util: Optimize _mesa_roundeven with SSE 4.1.

The SSE 4.1 ROUND instructions let us implement roundeven directly.
Otherwise we assume that the rounding mode has not been modified (as we
do in the rest of Mesa) and use rint().

glibc uses the ROUND instruction in rint() after a cpuid check. This
patch just lets us inline it directly when we're already building for
SSE 4.1.

Reviewed-by: Carl Worth <cworth@cworth.org>

src/util/rounding.h

@@ -23,6 +23,10 @@
 
 #include <math.h>
 
+#ifdef __SSE4_1__
+#include <smmintrin.h>
+#endif
+
 /* The C standard library has functions round()/rint()/nearbyint() that round
  * their arguments according to the rounding mode set in the floating-point
  * control register. While there are trunc()/ceil()/floor() functions that do
@@ -45,7 +49,15 @@
 static inline float
 _mesa_roundevenf(float x)
 {
+#ifdef __SSE4_1__
+   float ret;
+   __m128 m = _mm_load_ss(&x);
+   m = _mm_round_ss(m, m, _MM_FROUND_CUR_DIRECTION | _MM_FROUND_NO_EXC);
+   _mm_store_ss(&ret, m);
+   return ret;
+#else
    return rintf(x);
+#endif
 }
 
 /**
@@ -54,5 +66,13 @@ _mesa_roundevenf(float x)
 static inline double
 _mesa_roundeven(double x)
 {
+#ifdef __SSE4_1__
+   double ret;
+   __m128d m = _mm_load_sd(&x);
+   m = _mm_round_sd(m, m, _MM_FROUND_CUR_DIRECTION | _MM_FROUND_NO_EXC);
+   _mm_store_sd(&ret, m);
+   return ret;
+#else
    return rint(x);
+#endif
 }