From 130766a55c7626b80bfbf34f30b5c20ed84af5ef Mon Sep 17 00:00:00 2001
From: Ted Green <tedgreen99@inbox.com>
Date: Wed, 1 Jun 2016 08:51:21 -0600
Subject: [PATCH] (nw) MIP3 DRC handle branches in delay slot properly.

---
 src/devices/cpu/drcfe.cpp      | 11 ++++++++++-
 src/devices/cpu/drcfe.h        |  5 +++++
 src/devices/cpu/mips/mips3.cpp |  2 ++
 3 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/devices/cpu/drcfe.cpp b/src/devices/cpu/drcfe.cpp
index 87478de59e6..6eda93e95c7 100644
--- a/src/devices/cpu/drcfe.cpp
+++ b/src/devices/cpu/drcfe.cpp
@@ -55,7 +55,8 @@ drc_frontend::drc_frontend(device_t &cpu, UINT32 window_start, UINT32 window_end
 		m_cpudevice(downcast<cpu_device &>(cpu)),
 		m_program(m_cpudevice.space(AS_PROGRAM)),
 		m_pageshift(m_cpudevice.space_config(AS_PROGRAM)->m_page_shift),
-		m_desc_array(window_end + window_start + 2, nullptr)
+		m_desc_array(window_end + window_start + 2, nullptr),
+		m_allow_branch_in_delay(false)
 {
 }
 
@@ -196,6 +197,14 @@ opcode_desc *drc_frontend::describe_one(offs_t curpc, const opcode_desc *prevdes
 	{
 		// iterate over slots and describe them
 		offs_t delaypc = curpc + desc->length;
+
+		// If previous instruction is a branch use the target pc. Currently MIP3s only.
+		if (m_allow_branch_in_delay && prevdesc && (prevdesc->flags & OPFLAG_IS_BRANCH) && prevdesc->targetpc != BRANCH_TARGET_DYNAMIC) {
+			// We got here because the previous instruction is a branch and this instruction is a branch.
+			// So the PC of the delay slot for the this branch will be the target address of the previous branch.
+			delaypc = prevdesc->targetpc;
+			//printf("drc_frontend::describe_one: branch in delay slot. curpc=0x%08X, delaypc=0x%08X\n", curpc, delaypc);
+		}
 		opcode_desc *prev = desc;
 		for (UINT8 slotnum = 0; slotnum < desc->delayslots; slotnum++)
 		{
diff --git a/src/devices/cpu/drcfe.h b/src/devices/cpu/drcfe.h
index 09d86b2bd99..c7458e87bdb 100644
--- a/src/devices/cpu/drcfe.h
+++ b/src/devices/cpu/drcfe.h
@@ -134,6 +134,9 @@ public:
 	// describe a block
 	const opcode_desc *describe_code(offs_t startpc);
 
+	// Set the allow branches in delay slot flag
+	void set_allow_branch_in_delay(bool flag) { m_allow_branch_in_delay = flag; }
+
 protected:
 	// required overrides
 	virtual bool describe(opcode_desc &desc, const opcode_desc *prev) = 0;
@@ -159,6 +162,8 @@ private:
 	simple_list<opcode_desc> m_desc_live_list;      // list of live descriptions
 	fixed_allocator<opcode_desc> m_desc_allocator;  // fixed allocator for descriptions
 	std::vector<opcode_desc *> m_desc_array;      // array of descriptions in PC order
+
+	bool m_allow_branch_in_delay;  // Allow branches in delay slots
 };
 
 
diff --git a/src/devices/cpu/mips/mips3.cpp b/src/devices/cpu/mips/mips3.cpp
index 7ae1fd1f337..dbc510771fc 100644
--- a/src/devices/cpu/mips/mips3.cpp
+++ b/src/devices/cpu/mips/mips3.cpp
@@ -386,6 +386,8 @@ void mips3_device::device_start()
 
 	/* initialize the front-end helper */
 	m_drcfe = std::make_unique<mips3_frontend>(this, COMPILE_BACKWARDS_BYTES, COMPILE_FORWARDS_BYTES, SINGLE_INSTRUCTION_MODE ? 1 : COMPILE_MAX_SEQUENCE);
+	// Allow branches in delay slots
+	m_drcfe->set_allow_branch_in_delay(true);
 
 	/* allocate memory for cache-local state and initialize it */
 	memcpy(m_fpmode, fpmode_source, sizeof(fpmode_source));