
+/**
+ * ShortSegmentManager
+ *
+ * Since MIPS is limited to 32 bits per instruction, loading data that's further away than 16 bits
+ * takes several instructions. Thus, small global data (which is likely to be accessed a lot from
+ * multiple locations) is often put into a GP-relative area (GP standing for the global pointer register)
+ * in MIPS processors. This class manages these segments of small global data, and is used by the
+ * member functions of MIPSDLObject, which query in information from this manager in order to deal with
+ * this segment during the loading/unloading of plugins.
+ *
+ * Since there's no true dynamic linker to change the GP register between plugins and the main engine,
+ * custom ld linker scripts for both the main executable and the plugins ensure the GP-area is in the
+ * same place for both. The ShortSegmentManager accesses this place via the symbols __plugin_hole_start
+ * and __plugin_hole_end, which are defined in those custom ld linker scripts.
+ */
+class ShortSegmentManager : public Common::Singleton<ShortSegmentManager> {
+private:
+	char *_shortsStart;
+	char *_shortsEnd;
+
+public:
+	char *getShortsStart() {
+		return _shortsStart;
+	}
+
+	// Returns whether or not an absolute address is in the GP-relative section.
+	bool inGeneralSegment(char *addr) {
+		return (addr >= _shortsStart && addr < _shortsEnd);
+	}
+
+	class Segment {
+	private:
+		friend class ShortSegmentManager;
+		Segment(char *start, uint32 size, char *origAddr) :
+			_startAddress(start),
+			_size(size),
+			_origAddress(origAddr) {
+		}
+
+		virtual ~Segment() {
+		}
+
+		char *_startAddress;		// Start of shorts segment in memory
+		uint32 _size;				// Size of shorts segment
+		char *_origAddress;			// Original address this segment was supposed to be at
+
+	public:
+		char *getStart() {
+			return _startAddress;
+		}
+
+		char *getEnd() {
+			return (_startAddress + _size);
+		}
+
+		Elf32_Addr getOffset() {
+			return (Elf32_Addr)(_startAddress - _origAddress);
+		}
+
+		bool inSegment(char *addr) {
+			return (addr >= _startAddress && addr <= _startAddress + _size);
+		}
+	};
+
+	Segment *newSegment(uint32 size, char *origAddr);
+	void deleteSegment(Segment *);
+
+private:
+	ShortSegmentManager();
+	friend class Common::Singleton<ShortSegmentManager>;
+	Common::List<Segment *> _list;
+	char *_highestAddress;
+};
+
+#endif // defined(DYNAMIC_MODULES) && defined(USE_ELF_LOADER) && defined(MIPS_TARGET)
+
+#endif /* SHORTS_SEGMENT_MANAGER_H */