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glib-init: statically assert that "small" enums are all int-sized

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ISO C allows compilers to make enums smaller than int if their
enumerated values would all fit in the range of a smaller type.

In practice, I suspect that in relevant compilers, all enums whose
values fit in the range INT32_MIN to INT32_MAX (inclusive) are the same
size as int. ISO C allows compiler to break that assumption, but those
that do would break code that works fine in other compilers, making the
compiler look bad, for no significant benefit. I conjecture that such
compilers are not popular.

Let's statically assert that my assumption holds. If all goes well,
GLib will continue to compile on every relevant platform; if it
fails to compile on some platform as a result of this change, then
there are probably a lot of naive uses of enums that need auditing
for this assumption.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=730932
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Allison Lortie
This commit is contained in:
Simon McVittie 2014-05-29 12:20:24 +01:00
parent c99fe67817
commit 5227630077
1 changed files with 23 additions and 0 deletions
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23
glib/glib-init.c
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@ -45,6 +45,29 @@ G_STATIC_ASSERT (_g_alignof (gpointer) == _g_alignof (GFunc));
G_STATIC_ASSERT (sizeof (GFunc) == sizeof (GCompareDataFunc));
G_STATIC_ASSERT (_g_alignof (GFunc) == _g_alignof (GCompareDataFunc));
/* We assume that "small" enums (those where all values fit in INT32_MIN
* to INT32_MAX) are exactly int-sized. In particular, we assume that if
* an enum has no members that exceed the range of char/short, the
* compiler will make it int-sized anyway, so adding a member later that
* *does* exceed the range of char/short is not an ABI break. */
typedef enum {
TEST_CHAR_0 = 0
} TestChar;
typedef enum {
TEST_SHORT_0 = 0,
TEST_SHORT_256 = 256
} TestShort;
typedef enum {
TEST_INT32_MIN = G_MININT32,
TEST_INT32_MAX = G_MAXINT32
} TestInt;
G_STATIC_ASSERT (sizeof (TestChar) == sizeof (int));
G_STATIC_ASSERT (sizeof (TestShort) == sizeof (int));
G_STATIC_ASSERT (sizeof (TestInt) == sizeof (int));
G_STATIC_ASSERT (_g_alignof (TestChar) == _g_alignof (int));
G_STATIC_ASSERT (_g_alignof (TestShort) == _g_alignof (int));
G_STATIC_ASSERT (_g_alignof (TestInt) == _g_alignof (int));
/**
* g_mem_gc_friendly:
*