MIPS: Improve compare and branch combining.

src/compiler/mips/code-generator-mips.cc

Index: src/compiler/mips/code-generator-mips.cc
diff --git a/src/compiler/mips/code-generator-mips.cc b/src/compiler/mips/code-generator-mips.cc
index 2a079404730de74a0a71050174d2564002894a8e..03e0057dae09debbb5623fe3611b012daac0f465 100644
--- a/src/compiler/mips/code-generator-mips.cc
+++ b/src/compiler/mips/code-generator-mips.cc
@@ -241,8 +241,6 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
       break;
     case kMipsTst:
       // Psuedo-instruction used for tst/branch.

[paul.l..., 2014/11/04 19:04:27]

Suggest adding "No opcode emitted here." to comment, as below.

Please either add back the 'break' or put in a comment: // Fall-through.

I don't think either approach will significantly change the generated code, so
it probably best to use explicit break.

[dusmil.imgtec, 2014/11/04 19:46:11]

Done.

-      __ And(kCompareReg, i.InputRegister(0), i.InputOperand(1));
-      break;
     case kMipsCmp:
       // Psuedo-instruction used for cmp/branch. No opcode emitted here.
       break;
@@ -418,7 +416,6 @@ void CodeGenerator::AssembleArchBranch(Instruction* instr,
   //    not separated by other instructions.
 
   if (instr->arch_opcode() == kMipsTst) {
-    // The kMipsTst psuedo-instruction emits And to 'kCompareReg' register.
     switch (condition) {
       case kNotEqual:
         cc = ne;
@@ -430,7 +427,8 @@ void CodeGenerator::AssembleArchBranch(Instruction* instr,
         UNSUPPORTED_COND(kMipsTst, condition);
         break;
     }
-    __ Branch(tlabel, cc, kCompareReg, Operand(zero_reg));
+    __ And(at, i.InputRegister(0), i.InputOperand(1));
+    __ Branch(tlabel, cc, at, Operand(zero_reg));
 
   } else if (instr->arch_opcode() == kMipsAddOvf ||
              instr->arch_opcode() == kMipsSubOvf) {
@@ -557,7 +555,6 @@ void CodeGenerator::AssembleArchBoolean(Instruction* instr,
   // TODO(plind): Add CHECK() to ensure that test/cmp and this branch were
   //    not separated by other instructions.
   if (instr->arch_opcode() == kMipsTst) {
-    // The kMipsTst psuedo-instruction emits And to 'kCompareReg' register.
     switch (condition) {
       case kNotEqual:
         cc = ne;
@@ -569,7 +566,8 @@ void CodeGenerator::AssembleArchBoolean(Instruction* instr,
         UNSUPPORTED_COND(kMipsTst, condition);
         break;
     }
-    __ Branch(USE_DELAY_SLOT, &done, cc, kCompareReg, Operand(zero_reg));
+    __ And(at, i.InputRegister(0), i.InputOperand(1));
+    __ Branch(USE_DELAY_SLOT, &done, cc, at, Operand(zero_reg));
     __ li(result, Operand(1));  // In delay slot.
 
   } else if (instr->arch_opcode() == kMipsAddOvf ||