Hello Simon,
thanks for catching that!

It certainly wasn't my intention to introduce any new C language
requirements, and in this case it's also not quite the right thing to
do to begin with: we want to print IEEE 754 floats with the right
precision, no matter what our host floating-point format is.
Unfortunately, I can't find a way to do that, thus the DECIMAL_DIG
usage.

I'm attaching a patch to use a fixed value of 17, which is the right
thing to do for a 52-bit mantissa. It still won't do the right thing
on hosts whose double type is insufficient to represent IEEE 754
doubles.

Suggested change log entry:
opcodes/:
2017-04-06  Pip Cet  <pipcet@gmail.com>

    * wasm32-dis.c (print_insn_wasm32): Use a fixed value (valid for
    IEEE 754 doubles) instead of DECIMAL_DIG.

diff --git a/opcodes/wasm32-dis.c b/opcodes/wasm32-dis.c
index 80e4ffe81a..f150163a80 100644
--- a/opcodes/wasm32-dis.c
+++ b/opcodes/wasm32-dis.c
@@ -23,13 +23,22 @@
 #include "opintl.h"
 #include "safe-ctype.h"
 #include "floatformat.h"
-#include <float.h>
 #include "libiberty.h"
 #include "elf-bfd.h"
 #include "elf/internal.h"
 #include "elf/wasm32.h"
 #include <stdint.h>

+/* Number of decimal digits, n, such that any floating-point number in the
+   widest supported floating type with pmax radix b digits can be rounded
+   to a floating-point number with n decimal digits and back again without
+   change to the value,
+
+    pmax * log10(b)            if b is a power of 10
+    ceil(1 + pmax * log10(b))    otherwise
+*/
+#define DECIMAL_DIG_IEEE754 17
+
 /* Type names for blocks and signatures.  */
 #define BLOCK_TYPE_NONE              0x40
 #define BLOCK_TYPE_I32               0x7f
@@ -405,7 +414,7 @@ print_insn_wasm32 (bfd_vma pc, struct
disassemble_info *info)
           if (ret < 0)
             return -1;
           len += ret;
-          prin (stream, " %.*g", DECIMAL_DIG, fconstant);
+          prin (stream, " %.*g", DECIMAL_DIG_IEEE754, fconstant);
           break;

         case wasm_constant_f64:
@@ -413,7 +422,7 @@ print_insn_wasm32 (bfd_vma pc, struct
disassemble_info *info)
           if (ret < 0)
             return -1;
           len += ret;
-          prin (stream, " %.*g", DECIMAL_DIG, fconstant);
+          prin (stream, " %.*g", DECIMAL_DIG_IEEE754, fconstant);
           break;

         case wasm_call: