From cd8a3ea04b0e269ee55966173e94fd725dbbf38f Mon Sep 17 00:00:00 2001
From: Faith Ekstrand <faith.ekstrand@collabora.com>
Date: Wed, 17 Apr 2024 17:46:52 -0500
Subject: [PATCH] util/format: Handle denorms when converting to R11G11B10F

The spec allows denormals and the R11G11B20F decoder handles them but
the encoder always flushes them to zero.  We should be consistent and
handle denorms going both directions.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28793>
---
 src/util/format_r11g11b10f.h | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/src/util/format_r11g11b10f.h b/src/util/format_r11g11b10f.h
index 8c982e23771..acd6adef503 100644
--- a/src/util/format_r11g11b10f.h
+++ b/src/util/format_r11g11b10f.h
@@ -91,7 +91,7 @@ static inline uint32_t f32_to_uf11(float val)
        *      converted to 65024."
        */
       uf11 = UF11(30, 63);
-   } else if (exponent > -15) { /* Representable value */
+   } else if (exponent > -15) { /* Normal value */
       /* Dividing by 2^exponent gives us a number in the range [1, 2).
        * Multiplying by 2^6=64 gives us our mantissa, plus an extra 1 which
        * we'll mask off.
@@ -109,6 +109,19 @@ static inline uint32_t f32_to_uf11(float val)
       mantissa &= UF11_MANTISSA_BITS;
       exponent += UF11_EXPONENT_BIAS;
       uf11 = UF11(exponent, mantissa);
+   } else { /* Zero or denormal */
+      /* Since exponent <= -15, Multiplying by 2^14 gives us a number in the
+       * range [0, 1). Multiplying by 2^6=64 gives us our mantissa.
+       */
+      mantissa = _mesa_lroundevenf(ldexp(val, 6 + 14));
+
+      /* It's possible that we get a normal after rounding */
+      if ((mantissa >> UF11_EXPONENT_SHIFT) != 0) {
+         assert(mantissa == (1 << UF11_EXPONENT_SHIFT));
+         uf11 = UF11(1, 0);
+      } else {
+         uf11 = UF11(0, mantissa);
+      }
    }
 
    return uf11;
@@ -187,7 +200,7 @@ static inline uint32_t f32_to_uf10(float val)
        *      converted to 64512."
        */
       uf10 = UF10(30, 31);
-   } else if (exponent > -15) { /* Representable value */
+   } else if (exponent > -15) { /* Normal value */
       /* Dividing by 2^exponent gives us a number in the range [1, 2).
        * Multiplying by 2^5=32 gives us our mantissa, plus an extra 1 which
        * we'll mask off.
@@ -205,6 +218,19 @@ static inline uint32_t f32_to_uf10(float val)
       mantissa &= UF10_MANTISSA_BITS;
       exponent += UF10_EXPONENT_BIAS;
       uf10 = UF10(exponent, mantissa);
+   } else { /* Zero or denormal */
+      /* Since exponent <= -15, Multiplying by 2^14 gives us a number in the
+       * range [0, 1). Multiplying by 2^5=32 gives us our mantissa.
+       */
+      mantissa = _mesa_lroundevenf(ldexp(val, 5 + 14));
+
+      /* It's possible that we get a normal after rounding */
+      if ((mantissa >> UF10_EXPONENT_SHIFT) != 0) {
+         assert(mantissa == (1 << UF10_EXPONENT_SHIFT));
+         uf10 = UF10(1, 0);
+      } else {
+         uf10 = UF10(0, mantissa);
+      }
    }
 
    return uf10;