Fix SLTIU instruction in MIPS assembler, disassembler, and simulator.

runtime/vm/assembler_mips.h

Index: runtime/vm/assembler_mips.h
===================================================================
--- runtime/vm/assembler_mips.h	(revision 42354)
+++ runtime/vm/assembler_mips.h	(working copy)
@@ -561,7 +561,7 @@
 
   void lui(Register rt, const Immediate& imm) {
     ASSERT(Utils::IsUint(kImmBits, imm.value()));
-    uint16_t imm_value = static_cast<uint16_t>(imm.value());
+    const uint16_t imm_value = static_cast<uint16_t>(imm.value());
     EmitIType(LUI, R0, rt, imm_value);
   }
 
@@ -673,7 +673,7 @@
 
   void ori(Register rt, Register rs, const Immediate& imm) {
     ASSERT(Utils::IsUint(kImmBits, imm.value()));
-    uint16_t imm_value = static_cast<uint16_t>(imm.value());
+    const uint16_t imm_value = static_cast<uint16_t>(imm.value());
     EmitIType(ORI, rs, rt, imm_value);
   }
 
@@ -704,13 +704,16 @@
 
   void slti(Register rt, Register rs, const Immediate& imm) {
     ASSERT(Utils::IsInt(kImmBits, imm.value()));
-    int16_t imm_value = static_cast<int16_t>(imm.value());
+    const uint16_t imm_value = static_cast<uint16_t>(imm.value());
     EmitIType(SLTI, rs, rt, imm_value);
   }
 
+  // Although imm argument is int32_t, it is interpreted as an uint32_t.
+  // For example, -1 stands for 0xffffffffUL: it is encoded as 0xffff in the
+  // instruction imm field and is then sign extended back to 0xffffffffUL.
   void sltiu(Register rt, Register rs, const Immediate& imm) {

[zra, 2014/12/13 03:12:59]

This is a weird instruction.

-    ASSERT(Utils::IsUint(kImmBits, imm.value()));
-    uint16_t imm_value = static_cast<uint16_t>(imm.value());
+    ASSERT(Utils::IsInt(kImmBits, imm.value()));
+    const uint16_t imm_value = static_cast<uint16_t>(imm.value());
     EmitIType(SLTIU, rs, rt, imm_value);
   }
 
@@ -1082,7 +1085,7 @@
   void BranchUnsignedLess(Register rd, const Immediate& imm, Label* l) {
     ASSERT(!in_delay_slot_);
     ASSERT(imm.value() != 0);
-    if (Utils::IsUint(kImmBits, imm.value())) {
+    if (Utils::IsInt(kImmBits, imm.value())) {
       sltiu(CMPRES2, rd, imm);
       bne(CMPRES2, ZR, l);
     } else {