[arm64] Avoid padding poke when unnecessary

This also fixes a bug in 'InitializeCallBuffer', where it wouldn't claim enough
slots for each parameter. This caused the Simd128 instruction selector test to
only claim 3 slots (rather than 4) and then perform an unnecessary padding poke.

v8_Default_embedded_blob_size from the generated file gen/embedded.S
  Before: 4957056
   After: 4954368

This gives a 0.05% size decrease.

Change-Id: Ic9bb998fb8a9111fb90e1c3e537ea0f2a5fa7b33
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1617665
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#61649}

src/compiler/backend/arm64/instruction-selector-arm64.cc

@@ -1701,6 +1701,7 @@ void InstructionSelector::EmitPrepareArguments(
   // `arguments` includes alignment "holes". This means that slots bigger than
   // kSystemPointerSize, e.g. Simd128, will span across multiple arguments.
   int claim_count = static_cast<int>(arguments->size());
+  bool needs_padding = claim_count % 2 != 0;
   int slot = claim_count - 1;
   claim_count = RoundUp(claim_count, 2);
   // Bump the stack pointer.
@@ -1710,9 +1711,10 @@ void InstructionSelector::EmitPrepareArguments(
     //               and emit paired stores with increment for non c frames.
     Emit(kArm64Claim, g.NoOutput(), g.TempImmediate(claim_count));
 
-    // Store padding, which might be overwritten.
-    Emit(kArm64Poke, g.NoOutput(), g.UseImmediate(0),
-         g.TempImmediate(claim_count - 1));
+    if (needs_padding) {
+      Emit(kArm64Poke, g.NoOutput(), g.UseImmediate(0),
+           g.TempImmediate(claim_count - 1));
+    }
   }
 
   // Poke the arguments into the stack.

src/compiler/backend/instruction-selector.cc

@@ -1006,8 +1006,13 @@ void InstructionSelector::InitializeCallBuffer(Node* call, CallBuffer* buffer,
     UnallocatedOperand unallocated = UnallocatedOperand::cast(op);
     if (unallocated.HasFixedSlotPolicy() && !call_tail) {
       int stack_index = -unallocated.fixed_slot_index() - 1;
+      // This can insert empty slots before stack_index and will insert enough
+      // slots after stack_index to store the parameter.
       if (static_cast<size_t>(stack_index) >= buffer->pushed_nodes.size()) {
-        buffer->pushed_nodes.resize(stack_index + 1);
+        int num_slots = std::max(
+            1, (ElementSizeInBytes(location.GetType().representation()) /
+                kSystemPointerSize));
+        buffer->pushed_nodes.resize(stack_index + num_slots);
       }
       PushParameter param = {*iter, location};
       buffer->pushed_nodes[stack_index] = param;

test/unittests/compiler/arm64/instruction-selector-arm64-unittest.cc

@@ -4648,9 +4648,7 @@ void TestPokePair(InstructionSelectorTest::StreamBuilder& m, Zone* zone,
   }
 
   EXPECT_EQ(expected_poke_pair, num_poke_pair);
-  // Note: The `+ 1` here comes from the padding Poke in
-  // EmitPrepareArguments.
-  EXPECT_EQ(expected_poke + 1, num_poke);
+  EXPECT_EQ(expected_poke, num_poke);
 }
 }  // namespace
 
@@ -4670,7 +4668,9 @@ TEST_F(InstructionSelectorTest, PokePairPrepareArgumentsInt32) {
     };
 
     const int expected_poke_pair = 1;
-    const int expected_poke = 1;
+    // Note: The `+ 1` here comes from the padding Poke in
+    // EmitPrepareArguments.
+    const int expected_poke = 1 + 1;
 
     TestPokePair(m, zone(), builder, nodes, arraysize(nodes),
                  expected_poke_pair, expected_poke);
@@ -4795,7 +4795,10 @@ TEST_F(InstructionSelectorTest, PokePairPrepareArgumentsIntFloatMixed) {
                      m.Float64Constant(0.0f), m.Float64Constant(0.0f)};
 
     const int expected_poke_pair = 2;
-    const int expected_poke = 3;
+
+    // Note: The `+ 1` here comes from the padding Poke in
+    // EmitPrepareArguments.
+    const int expected_poke = 3 + 1;
 
     TestPokePair(m, zone(), builder, nodes, arraysize(nodes),
                  expected_poke_pair, expected_poke);