GMISC invsqrt() function.

Fix a 64bit processor bug.
Add support for processors where the floating point and integer endianness don't match.
Add support for processors with a non-standard floating point format.
Update gdisp_streaming demo with extra comments to reflect the above changes.

1 files changed, 41 insertions, 15 deletions

diff --git a/src/gmisc/trig.c b/src/gmisc/trig.c
index 00b6365a..cd35bdc0 100644
--- a/src/gmisc/trig.c
+++ b/src/gmisc/trig.c
@@ -143,21 +143,47 @@
 #endif
 
 #if GMISC_NEED_INVSQRT
-	// Algorithm based on Quake code
-	float invsqrt(float n) {
-		long i;
-		float x2, y;
-		const float threehalfs = 1.5F;
-
-		x2 = n * 0.5F;
-		y  = n;
-		i  = * ( long * ) &y;						// evil floating point bit level hacking
-		i  = 0x5f3759df - ( i >> 1 );				// what the?
-		y  = * ( float * ) &i;
-		y  = y * ( threehalfs - ( x2 * y * y ) );	// 1st iteration
-		//y  = y * ( threehalfs - ( x2 * y * y ) );	// 2nd iteration for extra precision, this can be removed
-		return y;
-	}
+	// Algorithm based on Quake code.
+	#if GDISP_INVSQRT_REAL_SLOW
+		#include <math.h>
+		float invsqrt(float n) {
+			return 1.0/sqrt(n);
+		}
+	#else
+		float invsqrt(float n) {
+			int32_t		i;
+			float		x2;
+
+			x2 = n * 0.5F;
+
+			// Convert into an int32 (no binary format conversion)
+			#if GDISP_INVSQRT_MIXED_ENDIAN
+				((char *)&i)[0] = ((char *)&n)[3];
+				((char *)&i)[1] = ((char *)&n)[2];
+				((char *)&i)[2] = ((char *)&n)[1];
+				((char *)&i)[3] = ((char *)&n)[0];
+			#else
+				i  = *(int32_t *)&n;
+			#endif
+
+			// evil floating point bit level hacking
+			i  = 0x5F3759DF - (i >> 1);
+
+			// Convert back to a float (no binary format conversion)
+			#if GDISP_INVSQRT_MIXED_ENDIAN
+				((char *)&n)[0] = ((char *)&i)[3];
+				((char *)&n)[1] = ((char *)&i)[2];
+				((char *)&n)[2] = ((char *)&i)[1];
+				((char *)&n)[3] = ((char *)&i)[0];
+			#else
+				n  = *(float *)&i;
+			#endif
+
+			n  = n * (1.5F - (x2 * n * n));				// 1st iteration
+			//n  = n * (1.5F - (x2 * n * n));			// 2nd iteration for extra precision, this can be removed
+			return n;
+		}
+	#endif
 #endif
 
 #endif /* GFX_USE_GMISC */