Fix optimized Math.pow on x64 with NaN exponents.

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
===================================================================
--- runtime/vm/intermediate_language_x64.cc	(revision 27949)
+++ runtime/vm/intermediate_language_x64.cc	(working copy)
@@ -4228,19 +4228,18 @@
     Register temp = locs()->temp(0).reg();
     XmmRegister zero_temp = locs()->temp(1).fpu_reg();
 
-    Label check_base_is_one;
     // Check if exponent is 0.0 -> return 1.0;
     __ LoadObject(temp, Double::ZoneHandle(Double::NewCanonical(0)), PP);
     __ movsd(zero_temp, FieldAddress(temp, Double::value_offset()));
     __ LoadObject(temp, Double::ZoneHandle(Double::NewCanonical(1)), PP);
     __ movsd(result, FieldAddress(temp, Double::value_offset()));
     // 'result' contains 1.0.
+    Label exp_is_nan;
     __ comisd(exp, zero_temp);
-    __ j(PARITY_EVEN, &check_base_is_one, Assembler::kNearJump);  // NaN.
+    __ j(PARITY_EVEN, &exp_is_nan, Assembler::kNearJump);  // NaN.
     __ j(EQUAL, &skip_call, Assembler::kNearJump);  // exp is 0, result is 1.0.
 
     Label base_is_nan;
-    __ Bind(&check_base_is_one);
     // Checks if base == 1.0.
     __ comisd(base, result);
     __ j(PARITY_EVEN, &base_is_nan, Assembler::kNearJump);
@@ -4251,7 +4250,14 @@
     // Returns NaN.
     __ movsd(result, base);
     __ jmp(&skip_call, Assembler::kNearJump);
-    // exp is Nan case is handled correctly in the C-library.
+
+    __ Bind(&exp_is_nan);
+    // Checks if base == 1.0.
+    __ comisd(base, result);
+    __ j(PARITY_EVEN, &base_is_nan, Assembler::kNearJump);
+    __ j(EQUAL, &skip_call, Assembler::kNearJump);  // base and result are 1.0
+    __ movsd(result, exp);  // result is NaN
+    __ jmp(&skip_call, Assembler::kNearJump);
   }
   __ Bind(&do_call);
   __ CallRuntime(TargetFunction(), InputCount());