From 74349e5d68f4663deff5b9df9d1ac52d069ff294 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Robert=20J=C3=B6rdens?= <rj@quartiq.de>
Date: Fri, 27 Nov 2020 10:36:30 +0100
Subject: [PATCH] iir: more generic math helpers, use core::intrinsics

---
 dsp/src/iir.rs | 54 +++++++++++++++++++++++++-------------------------
 dsp/src/lib.rs |  2 +-
 2 files changed, 28 insertions(+), 28 deletions(-)

diff --git a/dsp/src/iir.rs b/dsp/src/iir.rs
index 33d8fdf..c6f2100 100644
--- a/dsp/src/iir.rs
+++ b/dsp/src/iir.rs
@@ -1,4 +1,4 @@
-use core::ops::{Add, Mul};
+use core::ops::{Add, Mul, Neg};
 use serde::{Deserialize, Serialize};
 
 use core::f32;
@@ -8,16 +8,24 @@ use core::f32;
 // `compiler-intrinsics`/llvm should have better (robust, universal, and
 // faster) implementations.
 
-fn abs(x: f32) -> f32 {
-    if x >= 0. {
+fn abs<T>(x: T) -> T
+where
+    T: PartialOrd + Default + Neg<Output = T>,
+{
+    if x >= T::default() {
         x
     } else {
         -x
     }
 }
 
-fn copysign(x: f32, y: f32) -> f32 {
-    if (x >= 0. && y >= 0.) || (x <= 0. && y <= 0.) {
+fn copysign<T>(x: T, y: T) -> T
+where
+    T: PartialOrd + Default + Neg<Output = T>,
+{
+    if (x >= T::default() && y >= T::default())
+        || (x <= T::default() && y <= T::default())
+    {
         x
     } else {
         -x
@@ -25,7 +33,10 @@ fn copysign(x: f32, y: f32) -> f32 {
 }
 
 #[cfg(not(feature = "nightly"))]
-fn max(x: f32, y: f32) -> f32 {
+fn max<T>(x: T, y: T) -> T
+where
+    T: PartialOrd,
+{
     if x > y {
         x
     } else {
@@ -34,7 +45,10 @@ fn max(x: f32, y: f32) -> f32 {
 }
 
 #[cfg(not(feature = "nightly"))]
-fn min(x: f32, y: f32) -> f32 {
+fn min<T>(x: T, y: T) -> T
+where
+    T: PartialOrd,
+{
     if x < y {
         x
     } else {
@@ -44,26 +58,12 @@ fn min(x: f32, y: f32) -> f32 {
 
 #[cfg(feature = "nightly")]
 fn max(x: f32, y: f32) -> f32 {
-    let o: f32;
-    unsafe {
-        asm!("vmaxnm.f32 {}, {}, {}",
-            lateout(sreg) o, in(sreg) x, in(sreg) y,
-            options(pure, nomem, nostack, preserves_flags)
-        );
-    }
-    o
+    core::intrinsics::maxnumf32(x, y)
 }
 
 #[cfg(feature = "nightly")]
 fn min(x: f32, y: f32) -> f32 {
-    let o: f32;
-    unsafe {
-        asm!("vminnm.f32 {}, {}, {}",
-            lateout(sreg) o, in(sreg) x, in(sreg) y,
-            options(pure, nomem, nostack, preserves_flags)
-        );
-    }
-    o
+    core::intrinsics::minnumf32(x, y)
 }
 
 // Multiply-accumulate vectors `x` and `a`.
@@ -76,7 +76,7 @@ where
 {
     x.iter()
         .zip(a)
-        .map(|(&x, &a)| x * a)
+        .map(|(x, a)| *x * *a)
         .fold(y0, |y, xa| y + xa)
 }
 
@@ -84,10 +84,10 @@ where
 ///
 /// To represent the IIR state (input and output memory) during the filter update
 /// this contains the three inputs (x0, x1, x2) and the two outputs (y1, y2)
-/// concatenated.
+/// concatenated. Lower indices correspond to more recent samples.
 /// To represent the IIR coefficients, this contains the feed-forward
-/// coefficients (b0, b1, b2) followd by the feed-back coefficients (a1, a2),
-/// all normalized such that a0 = 1.
+/// coefficients (b0, b1, b2) followd by the negated feed-back coefficients
+/// (-a1, -a2), all five normalized such that a0 = 1.
 pub type IIRState = [f32; 5];
 
 /// IIR configuration.
diff --git a/dsp/src/lib.rs b/dsp/src/lib.rs
index ac25d1e..b2acf34 100644
--- a/dsp/src/lib.rs
+++ b/dsp/src/lib.rs
@@ -1,4 +1,4 @@
 #![no_std]
-#![cfg_attr(feature = "nightly", feature(asm))]
+#![cfg_attr(feature = "nightly", feature(asm, core_intrinsics))]
 
 pub mod iir;