py/formatfloat: Fix exact int formatting on 32-bit mingw.

dlech · dlech · commit 5232f702a858 · 2022-12-18T18:34:45.000-06:00
When compiler optimizations are enabled on the mingw version of gcc, we
are getting failing tests because of rounding issues, for example:

    print(float("1e24"))

would print

    9.999999999999999e+23

instead of

    1e+24

We can work around the issue by using `powl()` instead of `pow()` in
`mp_format_float()` on affected targets.

Signed-off-by: David Lechner &lt;david@pybricks.com&gt;
diff --git a/py/formatfloat.c b/py/formatfloat.c
@@ -98,6 +98,14 @@ static inline int fp_expval(FPTYPE x) {
     return (int)((fb.i >> MP_FLOAT_FRAC_BITS) & (~(0xFFFFFFFF << MP_FLOAT_EXP_BITS))) - MP_FLOAT_EXP_OFFSET;
 }
 
+#if MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_DOUBLE && defined(__GNUC__) && defined(__MINGW32__) && defined(__i386__)
+// When optimizations are enabled using mingw's gcc, it breaks pow(double, double)
+// and gives wrong results.
+#define POW powl 
+#else
+#define POW MICROPY_FLOAT_C_FUN(pow)
+#endif
+
 int mp_format_float(FPTYPE f, char *buf, size_t buf_size, char fmt, int prec, char sign) {
 
     char *s = buf;
@@ -184,10 +192,10 @@ int mp_format_float(FPTYPE f, char *buf, size_t buf_size, char fmt, int prec, ch
         FPTYPE f_entry = f;  // Save f in case we go to 'f' format.
         // Build negative exponent
         e = -e_guess;
-        FPTYPE u_base = MICROPY_FLOAT_C_FUN(pow)(10, -e);
+        FPTYPE u_base = POW(10, -e);
         while (u_base > f) {
             ++e;
-            u_base = MICROPY_FLOAT_C_FUN(pow)(10, -e);
+            u_base = POW(10, -e);
         }
         // Normalize out the inferred unit.  Use divide because
         // pow(10, e) * pow(10, -e) is slightly < 1 for some e in float32
@@ -235,10 +243,10 @@ int mp_format_float(FPTYPE f, char *buf, size_t buf_size, char fmt, int prec, ch
         // that is not greater than it, and use that to start the
         // mantissa.
         e = e_guess;
-        FPTYPE next_u = MICROPY_FLOAT_C_FUN(pow)(10, e + 1);
+        FPTYPE next_u = POW(10, e + 1);
         while (f >= next_u) {
             ++e;
-            next_u = MICROPY_FLOAT_C_FUN(pow)(10, e + 1);
+            next_u = POW(10, e + 1);
         }
 
         // If the user specified fixed format (fmt == 'f') and e makes the
@@ -310,7 +318,7 @@ int mp_format_float(FPTYPE f, char *buf, size_t buf_size, char fmt, int prec, ch
         FPTYPE u_base = FPCONST(1.0);
         if (digit_index > 0) {
             // Generate 10^digit_index for positive digit_index.
-            u_base = MICROPY_FLOAT_C_FUN(pow)(10, digit_index);
+            u_base = POW(10, digit_index);
         }
         for (d = 0; d < 9; ++d) {
             if (f < u_base) {
