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

dlech · dlech · commit e58d36855488 · 2022-12-18T23:31:29.000-06:00
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

It turns out special compiler options are needed to get GCC to use the
SSE instruction set instead of the 387 coprocessor.

Signed-off-by: David Lechner &lt;david@pybricks.com&gt;
diff --git a/ports/windows/Makefile b/ports/windows/Makefile
@@ -84,6 +84,15 @@ ifneq ($(FROZEN_MANIFEST),)
 CFLAGS += -DMICROPY_QSTR_EXTRA_POOL=mp_qstr_frozen_const_pool -DMICROPY_MODULE_FROZEN_MPY=1 -DMPZ_DIG_SIZE=16
 endif
 
+ifeq ($(shell $(CC) -dumpmachine),i686-w64-mingw32)
+# GCC disables the SSE instruction set by default on i366 targets and we have
+# to specify all three of these options to enable it.
+# https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html (see -mfpmath=unit section)
+# Enabling the SSE instruction set is necessary to get correct rounding of floating points.
+# https://lemire.me/blog/2020/06/26/gcc-not-nearest
+CFLAGS += -msse -mfpmath=sse -march=pentium4
+endif
+
 CXXFLAGS += $(filter-out -std=gnu99,$(CFLAGS))
 
 include $(TOP)/py/mkrules.mk
