[wasm][asm.js] Use x&(x-1) trick to speed up life.

Using x&(x-1) to check for power of two masks usable at runtime
speeds up the life benchmark.

Borrowing this from SimplifiedLowering for the AsmJsRemS internal
wasm opcode.

Leaving this out for general wasm as we should be doing this optimization
in LLVM.

BUG=v8:4203
TEST=None
R=bmeurer@chromium.org

Review-Url: https://codereview.chromium.org/2556963005
Cr-Commit-Position: refs/heads/master@{#41581}

src/compiler/wasm-compiler.cc

@@ -1920,36 +1920,101 @@ Node* WasmGraphBuilder::BuildI32AsmjsDivS(Node* left, Node* right) {
 }
 
 Node* WasmGraphBuilder::BuildI32AsmjsRemS(Node* left, Node* right) {
+  CommonOperatorBuilder* c = jsgraph()->common();
   MachineOperatorBuilder* m = jsgraph()->machine();
+  Node* const zero = jsgraph()->Int32Constant(0);
 
   Int32Matcher mr(right);
   if (mr.HasValue()) {
-    if (mr.Value() == 0) {
-      return jsgraph()->Int32Constant(0);
-    } else if (mr.Value() == -1) {
-      return jsgraph()->Int32Constant(0);
+    if (mr.Value() == 0 || mr.Value() == -1) {
+      return zero;
     }
     return graph()->NewNode(m->Int32Mod(), left, right, *control_);
   }
 
-  // asm.js semantics return 0 on divide or mod by zero.
-  // Explicit check for x % 0.
-  Diamond z(
-      graph(), jsgraph()->common(),
-      graph()->NewNode(m->Word32Equal(), right, jsgraph()->Int32Constant(0)),
-      BranchHint::kFalse);
+  // General case for signed integer modulus, with optimization for (unknown)
+  // power of 2 right hand side.
+  //
+  //   if 0 < right then
+  //     msk = right - 1
+  //     if right & msk != 0 then
+  //       left % right
+  //     else
+  //       if left < 0 then
+  //         -(-left & msk)
+  //       else
+  //         left & msk
+  //   else
+  //     if right < -1 then
+  //       left % right
+  //     else
+  //       zero
+  //
+  // Note: We do not use the Diamond helper class here, because it really hurts
+  // readability with nested diamonds.
+  Node* const minus_one = jsgraph()->Int32Constant(-1);
 
-  // Explicit check for x % -1.
-  Diamond d(
-      graph(), jsgraph()->common(),
-      graph()->NewNode(m->Word32Equal(), right, jsgraph()->Int32Constant(-1)),
-      BranchHint::kFalse);
-  d.Chain(z.if_false);
+  const Operator* const merge_op = c->Merge(2);
+  const Operator* const phi_op = c->Phi(MachineRepresentation::kWord32, 2);
+
+  Node* check0 = graph()->NewNode(m->Int32LessThan(), zero, right);
+  Node* branch0 =
+      graph()->NewNode(c->Branch(BranchHint::kTrue), check0, graph()->start());
+
+  Node* if_true0 = graph()->NewNode(c->IfTrue(), branch0);
+  Node* true0;
+  {
+    Node* msk = graph()->NewNode(m->Int32Add(), right, minus_one);
+
+    Node* check1 = graph()->NewNode(m->Word32And(), right, msk);
+    Node* branch1 = graph()->NewNode(c->Branch(), check1, if_true0);
+
+    Node* if_true1 = graph()->NewNode(c->IfTrue(), branch1);
+    Node* true1 = graph()->NewNode(m->Int32Mod(), left, right, if_true1);
+
+    Node* if_false1 = graph()->NewNode(c->IfFalse(), branch1);
+    Node* false1;
+    {
+      Node* check2 = graph()->NewNode(m->Int32LessThan(), left, zero);
+      Node* branch2 =
+          graph()->NewNode(c->Branch(BranchHint::kFalse), check2, if_false1);
+
+      Node* if_true2 = graph()->NewNode(c->IfTrue(), branch2);
+      Node* true2 = graph()->NewNode(
+          m->Int32Sub(), zero,
+          graph()->NewNode(m->Word32And(),
+                           graph()->NewNode(m->Int32Sub(), zero, left), msk));
+
+      Node* if_false2 = graph()->NewNode(c->IfFalse(), branch2);
+      Node* false2 = graph()->NewNode(m->Word32And(), left, msk);
+
+      if_false1 = graph()->NewNode(merge_op, if_true2, if_false2);
+      false1 = graph()->NewNode(phi_op, true2, false2, if_false1);
+    }
+
+    if_true0 = graph()->NewNode(merge_op, if_true1, if_false1);
+    true0 = graph()->NewNode(phi_op, true1, false1, if_true0);
+  }
+
+  Node* if_false0 = graph()->NewNode(c->IfFalse(), branch0);
+  Node* false0;
+  {
+    Node* check1 = graph()->NewNode(m->Int32LessThan(), right, minus_one);
+    Node* branch1 =
+        graph()->NewNode(c->Branch(BranchHint::kTrue), check1, if_false0);
+
+    Node* if_true1 = graph()->NewNode(c->IfTrue(), branch1);
+    Node* true1 = graph()->NewNode(m->Int32Mod(), left, right, if_true1);
+
+    Node* if_false1 = graph()->NewNode(c->IfFalse(), branch1);
+    Node* false1 = zero;
+
+    if_false0 = graph()->NewNode(merge_op, if_true1, if_false1);
+    false0 = graph()->NewNode(phi_op, true1, false1, if_false0);
+  }
 
-  return z.Phi(
-      MachineRepresentation::kWord32, jsgraph()->Int32Constant(0),
-      d.Phi(MachineRepresentation::kWord32, jsgraph()->Int32Constant(0),
-            graph()->NewNode(m->Int32Mod(), left, right, d.if_false)));
+  Node* merge0 = graph()->NewNode(merge_op, if_true0, if_false0);
+  return graph()->NewNode(phi_op, true0, false0, merge0);
 }
 
 Node* WasmGraphBuilder::BuildI32AsmjsDivU(Node* left, Node* right) {