From b3b7c684c68e767923446eac347d21206f20d52c Mon Sep 17 00:00:00 2001
From: David Given <dg@cowlark.com>
Date: Sun, 9 Sep 2018 12:23:59 +0200
Subject: [PATCH] Simplify MIPS relocations. Add RELS2 (for generic high-word
 relocations). Add support for MIPS branch-and-jump relocations.

---
 h/out.h              |  4 +--
 mach/mips/as/mach3.c |  4 +--
 mach/mips/as/mach4.c |  4 +--
 util/led/relocate.c  | 71 ++++++++++++++++++++++++++++++++++++++++++--
 4 files changed, 75 insertions(+), 8 deletions(-)

diff --git a/h/out.h b/h/out.h
index 4176fd341..cb7995133 100644
--- a/h/out.h
+++ b/h/out.h
@@ -68,9 +68,9 @@ struct outname {
 #define RELOPPC      4          /* PowerPC 26-bit address */
 #define RELOPPC_LIS  5          /* PowerPC lis */
 #define RELOVC4      6          /* VideoCore IV address in 32-bit instruction */
-#define RELOMIPS     7          /* MIPS, low half of word or other*/
-#define RELOMIPSHI   8          /* MIPS, high half of word */
+#define RELOMIPS     7          /* MIPS */
 
+#define RELS2   0x1000          /* shift result right 16 bits before writing back */
 #define RELPC	0x2000			/* pc relative */
 #define RELBR	0x4000			/* High order byte lowest address. */
 #define RELWR	0x8000			/* High order word lowest address. */
diff --git a/mach/mips/as/mach3.c b/mach/mips/as/mach3.c
index 519bb02e3..98e62c044 100644
--- a/mach/mips/as/mach3.c
+++ b/mach/mips/as/mach3.c
@@ -76,8 +76,8 @@
 
 0,     OP_LI,      0,          "li",
 0,     OP_LA,      0,          "la",
-0,     HI,         RELOMIPSHI, "hi",
-0,     LO,         RELOMIPS,   "lo",
+0,     HI,         RELO2|RELS2,"hi",
+0,     LO,         RELO2,      "lo",
 
 #include "tokens.y"
 
diff --git a/mach/mips/as/mach4.c b/mach/mips/as/mach4.c
index 7b2e7e2e0..45c1bedc6 100644
--- a/mach/mips/as/mach4.c
+++ b/mach/mips/as/mach4.c
@@ -24,10 +24,10 @@
 		word_t val = $4.val;
 
 		if (type != S_ABS)
-			newrelo($4.typ, RELOMIPSHI | FIXUPFLAGS);
+			newrelo($4.typ, RELO2 | RELS2 | FIXUPFLAGS);
 		emit4(0x3c000000 | (reg<<16) | (val>>16)); /* lui reg, value */
 		if (type != S_ABS)
-			newrelo($4.typ, RELOMIPS | FIXUPFLAGS);
+			newrelo($4.typ, RELO2 | FIXUPFLAGS);
 		emit4(0x34000000 | (reg<<16) | (reg<<21) | (val & 0xffff)); /* ori reg, reg, value */
 	}
 
diff --git a/util/led/relocate.c b/util/led/relocate.c
index f2e9f784d..8b34e8ec2 100644
--- a/util/led/relocate.c
+++ b/util/led/relocate.c
@@ -179,6 +179,30 @@ static uint32_t get_lis_valu(char *addr, uint16_t type)
 	return valu;
 }
 
+/* RELOMIPS is used for j and b instructions only. */
+static uint32_t get_mips_valu(char* addr)
+{
+	uint32_t value = read4(addr, 0);
+	switch (value >> 26)
+	{
+		case 2: /* j */
+		case 3: /* jal */
+		case 29: /* jalx */
+			/* Unsigned 26-bit payload. */
+			value = value & ((1<<26)-1);
+			break;
+
+		default: /* assume everything else is a b, there are lots */
+			/* Signed 16-bit payload. */
+			value = ((int32_t)value << 16) >> 16;
+			break;
+	}
+
+	/* The value has two implicit zero bits on the bottom. */
+	value <<= 2;
+	return value;
+}
+
 /*
  * The bits in type indicate how many bytes the value occupies and what
  * significance should be attributed to each byte.
@@ -198,8 +222,10 @@ static uint32_t getvalu(char* addr, uint16_t type)
 		return get_lis_valu(addr, type);
 	case RELOVC4:
 		return get_vc4_valu(addr);
+	case RELOMIPS:
+		return get_mips_valu(addr);
 	default:
-		fatal("bad relocation type %x", type & RELSZ);
+		fatal("can't read relocation type %x", type & RELSZ);
 	}
 	/* NOTREACHED */
 }
@@ -369,6 +395,36 @@ static void put_lis_valu(char* addr, uint32_t value, uint16_t type)
 	write4(opcode, addr, type);
 }
 
+/* RELOMIPS is used for j and b instructions only. */
+static void put_mips_valu(char* addr, uint32_t value)
+{
+	uint32_t opcode = read4(addr, 0);
+
+	/* The two bottom zero bits are implicit. */
+	if (value & 3)
+		fatal("invalid MIPS relocation value 0x%x", value);
+	value >>= 2;
+
+	switch (value >> 26)
+	{
+		case 2: /* j */
+		case 3: /* jal */
+		case 29: /* jalx */
+			/* Unsigned 26-bit payload. */
+			value = value & ((1<<26)-1);
+			opcode = opcode & ~((1<<26)-1);
+			break;
+
+		default: /* assume everything else is a b, there are lots */
+			/* Signed 16-bit payload. */
+			value = value & ((1<<16)-1);
+			opcode = opcode & ~((1<<16)-1);
+			break;
+	}
+
+	write4(opcode | value, addr, 0);
+}
+
 /*
  * The bits in type indicate how many bytes the value occupies and what
  * significance should be attributed to each byte.
@@ -396,8 +452,11 @@ static putvalu(uint32_t valu, char* addr, uint16_t type)
 	case RELOVC4:
 		put_vc4_valu(addr, valu);
 		break;
+	case RELOMIPS:
+		put_mips_valu(addr, valu);
+		break;
 	default:
-		fatal("bad relocation type %x", type & RELSZ);
+		fatal("can't write relocation type %x", type & RELSZ);
 	}
 }
 
@@ -506,6 +565,14 @@ relocate(head, emit, names, relo, off)
 	if (relo->or_type & RELPC)
 		valu -=	relorig[sectindex].org_size+outsect[sectindex].os_base;
 
+	/*
+	 * If RELS2 is set, right shift the value by sixteen bits; this
+	 * allows 32-bit values to be fixed up as a high word and a low
+	 * word.
+	 */
+	if (relo->or_type & RELS2)
+		valu >>= 16;
+
 	/*
 	 * Now put the value back.
 	 */