nir: fix nir_round_int_to_float for fp16

fp16 has quite the limited value range and with bigger integers
nir_round_int_to_float might return Inf where it shouldn't depending on
the rounding mode.

Fixes conversions half_rt[npz]_(u)?(int|long) CL CTS tests.

Cc: mesa-stable
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Rob Clark <rob.clark@oss.qualcomm.com>
(cherry picked from commit e1ed7de274)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40359>

.pick_status.json

@@ -3444,7 +3444,7 @@
         "description": "nir: fix nir_round_int_to_float for fp16",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null,
         "notes": null

src/compiler/nir/nir_conversion_builder.h

@@ -24,6 +24,7 @@
 #ifndef NIR_CONVERSION_BUILDER_H
 #define NIR_CONVERSION_BUILDER_H
 
+#include "util/half_float.h"
 #include "util/u_math.h"
 #include "nir_builder.h"
 #include "nir_builtin_builder.h"
@@ -162,6 +163,29 @@ nir_round_int_to_float(nir_builder *b, nir_def *src,
       }
       UNREACHABLE("unexpected rounding mode");
    } else {
+      /* For conversions to FP16 we need to clamp the input against the fp16
+       * max value when rounding towards zero or down. The reason for that is
+       * that for integer values outside of FP16 finite value range we could
+       * get Infinity, which would be incorrect rounding in those cases.
+       *
+       * Furthermore, we only need to do the clamping for integers bigger than
+       * 32 bits, because the lowering below will already clamp 16 bit integers
+       * correctly.
+       *
+       * This isn't a problem for FP32 or FP64 floats as integers can't exceed
+       * the finite value ranges.
+       */
+      if (dest_bit_size == 16 && src->bit_size >= 32) {
+         switch (round) {
+         case nir_rounding_mode_rtz:
+         case nir_rounding_mode_rd:
+            src = nir_umin_imm(b, src, FP16_MAX_F);
+            break;
+         default:
+            break;
+         }
+      }
+
       nir_def *mantissa_bit_size = nir_imm_int(b, mantissa_bits);
       nir_def *msb = nir_imax(b, nir_ufind_msb(b, src), mantissa_bit_size);
       nir_def *bits_to_lose = nir_isub(b, msb, mantissa_bit_size);

src/util/half_float.h

@@ -43,6 +43,7 @@ extern "C" {
 
 #define FP16_ONE     ((uint16_t) 0x3c00)
 #define FP16_ZERO    ((uint16_t) 0)
+#define FP16_MAX_F   65504.0
 
 uint16_t _mesa_float_to_half_slow(float val);
 float _mesa_half_to_float_slow(uint16_t val);