From: Joshua Simmons <josh@nega.tv>
Date: Sun, 5 May 2024 14:16:18 +0000 (+0200)
Subject: narcissus-maths: Add `exp_f32` implementation
X-Git-Url: https://git.nega.tv//gitweb.cgi?a=commitdiff_plain;h=b60fb0c77e392bacb8f40bde23a3a76aaae26285;p=josh%2Fnarcissus

narcissus-maths: Add `exp_f32` implementation

Based on Norbert Juffa's implemention for CUDA from
https://forums.developer.nvidia.com/t/a-more-accurate-performance-competitive-implementation-of-expf/47528
---

diff --git a/engine/narcissus-maths/src/exp.rs b/engine/narcissus-maths/src/exp.rs
new file mode 100644
index 0000000..226891f
--- /dev/null
+++ b/engine/narcissus-maths/src/exp.rs
@@ -0,0 +1,53 @@
+// Norbert Juffa's exp for CUDA, incompletely translated.
+// https://forums.developer.nvidia.com/t/a-more-accurate-performance-competitive-implementation-of-expf/47528
+
+use crate::f32_to_i32;
+
+pub fn exp_f32(a: f32) -> f32 {
+    // exp(a) = 2**i * exp(f); i = rintf (a / log(2))
+    let j = a.mul_add(std::f32::consts::LOG2_E, 12582912.0) - 12582912.0; // 0x1.715476p0, 0x1.8p23
+    let f = j.mul_add(-6.931_457_5e-1, a); // -0x1.62e400p-1  // log_2_hi
+    let f = j.mul_add(-1.428_606_8e-6, f); // -0x1.7f7d1cp-20 // log_2_lo
+
+    let i = f32_to_i32(j);
+
+    // approximate r = exp(f) on interval [-log(2)/2, +log(2)/2]
+    let r = 1.378_059_4e-3_f32; // 0x1.694000p-10
+    let r = r.mul_add(f, 8.373_124_5e-3); // 0x1.125edcp-7
+    let r = r.mul_add(f, 4.166_953_6e-2); // 0x1.555b5ap-5
+    let r = r.mul_add(f, 1.666_647_2e-1); // 0x1.555450p-3
+    let r = r.mul_add(f, 4.999_998_5e-1); // 0x1.fffff6p-2
+    let r = r.mul_add(f, 1.0);
+    let r = r.mul_add(f, 1.0);
+
+    // exp(a) = 2**i * r
+    let ia = if i > 0 { 0_u32 } else { 0x83000000 };
+    let s = f32::from_bits(0x7f000000_u32.wrapping_add(ia));
+    let t = f32::from_bits(((i as u32) << 23).wrapping_sub(ia) as u32);
+    let r = r * s;
+    let r = r * t;
+
+    // handle special cases: severe overflow / underflow
+    if a.abs() >= 104.0 {
+        if a > 0.0 {
+            f32::INFINITY
+        } else {
+            0.0
+        }
+    } else {
+        r
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::exp_f32;
+
+    #[test]
+    fn basics() {
+        assert_eq!(
+            exp_f32(255544082189650076565756907338896244736.0),
+            f32::INFINITY
+        );
+    }
+}
diff --git a/engine/narcissus-maths/src/lib.rs b/engine/narcissus-maths/src/lib.rs
index 45b0541..0337a8a 100644
--- a/engine/narcissus-maths/src/lib.rs
+++ b/engine/narcissus-maths/src/lib.rs
@@ -1,5 +1,6 @@
 mod affine2;
 mod affine3;
+mod exp;
 mod mat2;
 mod mat3;
 mod mat4;
@@ -16,6 +17,7 @@ mod vec4;
 
 pub use affine2::Affine2;
 pub use affine3::Affine3;
+pub use exp::exp_f32;
 pub use mat2::Mat2;
 pub use mat3::Mat3;
 pub use mat4::Mat4;
diff --git a/engine/narcissus-maths/tests/exhaustive_f32.rs b/engine/narcissus-maths/tests/exhaustive_f32.rs
index af505c2..385b0db 100644
--- a/engine/narcissus-maths/tests/exhaustive_f32.rs
+++ b/engine/narcissus-maths/tests/exhaustive_f32.rs
@@ -5,7 +5,7 @@ use std::{
     sync::atomic::{AtomicUsize, Ordering},
 };
 
-use narcissus_maths::{next_after_f32, sin_cos_pi_f32, tan_pi_f32};
+use narcissus_maths::{exp_f32, next_after_f32, sin_cos_pi_f32, tan_pi_f32};
 
 use gmp_mpfr_sys::mpfr;
 
@@ -124,6 +124,10 @@ impl Float {
         unsafe { mpfr::tan(&mut self.0, &self.0, round.to_raw()) }
     }
 
+    pub fn exp_mut(&mut self, round: Round) -> i32 {
+        unsafe { mpfr::exp(&mut self.0, &self.0, round.to_raw()) }
+    }
+
     pub fn abs_mut(&mut self, round: Round) -> i32 {
         unsafe { mpfr::abs(&mut self.0, &self.0, round.to_raw()) }
     }
@@ -418,6 +422,10 @@ fn ref_tan_pi_f32(x: &mut Float, pi: &Float) -> i32 {
     x.tan_mut(Round::TiesToEven)
 }
 
+fn ref_exp_f32(x: &mut Float, _: &Float) -> i32 {
+    x.exp_mut(Round::TiesToEven)
+}
+
 #[test]
 #[ignore]
 pub fn exhaustive_sin_pi() {
@@ -444,3 +452,11 @@ pub fn exhaustive_tan_pi() {
     assert_eq!(errors.num_errors, 100_555_422);
     assert_eq!(errors.max_error_ulp, 2);
 }
+
+#[test]
+#[ignore]
+pub fn exhaustive_exp() {
+    let errors = check_exhaustive_f32(ref_exp_f32, |a| exp_f32(a), false);
+    println!("EXP: {errors:?}");
+    assert_eq!(errors.max_error_ulp, 1);
+}