MIPS[64]: Optimize the size of deoptimizer table on MIPS R6

On MIPS R6 we have BC instruction that has 26bit offset. We exploit this
in order to generate deoptimizer table with two instructions per entry
instead of earlier three instructions per entry.

Bug: 
Change-Id: If6638a9d4aa1515bfaa61530767d26ce5a78b0b0
Reviewed-on: https://chromium-review.googlesource.com/536972Reviewed-by: Miran Karić <Miran.Karic@imgtec.com>
Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Cr-Commit-Position: refs/heads/master@{#46466}

src/mips/deoptimizer-mips.cc

@@ -306,7 +306,11 @@ void Deoptimizer::TableEntryGenerator::Generate() {
 
 
 // Maximum size of a table entry generated below.
+#ifdef _MIPS_ARCH_MIPS32R6
+const int Deoptimizer::table_entry_size_ = 2 * Assembler::kInstrSize;
+#else
 const int Deoptimizer::table_entry_size_ = 3 * Assembler::kInstrSize;
+#endif
 
 void Deoptimizer::TableEntryGenerator::GeneratePrologue() {
   Assembler::BlockTrampolinePoolScope block_trampoline_pool(masm());
@@ -315,8 +319,14 @@ void Deoptimizer::TableEntryGenerator::GeneratePrologue() {
   // Note that registers are still live when jumping to an entry.
   Label table_start, done, trampoline_jump;
   __ bind(&table_start);
+
+#ifdef _MIPS_ARCH_MIPS32R6
+  int kMaxEntriesBranchReach =
+      (1 << (kImm26Bits - 2)) / (table_entry_size_ / Assembler::kInstrSize);
+#else
   int kMaxEntriesBranchReach = (1 << (kImm16Bits - 2))/
      (table_entry_size_ /  Assembler::kInstrSize);
+#endif
 
   if (count() <= kMaxEntriesBranchReach) {
     // Common case.
@@ -324,10 +334,14 @@ void Deoptimizer::TableEntryGenerator::GeneratePrologue() {
       Label start;
       __ bind(&start);
       DCHECK(is_int16(i));
-      __ BranchShort(USE_DELAY_SLOT, &done);  // Expose delay slot.
-      __ li(at, i);  // In the delay slot.
-      __ nop();
-
+      if (IsMipsArchVariant(kMips32r6)) {
+        __ li(at, i);
+        __ BranchShort(PROTECT, &done);
+      } else {
+        __ BranchShort(USE_DELAY_SLOT, &done);  // Expose delay slot.
+        __ li(at, i);                           // In the delay slot.
+        __ nop();
+      }
       DCHECK_EQ(table_entry_size_, masm()->SizeOfCodeGeneratedSince(&start));
     }
 
@@ -336,6 +350,7 @@ void Deoptimizer::TableEntryGenerator::GeneratePrologue() {
     __ bind(&done);
     __ Push(at);
   } else {
+    DCHECK(!IsMipsArchVariant(kMips32r6));
     // Uncommon case, the branch cannot reach.
     // Create mini trampoline to reach the end of the table
     for (int i = 0, j = 0; i < count(); i++, j++) {

src/mips64/deoptimizer-mips64.cc

@@ -306,7 +306,11 @@ void Deoptimizer::TableEntryGenerator::Generate() {
 
 
 // Maximum size of a table entry generated below.
+#ifdef _MIPS_ARCH_MIPS64R6
+const int Deoptimizer::table_entry_size_ = 2 * Assembler::kInstrSize;
+#else
 const int Deoptimizer::table_entry_size_ = 3 * Assembler::kInstrSize;
+#endif
 
 void Deoptimizer::TableEntryGenerator::GeneratePrologue() {
   Assembler::BlockTrampolinePoolScope block_trampoline_pool(masm());
@@ -315,8 +319,13 @@ void Deoptimizer::TableEntryGenerator::GeneratePrologue() {
   // Note that registers are still live when jumping to an entry.
   Label table_start, done, trampoline_jump;
   __ bind(&table_start);
+#ifdef _MIPS_ARCH_MIPS64R6
+  int kMaxEntriesBranchReach =
+      (1 << (kImm26Bits - 2)) / (table_entry_size_ / Assembler::kInstrSize);
+#else
   int kMaxEntriesBranchReach =
       (1 << (kImm16Bits - 2)) / (table_entry_size_ / Assembler::kInstrSize);
+#endif
 
   if (count() <= kMaxEntriesBranchReach) {
     // Common case.
@@ -324,9 +333,14 @@ void Deoptimizer::TableEntryGenerator::GeneratePrologue() {
       Label start;
       __ bind(&start);
       DCHECK(is_int16(i));
-      __ BranchShort(USE_DELAY_SLOT, &done);  // Expose delay slot.
-      __ li(at, i);                      // In the delay slot.
-      __ nop();
+      if (kArchVariant == kMips64r6) {
+        __ li(at, i);
+        __ BranchShort(PROTECT, &done);
+      } else {
+        __ BranchShort(USE_DELAY_SLOT, &done);  // Expose delay slot.
+        __ li(at, i);                           // In the delay slot.
+        __ nop();
+      }
 
       DCHECK_EQ(table_entry_size_, masm()->SizeOfCodeGeneratedSince(&start));
     }
@@ -336,6 +350,7 @@ void Deoptimizer::TableEntryGenerator::GeneratePrologue() {
     __ bind(&done);
     __ Push(at);
   } else {
+    DCHECK(kArchVariant != kMips64r6);
     // Uncommon case, the branch cannot reach.
     // Create mini trampoline to reach the end of the table
     for (int i = 0, j = 0; i < count(); i++, j++) {