util/half: Use explicit RTNE rounding for the C++ float16_t

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41295>

src/util/double.h

@@ -25,7 +25,6 @@
 #ifndef _DOUBLE_H_
 #define _DOUBLE_H_
 
-#include "half_float.h"
 #include "u_math.h"
 
 #ifdef __cplusplus
@@ -48,68 +47,6 @@ _mesa_double_to_float_rtne(double val)
    return _mesa_double_to_float(val);
 }
 
-/*
- * We round down from double to half float by going through float in between,
- * but this can give us inaccurate results in some cases.
- * One such case is 0x40ee6a0000000001, which should round to 0x7b9b, but
- * going through float first turns into 0x7b9a instead. This is because the
- * first non-fitting bit is set, so we get a tie, but with the least
- * significant bit of the original number set, the tie should break rounding
- * up.
- * The cast to float, however, turns into 0x47735000, which when going to half
- * still ties, but now we lost the tie-up bit, and instead we round to the
- * nearest even, which in this case is down.
- *
- * To fix this, we check if the original would have tied, and if the tie would
- * have rounded up, and if both are true, set the least significant bit of the
- * intermediate float to 1, so that a tie on the next cast rounds up as well.
- * If the rounding already got rid of the tie, that set bit will just be
- * truncated anyway and the end result doesn't change.
- *
- * Another failing case is 0x40effdffffffffff. This one doesn't have the tie
- * from double to half, so it just rounds down to 0x7bff (65504.0), but going
- * through float first, it turns into 0x477ff000, which does have the tie bit
- * for half set, and when that one gets rounded it turns into 0x7c00
- * (Infinity).
- * The fix for that one is to make sure the intermediate float does not have
- * the tie bit set if the original didn't have it.
- */
-static inline uint16_t
-_mesa_double_to_float16_rtne(double val)
-{
-   int significand_bits16 = 10;
-   int significand_bits32 = 23;
-   int significand_bits64 = 52;
-   int f64_to_16_tie_bit = significand_bits64 - significand_bits16 - 1;
-   int f32_to_16_tie_bit = significand_bits32 - significand_bits16 - 1;
-   uint64_t f64_rounds_up_mask = ((1ULL << f64_to_16_tie_bit) - 1);
-
-   union di src;
-   union fi dst;
-
-   src.d = val;
-   dst.f = val;
-
-   bool f64_has_tie = (src.ui & (1ULL << f64_to_16_tie_bit)) != 0;
-   bool f64_rounds_up = (src.ui & f64_rounds_up_mask) != 0;
-
-   dst.ui |= (f64_has_tie && f64_rounds_up);
-   if (!f64_has_tie)
-      dst.ui &= ~(1U << f32_to_16_tie_bit);
-
-   return _mesa_float_to_float16_rtne(dst.f);
-}
-
-/*
- * double -> float -> half with RTZ doesn't have as many complications as
- * RTNE, but we do need to ensure that the double -> float cast also uses RTZ.
- */
-static inline uint16_t
-_mesa_double_to_float16_rtz(double val)
-{
-   return _mesa_float_to_float16_rtz(_mesa_double_to_float_rtz(val));
-}
-
 #ifdef __cplusplus
 } /* extern C */
 #endif

src/util/half_float.h

@@ -31,6 +31,7 @@
 #include <string.h>
 #include "util/detect_arch.h"
 #include "util/detect_cc.h"
+#include "util/double.h"
 #include "util/u_cpu_detect.h"
 
 #if DETECT_ARCH_X86_64
@@ -144,6 +145,67 @@ _mesa_float_is_half(double val)
    return val == (double) _mesa_half_to_float(fp16_val) && !is_denorm;
 }
 
+/*
+ * We round down from double to half float by going through float in between,
+ * but this can give us inaccurate results in some cases.
+ * One such case is 0x40ee6a0000000001, which should round to 0x7b9b, but
+ * going through float first turns into 0x7b9a instead. This is because the
+ * first non-fitting bit is set, so we get a tie, but with the least
+ * significant bit of the original number set, the tie should break rounding
+ * up.
+ * The cast to float, however, turns into 0x47735000, which when going to half
+ * still ties, but now we lost the tie-up bit, and instead we round to the
+ * nearest even, which in this case is down.
+ *
+ * To fix this, we check if the original would have tied, and if the tie would
+ * have rounded up, and if both are true, set the least significant bit of the
+ * intermediate float to 1, so that a tie on the next cast rounds up as well.
+ * If the rounding already got rid of the tie, that set bit will just be
+ * truncated anyway and the end result doesn't change.
+ *
+ * Another failing case is 0x40effdffffffffff. This one doesn't have the tie
+ * from double to half, so it just rounds down to 0x7bff (65504.0), but going
+ * through float first, it turns into 0x477ff000, which does have the tie bit
+ * for half set, and when that one gets rounded it turns into 0x7c00
+ * (Infinity).
+ * The fix for that one is to make sure the intermediate float does not have
+ * the tie bit set if the original didn't have it.
+ */
+static inline uint16_t
+_mesa_double_to_float16_rtne(double val)
+{
+   int significand_bits16 = 10;
+   int significand_bits32 = 23;
+   int significand_bits64 = 52;
+   int f64_to_16_tie_bit = significand_bits64 - significand_bits16 - 1;
+   int f32_to_16_tie_bit = significand_bits32 - significand_bits16 - 1;
+   uint64_t f64_rounds_up_mask = ((1ULL << f64_to_16_tie_bit) - 1);
+
+   union di src;
+   union fi dst;
+
+   src.d = val;
+   dst.f = val;
+
+   bool f64_has_tie = (src.ui & (1ULL << f64_to_16_tie_bit)) != 0;
+   bool f64_rounds_up = (src.ui & f64_rounds_up_mask) != 0;
+
+   dst.ui |= (f64_has_tie && f64_rounds_up);
+   if (!f64_has_tie)
+      dst.ui &= ~(1U << f32_to_16_tie_bit);
+
+   return _mesa_float_to_float16_rtne(dst.f);
+}
+
+/*
+ * double -> float -> half with RTZ doesn't have as many complications as
+ * RTNE, but we do need to ensure that the double -> float cast also uses RTZ.
+ */
+static inline uint16_t
+_mesa_double_to_float16_rtz(double val)
+{
+   return _mesa_float_to_float16_rtz(_mesa_double_to_float_rtz(val));
+}
 
 #ifdef __cplusplus
 
@@ -154,8 +216,8 @@ namespace mesa
 
 struct float16_t {
    uint16_t bits;
-   float16_t(float f) : bits(_mesa_float_to_half(f)) {}
-   float16_t(double d) : bits(_mesa_float_to_half((float)d)) {}
+   float16_t(float f) : bits(_mesa_float_to_float16_rtne(f)) {}
+   float16_t(double d) : bits(_mesa_double_to_float16_rtne(d)) {}
    float16_t(uint16_t raw_bits) : bits(raw_bits) {}
    static float16_t one() { return float16_t(FP16_ONE); }
    static float16_t zero() { return float16_t(FP16_ZERO); }