[Mesa-dev] [PATCH] util: don't use __builtin_clz unconditionally

[Marek Olšák]

From: Marek Olšák <marek.olsak at amd.com>

This fixes the build if __builtin_clz is unsupported.
---
 src/util/half_float.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/util/half_float.c b/src/util/half_float.c
index 588f050d924..2eff2c84f51 100644
--- a/src/util/half_float.c
+++ b/src/util/half_float.c
@@ -208,21 +208,31 @@ uint8_t _mesa_half_to_unorm8(uint16_t val)
   * Takes a uint16_t, divides by 65536, converts the infinite-precision
   * result to fp16 with round-to-zero. Used by the ASTC decoder.
   */
 uint16_t _mesa_uint16_div_64k_to_half(uint16_t v)
 {
    /* Zero or subnormal. Set the mantissa to (v << 8) and return. */
    if (v < 4)
       return v << 8;
 
    /* Count the leading 0s in the uint16_t */
-   int n = __builtin_clz(v) - (sizeof(unsigned int) - sizeof(uint16_t)) * 8;
+#ifdef HAVE___BUILTIN_CLZ
+   int n = __builtin_clz(v) - 16;
+#else
+   int n = 16;
+   for (int i = 15; i >= 0; i--) {
+      if (v & (1 << i)) {
+         n = 15 - i;
+         break;
+      }
+   }
+#endif
 
    /* Shift the mantissa up so bit 16 is the hidden 1 bit,
     * mask it off, then shift back down to 10 bits
     */
    int m = ( ((uint32_t)v << (n + 1)) & 0xffff ) >> 6;
 
    /*  (0{n} 1 X{15-n}) * 2^-16
     * = 1.X * 2^(15-n-16)
     * = 1.X * 2^(14-n - 15)
     * which is the FP16 form with e = 14 - n
-- 
2.17.1