Cycles: Fix failing avxf cross test on AVX2

cycles_util_avxf_avx2_test failed on the cross test, since
it wasn't immediately clear why, the test was disabled.

After looking into it, this test when build for AVX2 is
generating FMA instructions where the intermediate results
have "infinite" precision [1] leading to slightly different
results.

This diff re-enables the cross test and allows for a small error
in the results.

[1] https://www.felixcloutier.com/x86/vfmadd132ps:vfmadd213ps:vfmadd231ps

Differential Revision: https://developer.blender.org/D6873

Reviewers: brecht

intern/cycles/test/util_avxf_test.h

@@ -44,6 +44,10 @@ bool validate_cpu_capabilities()
   for (size_t index = 0; index < a.size; index++) \
     EXPECT_FLOAT_EQ(a[index], b[index]);
 
+#define compare_vector_vector_near(a, b, abserror) \
+  for (size_t index = 0; index < a.size; index++) \
+    EXPECT_NEAR(a[index], b[index], abserror);
+
 #define basic_test_vv(a, b, op) \
   VALIDATECPU \
   avxf c = a op b; \
@@ -190,23 +194,21 @@ TEST(util_avx, avxf_shuffle)
   compare_vector_vector(res, avxf(0.4f, 0.2f, 0.1f, 0.3f, 0.5f, 0.6f, 0.7f, 0.8f));
 }
 
-/* XXX Test Fails on AVX2, needs further investigation before it can be enabled */
-#if 0
 TEST(util_avx, avxf_cross)
 {
-	VALIDATECPU
-	avxf res = cross(avxf_b, avxf_c);
-	compare_vector_vector(res,
-	                      avxf(0.0f,
-	                           -9.5367432e-07f,
-	                           0.0f,
-	                           4.7683716e-07f,
-	                           0.0f,
-	                           -3.8146973e-06f,
-	                           3.8146973e-06f,
-	                           3.8146973e-06f));
+  VALIDATECPU
+  avxf res = cross(avxf_b, avxf_c);
+  compare_vector_vector_near(res,
+                             avxf(0.0f,
+                                  -9.5367432e-07f,
+                                  0.0f,
+                                  4.7683716e-07f,
+                                  0.0f,
+                                  -3.8146973e-06f,
+                                  3.8146973e-06f,
+                                  3.8146973e-06f),
+                             0.000002000f);
 }
-#endif
 
 TEST(util_avx, avxf_dot3)
 {