[crankshaft][arm][mips][mips64] Fix Math.min(0, 0)

for the special case where the same register is used as both left and
right input.

Review URL: https://codereview.chromium.org/1695283002

Cr-Commit-Position: refs/heads/master@{#33996}

src/crankshaft/arm/lithium-codegen-arm.cc

@@ -1932,8 +1932,14 @@ void LCodeGen::DoMathMinMax(LMathMinMax* instr) {
     // At this point, both left and right are either 0 or -0.
     if (operation == HMathMinMax::kMathMin) {
       // We could use a single 'vorr' instruction here if we had NEON support.
+      // The algorithm is: -((-L) + (-R)), which in case of L and R being
+      // different registers is most efficiently expressed as -((-L) - R).
       __ vneg(left_reg, left_reg);
-      __ vsub(result_reg, left_reg, right_reg);
+      if (left_reg.is(right_reg)) {
+        __ vadd(result_reg, left_reg, right_reg);
+      } else {
+        __ vsub(result_reg, left_reg, right_reg);
+      }
       __ vneg(result_reg, result_reg);
     } else {
       // Since we operate on +0 and/or -0, vadd and vand have the same effect;

src/crankshaft/mips/lithium-codegen-mips.cc

@@ -1789,8 +1789,14 @@ void LCodeGen::DoMathMinMax(LMathMinMax* instr) {
     __ BranchF(&return_left, NULL, ne, left_reg, kDoubleRegZero);
     // At this point, both left and right are either 0 or -0.
     if (operation == HMathMinMax::kMathMin) {
+      // The algorithm is: -((-L) + (-R)), which in case of L and R being
+      // different registers is most efficiently expressed as -((-L) - R).
       __ neg_d(left_reg, left_reg);
-      __ sub_d(result_reg, left_reg, right_reg);
+      if (left_reg.is(right_reg)) {
+        __ add_d(result_reg, left_reg, right_reg);
+      } else {
+        __ sub_d(result_reg, left_reg, right_reg);
+      }
       __ neg_d(result_reg, result_reg);
     } else {
       __ add_d(result_reg, left_reg, right_reg);

src/crankshaft/mips64/lithium-codegen-mips64.cc

@@ -1906,8 +1906,14 @@ void LCodeGen::DoMathMinMax(LMathMinMax* instr) {
     __ BranchF(&return_left, NULL, ne, left_reg, kDoubleRegZero);
     // At this point, both left and right are either 0 or -0.
     if (operation == HMathMinMax::kMathMin) {
+      // The algorithm is: -((-L) + (-R)), which in case of L and R being
+      // different registers is most efficiently expressed as -((-L) - R).
       __ neg_d(left_reg, left_reg);
-      __ sub_d(result_reg, left_reg, right_reg);
+      if (left_reg.is(right_reg)) {
+        __ add_d(result_reg, left_reg, right_reg);
+      } else {
+        __ sub_d(result_reg, left_reg, right_reg);
+      }
       __ neg_d(result_reg, result_reg);
     } else {
       __ add_d(result_reg, left_reg, right_reg);

test/mjsunit/regress/math-min.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+var a = new Float64Array(4);
+a[2] *= -1;
+a[3] *= -1;
+assertEquals(0, a[0]);
+assertEquals(0, a[1]);
+assertEquals(-0, a[2]);
+assertEquals(-0, a[3]);
+
+function f1() {
+  var z = a[0];
+  // Same register.
+  assertEquals(0, Math.min(z, z));
+}
+
+function f2() {
+  // Different registers.
+  assertEquals(0, Math.min(a[0], a[1]));
+}
+
+function f3() {
+  // Zero and minus zero.
+  assertEquals(-0, Math.min(a[1], a[2]));
+}
+
+function f4() {
+  // Zero and minus zero, reversed order.
+  assertEquals(-0, Math.min(a[2], a[1]));
+}
+
+function f5() {
+  // Minus zero, same register.
+  var m_z = a[2];
+  assertEquals(-0, Math.min(m_z, m_z));
+}
+
+function f6() {
+  // Minus zero, different registers.
+  assertEquals(-0, Math.min(a[2], a[3]));
+}
+
+for (var i = 0; i < 3; i++) {
+  f1();
+  f2();
+  f3();
+  f4();
+  f5();
+  f6();
+}
+%OptimizeFunctionOnNextCall(f1);
+%OptimizeFunctionOnNextCall(f2);
+%OptimizeFunctionOnNextCall(f3);
+%OptimizeFunctionOnNextCall(f4);
+%OptimizeFunctionOnNextCall(f5);
+%OptimizeFunctionOnNextCall(f6);
+f1();
+f2();
+f3();
+f4();
+f5();
+f6();