From 68e6650df737450bfc469ce5228e9babea4a8966 Mon Sep 17 00:00:00 2001
From: Scott Lahteine <github@thinkyhead.com>
Date: Wed, 4 Jul 2018 18:41:41 -0500
Subject: [PATCH] Remove Quake Fast Inverse SQRT (it isn't faster)

---
 Marlin/src/inc/Conditionals_post.h | 19 -------------------
 Marlin/src/module/delta.cpp        | 23 -----------------------
 Marlin/src/module/delta.h          | 13 +------------
 3 files changed, 1 insertion(+), 54 deletions(-)

diff --git a/Marlin/src/inc/Conditionals_post.h b/Marlin/src/inc/Conditionals_post.h
index 417fc59d96..b9972e2bcc 100644
--- a/Marlin/src/inc/Conditionals_post.h
+++ b/Marlin/src/inc/Conditionals_post.h
@@ -1353,25 +1353,6 @@
   #endif
 #endif
 
-// Use float instead of double. Needs profiling.
-#if defined(ARDUINO_ARCH_SAM) && ENABLED(DELTA_FAST_SQRT)
-  #undef ATAN2
-  #undef FABS
-  #undef POW
-  #undef SQRT
-  #undef CEIL
-  #undef FLOOR
-  #undef LROUND
-  #undef FMOD
-  #define ATAN2(y, x) atan2f(y, x)
-  #define POW(x, y) powf(x, y)
-  #define SQRT(x) sqrtf(x)
-  #define CEIL(x) ceilf(x)
-  #define FLOOR(x) floorf(x)
-  #define LROUND(x) lroundf(x)
-  #define FMOD(x, y) fmodf(x, y)
-#endif
-
 // Number of VFAT entries used. Each entry has 13 UTF-16 characters
 #if ENABLED(SCROLL_LONG_FILENAMES)
   #define MAX_VFAT_ENTRIES (5)
diff --git a/Marlin/src/module/delta.cpp b/Marlin/src/module/delta.cpp
index f6e2d65a2b..2fe924a427 100644
--- a/Marlin/src/module/delta.cpp
+++ b/Marlin/src/module/delta.cpp
@@ -90,31 +90,8 @@ void recalc_delta_settings() {
  *
  * - Disable the home_offset (M206) and/or position_shift (G92)
  *   features to remove up to 12 float additions.
- *
- * - Use a fast-inverse-sqrt function and add the reciprocal.
- *   (see above)
  */
 
-#if ENABLED(DELTA_FAST_SQRT) && defined(__AVR__)
-  /**
-   * Fast inverse sqrt from Quake III Arena
-   * See: https://en.wikipedia.org/wiki/Fast_inverse_square_root
-   */
-  float Q_rsqrt(float number) {
-    long i;
-    float x2, y;
-    const float threehalfs = 1.5f;
-    x2 = number * 0.5f;
-    y  = number;
-    i  = * ( long * ) &y;                       // evil floating point bit level hacking
-    i  = 0x5F3759DF - ( i >> 1 );               // what the f***?
-    y  = * ( float * ) &i;
-    y  = y * ( threehalfs - ( x2 * y * y ) );   // 1st iteration
-    // y  = y * ( threehalfs - ( x2 * y * y ) );   // 2nd iteration, this can be removed
-    return y;
-  }
-#endif
-
 #define DELTA_DEBUG(VAR) do { \
     SERIAL_ECHOPAIR("cartesian X:", VAR[X_AXIS]); \
     SERIAL_ECHOPAIR(" Y:", VAR[Y_AXIS]);          \
diff --git a/Marlin/src/module/delta.h b/Marlin/src/module/delta.h
index 2c367fa8d4..cf62b5e621 100644
--- a/Marlin/src/module/delta.h
+++ b/Marlin/src/module/delta.h
@@ -64,19 +64,8 @@ void recalc_delta_settings();
  *   (see above)
  */
 
-#if ENABLED(DELTA_FAST_SQRT) && defined(__AVR__)
-  /**
-   * Fast inverse sqrt from Quake III Arena
-   * See: https://en.wikipedia.org/wiki/Fast_inverse_square_root
-   */
-  float Q_rsqrt(float number);
-  #define _SQRT(n) (1.0f / Q_rsqrt(n))
-#else
-  #define _SQRT(n) SQRT(n)
-#endif
-
 // Macro to obtain the Z position of an individual tower
-#define DELTA_Z(V,T) V[Z_AXIS] + _SQRT(   \
+#define DELTA_Z(V,T) V[Z_AXIS] + SQRT(   \
   delta_diagonal_rod_2_tower[T] - HYPOT2( \
       delta_tower[T][X_AXIS] - V[X_AXIS], \
       delta_tower[T][Y_AXIS] - V[Y_AXIS]  \