From 5e432754b352f75c3ea0caa7287fbb3f4767950a Mon Sep 17 00:00:00 2001
From: "yangguo@chromium.org"
 <yangguo@chromium.org@ce2b1a6d-e550-0410-aec6-3dcde31c8c00>
Date: Tue, 6 Dec 2011 09:20:28 +0000
Subject: [PATCH] Skip check for +/-0.5 in optimized Math.pow (ia32).

Review URL: http://codereview.chromium.org/8820007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10168 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
---
 src/ia32/code-stubs-ia32.cc | 65 ++++++++++++++++++++-----------------
 1 file changed, 35 insertions(+), 30 deletions(-)

diff --git a/src/ia32/code-stubs-ia32.cc b/src/ia32/code-stubs-ia32.cc
index 0afc9cc6e0..c9d8cfd4d5 100644
--- a/src/ia32/code-stubs-ia32.cc
+++ b/src/ia32/code-stubs-ia32.cc
@@ -3025,38 +3025,43 @@ void MathPowStub::Generate(MacroAssembler* masm) {
     __ ucomisd(xmm2, xmm4);
     __ j(equal, &int_exponent);
 
-    // Detect square root case.
-    // Test for -0.5.
-    // Load xmm4 with -0.5.
-    __ mov(ecx, Immediate(0xBF000000u));
-    __ movd(xmm4, ecx);
-    __ cvtss2sd(xmm4, xmm4);
-    // xmm3 now has -0.5.
-    __ ucomisd(xmm4, xmm2);
-    __ j(not_equal, &not_minus_half, Label::kNear);
+    if (exponent_type_ == ON_STACK) {
+      // Detect square root case.  Crankshaft detects constant +/-0.5 at
+      // compile time and uses DoMathPowHalf instead.  We then skip this check
+      // for non-constant cases of +/-0.5 as these hardly occur.
 
-    // Calculates reciprocal of square root.eax
-    // sqrtsd returns -0 when input is -0.  ECMA spec requires +0.
-    __ xorps(xmm2, xmm2);
-    __ addsd(xmm2, xmm1);
-    __ sqrtsd(xmm2, xmm2);
-    __ divsd(xmm3, xmm2);
-    __ jmp(&done);
+      // Test for -0.5.
+      // Load xmm4 with -0.5.
+      __ mov(ecx, Immediate(0xBF000000u));
+      __ movd(xmm4, ecx);
+      __ cvtss2sd(xmm4, xmm4);
+      // xmm3 now has -0.5.
+      __ ucomisd(xmm4, xmm2);
+      __ j(not_equal, &not_minus_half, Label::kNear);
 
-    // Test for 0.5.
-    __ bind(&not_minus_half);
-    // Load xmm2 with 0.5.
-    // Since xmm3 is 1 and xmm4 is -0.5 this is simply xmm4 + xmm3.
-    __ addsd(xmm4, xmm3);
-    // xmm2 now has 0.5.
-    __ ucomisd(xmm4, xmm2);
-    __ j(not_equal, &fast_power, Label::kNear);
-    // Calculates square root.
-    // sqrtsd returns -0 when input is -0.  ECMA spec requires +0.
-    __ xorps(xmm4, xmm4);
-    __ addsd(xmm4, xmm1);
-    __ sqrtsd(xmm3, xmm4);
-    __ jmp(&done);
+      // Calculates reciprocal of square root.eax
+      // sqrtsd returns -0 when input is -0.  ECMA spec requires +0.
+      __ xorps(xmm2, xmm2);
+      __ addsd(xmm2, xmm1);
+      __ sqrtsd(xmm2, xmm2);
+      __ divsd(xmm3, xmm2);
+      __ jmp(&done);
+
+      // Test for 0.5.
+      __ bind(&not_minus_half);
+      // Load xmm2 with 0.5.
+      // Since xmm3 is 1 and xmm4 is -0.5 this is simply xmm4 + xmm3.
+      __ addsd(xmm4, xmm3);
+      // xmm2 now has 0.5.
+      __ ucomisd(xmm4, xmm2);
+      __ j(not_equal, &fast_power, Label::kNear);
+      // Calculates square root.
+      // sqrtsd returns -0 when input is -0.  ECMA spec requires +0.
+      __ xorps(xmm4, xmm4);
+      __ addsd(xmm4, xmm1);
+      __ sqrtsd(xmm3, xmm4);
+      __ jmp(&done);
+    }
 
     // Using FPU instructions to calculate power.
     Label fast_power_failed;