From: Steinar H. Gunderson <sgunderson@bigfoot.com>
Date: Mon, 23 Feb 2015 19:17:49 +0000 (+0100)
Subject: Use the F16C instruction set when available.
X-Git-Tag: 1.1.3~6
X-Git-Url: https://git.sesse.net/?p=movit;a=commitdiff_plain;h=6be20704cdc7b64e37cc886b7872df58ef66eb1f

Use the F16C instruction set when available.

For most users, this is mostly theoretical, as it requires compiling
with -march=native or similar. And these are definitely meant for
vectorizing, although it's still 2-3x as fast to use them as our own
software fallback.

These are supported starting from Haswell, and also by some AMD CPUs.
---

diff --git a/fp16.cpp b/fp16.cpp
index 3738f5c..fc5800e 100644
--- a/fp16.cpp
+++ b/fp16.cpp
@@ -209,6 +209,8 @@ const int FP16_MAX_EXPONENT = (1 << FP16_EXPONENT_BITS) - 1;
 
 }  // namespace
 
+#ifndef __F16C__
+
 double fp16_to_fp64(fp16_int_t x)
 {
 	return fp_upconvert<fp16_int_t,
@@ -223,6 +225,8 @@ fp16_int_t fp64_to_fp16(double x)
 	       FP64_BIAS, FP64_MANTISSA_BITS, FP64_EXPONENT_BITS, FP64_MAX_EXPONENT>(x);
 }
 
+#endif
+
 double fp32_to_fp64(fp32_int_t x)
 {
 	return fp_upconvert<fp32_int_t,
diff --git a/fp16.h b/fp16.h
index 4e6652f..5417e02 100644
--- a/fp16.h
+++ b/fp16.h
@@ -1,6 +1,10 @@
 #ifndef _MOVIT_FP16_H
 #define _MOVIT_FP16_H 1
 
+#ifdef __F16C__
+#include <immintrin.h>
+#endif
+
 // Code for converting to and from fp16 (from fp64), without any particular
 // machine support, with proper IEEE round-to-even behavior (and correct
 // handling of NaNs and infinities). This is needed because some OpenGL
@@ -13,9 +17,29 @@ namespace movit {
 typedef unsigned int fp32_int_t;
 typedef unsigned short fp16_int_t;
 
+#ifdef __F16C__
+
+// Use the f16c instructions from Haswell if available (and we know that they
+// are at compile time).
+static inline double fp16_to_fp64(fp16_int_t x)
+{
+	return _cvtsh_ss(x);
+}
+
+static inline fp16_int_t fp64_to_fp16(double x)
+{
+	// NOTE: Strictly speaking, there are some select values where this isn't correct,
+	// since we first round to fp32 and then to fp16.
+	return _cvtss_sh(x, 0);
+}
+
+#else
+
 double fp16_to_fp64(fp16_int_t x);
 fp16_int_t fp64_to_fp16(double x);
 
+#endif
+
 // These are not very useful by themselves, but are implemented using the same
 // code as the fp16 ones (just with different constants), so they are useful
 // for verifying against the FPU in unit tests.