ARM: Correctness fix to Math.pow optimization

The change in r4990 contained a bug in Math.pow when then exponent was a large negative smi. In that case calculating 1/Math.pow(x,-y) did not provide the correct result as Math.pow(x,-y) would overflow ti infinity. This was caught by Sputnik test S8.5_A13_T1.
Review URL: http://codereview.chromium.org/2815039

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4994 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/arm/codegen-arm.cc

@@ -4346,10 +4346,20 @@ void CodeGenerator::GenerateMathPow(ZoneList<Expression*>* args) {
     __ vmul(d1, d1, d1, ne);  // Don't bother calculating next d1 if done.
     __ b(ne, &more_bits);
 
-    // If exponent is negative result is 1/result (d2 already holds 1.0 in that
-    // case).
+    // If exponent is positive we are done.
     __ cmp(exponent, Operand(0));
-    __ vdiv(d0, d2, d0, mi);
+    __ b(ge, &allocate_return);
+
+    // If exponent is negative result is 1/result (d2 already holds 1.0 in that
+    // case). However if d0 has reached infinity this will not provide the
+    // correct result, so call runtime if that is the case.
+    __ mov(scratch2, Operand(0x7FF00000));
+    __ mov(scratch1, Operand(0));
+    __ vmov(d1, scratch1, scratch2);  // Load infinity into d1.
+    __ vcmp(d0, d1);
+    __ vmrs(pc);
+    runtime.Branch(eq);  // d0 reached infinity.
+    __ vdiv(d0, d2, d0);
     __ b(&allocate_return);
 
     __ bind(&exponent_nonsmi);

test/mjsunit/math-pow.js

@@ -122,3 +122,8 @@ assertEquals(NaN, Math.pow(-2, 1.1));
 assertEquals(NaN, Math.pow(-2, -1.1));
 assertEquals(NaN, Math.pow(-1000, 1.1));
 assertEquals(NaN, Math.pow(-1000, -1.1));
+
+// Tests from Sputnik S8.5_A13_T1.
+assertTrue((1*((Math.pow(2,53))-1)*(Math.pow(2,-1074))) === 4.4501477170144023e-308);
+assertTrue((1*(Math.pow(2,52))*(Math.pow(2,-1074))) === 2.2250738585072014e-308);
+assertTrue((-1*(Math.pow(2,52))*(Math.pow(2,-1074))) === -2.2250738585072014e-308);