[asmjs] Do constant folding for I32Asmjs(Div|Rem)S to avoid checks of constant divisors

This change makes the embenchen/copy benchmark a factor of 2 faster and
brings back the performance loss through graph trimming.

R=titzer@chromium.org
CC=bradnelson@chromium.org

Review-Url: https://codereview.chromium.org/2453343002
Cr-Commit-Position: refs/heads/master@{#40628}

src/compiler/wasm-compiler.cc

@@ -1867,6 +1867,18 @@ Node* WasmGraphBuilder::BuildI32RemU(Node* left, Node* right,
 
 Node* WasmGraphBuilder::BuildI32AsmjsDivS(Node* left, Node* right) {
   MachineOperatorBuilder* m = jsgraph()->machine();
+
+  Int32Matcher mr(right);
+  if (mr.HasValue()) {
+    if (mr.Value() == 0) {
+      return jsgraph()->Int32Constant(0);
+    } else if (mr.Value() == -1) {
+      // The result is the negation of the left input.
+      return graph()->NewNode(m->Int32Sub(), jsgraph()->Int32Constant(0), left);
+    }
+    return graph()->NewNode(m->Int32Div(), left, right, *control_);
+  }
+
   // asm.js semantics return 0 on divide or mod by zero.
   if (m->Int32DivIsSafe()) {
     // The hardware instruction does the right thing (e.g. arm).
@@ -1896,6 +1908,17 @@ Node* WasmGraphBuilder::BuildI32AsmjsDivS(Node* left, Node* right) {
 
 Node* WasmGraphBuilder::BuildI32AsmjsRemS(Node* left, Node* right) {
   MachineOperatorBuilder* m = jsgraph()->machine();
+
+  Int32Matcher mr(right);
+  if (mr.HasValue()) {
+    if (mr.Value() == 0) {
+      return jsgraph()->Int32Constant(0);
+    } else if (mr.Value() == -1) {
+      return jsgraph()->Int32Constant(0);
+    }
+    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(

src/wasm/wasm-macro-gen.h

@@ -494,6 +494,14 @@ class LocalDeclEncoder {
 #define WASM_I32_POPCNT(x) x, kExprI32Popcnt
 #define WASM_I32_EQZ(x) x, kExprI32Eqz
 
+//------------------------------------------------------------------------------
+// Asmjs Int32 operations
+//------------------------------------------------------------------------------
+#define WASM_I32_ASMJS_DIVS(x, y) x, y, kExprI32AsmjsDivS
+#define WASM_I32_ASMJS_REMS(x, y) x, y, kExprI32AsmjsRemS
+#define WASM_I32_ASMJS_DIVU(x, y) x, y, kExprI32AsmjsDivU
+#define WASM_I32_ASMJS_REMU(x, y) x, y, kExprI32AsmjsRemU
+
 //------------------------------------------------------------------------------
 // Int64 operations
 //------------------------------------------------------------------------------

test/cctest/wasm/test-run-wasm.cc

@@ -448,6 +448,42 @@ WASM_EXEC_TEST(Int32DivS_byzero_const) {
   }
 }
 
+WASM_EXEC_TEST(Int32AsmjsDivS_byzero_const) {
+  for (int8_t denom = -2; denom < 8; ++denom) {
+    TestingModule module(execution_mode);
+    module.ChangeOriginToAsmjs();
+    WasmRunner<int32_t> r(&module, MachineType::Int32());
+    BUILD(r, WASM_I32_ASMJS_DIVS(WASM_GET_LOCAL(0), WASM_I8(denom)));
+    FOR_INT32_INPUTS(i) {
+      if (denom == 0) {
+        CHECK_EQ(0, r.Call(*i));
+      } else if (denom == -1 && *i == std::numeric_limits<int32_t>::min()) {
+        CHECK_EQ(std::numeric_limits<int32_t>::min(), r.Call(*i));
+      } else {
+        CHECK_EQ(*i / denom, r.Call(*i));
+      }
+    }
+  }
+}
+
+WASM_EXEC_TEST(Int32AsmjsRemS_byzero_const) {
+  for (int8_t denom = -2; denom < 8; ++denom) {
+    TestingModule module(execution_mode);
+    module.ChangeOriginToAsmjs();
+    WasmRunner<int32_t> r(&module, MachineType::Int32());
+    BUILD(r, WASM_I32_ASMJS_REMS(WASM_GET_LOCAL(0), WASM_I8(denom)));
+    FOR_INT32_INPUTS(i) {
+      if (denom == 0) {
+        CHECK_EQ(0, r.Call(*i));
+      } else if (denom == -1 && *i == std::numeric_limits<int32_t>::min()) {
+        CHECK_EQ(0, r.Call(*i));
+      } else {
+        CHECK_EQ(*i % denom, r.Call(*i));
+      }
+    }
+  }
+}
+
 WASM_EXEC_TEST(Int32DivU_byzero_const) {
   for (uint32_t denom = 0xfffffffe; denom < 8; ++denom) {
     WasmRunner<uint32_t> r(execution_mode, MachineType::Uint32());

test/cctest/wasm/wasm-run-utils.h

@@ -95,6 +95,8 @@ class TestingModule : public ModuleEnv {
     if (interpreter_) delete interpreter_;
   }
 
+  void ChangeOriginToAsmjs() { origin = kAsmJsOrigin; }
+
   byte* AddMemory(uint32_t size) {
     CHECK_NULL(instance->mem_start);
     CHECK_EQ(0, instance->mem_size);