agx: Add 8-bit AGX minifloat routines

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10582>

src/asahi/compiler/agx_minifloat.h

@@ -0,0 +1,113 @@
+/*
+ * Copyright (C) 2021 Alyssa Rosenzweig <alyssa@rosenzweig.io>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#ifndef __AGX_MINIFLOAT_H_
+#define __AGX_MINIFLOAT_H_
+
+#include <math.h>
+#include "util/macros.h"
+
+/* AGX includes an 8-bit floating-point format for small dyadic immediates,
+ * consisting of 3 bits for the exponent, 4 bits for the mantissa, and 1-bit
+ * for sign, in the usual order. Zero exponent has special handling. */
+
+static inline float
+agx_minifloat_decode(uint8_t imm)
+{
+   float sign = (imm & 0x80) ? -1.0 : 1.0;
+   signed exp = (imm & 0x70) >> 4;
+   unsigned mantissa = (imm & 0xF);
+
+   if (exp)
+      return ldexpf(sign * (float) (mantissa | 0x10), exp - 7);
+   else
+      return ldexpf(sign * ((float) mantissa), -6);
+}
+
+/* Encodes a float. Results are only valid if the float can be represented
+ * exactly, if not the result of this function is UNDEFINED. signbit() is used
+ * to ensure -0.0 is handled correctly. */
+
+static inline uint8_t
+agx_minifloat_encode(float f)
+{
+   unsigned sign = signbit(f) ? 0x80 : 0;
+   f = fabsf(f);
+
+   /* frac is in [0.5, 1) and f = frac * 2^exp */
+   int exp = 0;
+   float frac = frexpf(f, &exp);
+
+   if (f >= 0.25) {
+      unsigned mantissa = (frac * 32.0);
+      exp -= 5; /* 2^5 = 32 */
+      exp = CLAMP(exp + 7, 0, 7);
+
+      assert(mantissa >= 0x10 && mantissa < 0x20);
+      assert(exp >= 1);
+
+      return sign | (exp << 4) | (mantissa & 0xF);
+   } else {
+      unsigned mantissa = (f * 64.0f);
+      assert(mantissa < 0x10);
+
+      return sign | mantissa;
+   }
+}
+
+static inline bool
+agx_minifloat_exact(float f)
+{
+   float f_ = agx_minifloat_decode(agx_minifloat_encode(f));
+   return memcmp(&f, &f_, sizeof(float)) == 0;
+}
+
+#ifndef NDEBUG
+static inline void
+agx_minifloat_tests(void)
+{
+   /* Decode some representative values */
+   assert(agx_minifloat_decode(0) == 0.0f);
+   assert(agx_minifloat_decode(25) == 0.390625f);
+   assert(agx_minifloat_decode(135) == -0.109375f);
+   assert(agx_minifloat_decode(255) == -31.0);
+
+   /* Verify exactness */
+   assert(agx_minifloat_exact(0.0f));
+   assert(agx_minifloat_exact(0.390625f));
+   assert(agx_minifloat_exact(-0.109375f));
+   assert(agx_minifloat_exact(-31.0));
+   assert(!agx_minifloat_exact(3.141f));
+   assert(!agx_minifloat_exact(2.718f));
+   assert(!agx_minifloat_exact(1.618f));
+
+   /* Check that all values round trip */
+   for (unsigned i = 0; i < 0x100; ++i) {
+      float f = agx_minifloat_decode(i);
+      assert(agx_minifloat_encode(f) == i);
+      assert(agx_minifloat_exact(f));
+   }
+}
+#endif
+
+#endif