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Automatically disable cast checks when building with optimization

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Cast checks are slow. We seem to have some rough consensus that they are
important for debug builds, but not for release builds. Problem is, very
few apps define G_DISABLE_CAST_CHECKS for release builds. Worse, it's
undocumented, so there's no way apps could even be expected to know
about it.

We can get the right default is almost all situations by making this
depend on the __OPTIMIZE__ preprocessor definition. This is a GCC-specific
thing, although Clang supports it too. If the compiler does not define
__OPTIMIZE__, then this commit does no harm: you can still use
G_DISABLE_CAST_CHECKS as before. When checking __OPTIMIZE__, we are
supposed to ensure our code has the same behavior as it would if we do
not, which will be true except in case the check fails (which is
programmer error).

Downside: this will not automatically do the right thing with -Og,
because __OPTIMIZE__ is always defined to 1. We don't want to disable
cast checks automatically if using -O0 or -Og. There's no way to
automatically fix this, but we can create an escape hatch by allowing
you to define G_DISABLE_CAST_CHECKS=0 to force-enable cast checks. In
practice, I don't think this matters much because -Og kinda failed:
GCC's man page says it should be a superior debugging experience to -O0,
but it optimizes variables away so it's definitely not.

Another downside: this is bad if you really *do* want cast checks in
release builds. The same solution applies: define
G_DISABLE_CAST_CHECKS=0 and you'll get your cast checks.
This commit is contained in:
Michael Catanzaro 2022-08-03 15:31:30 -05:00
parent 08715a4cd1
commit b8e1ecdd6b
1 changed files with 6 additions and 5 deletions
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11
gobject/gtype.h
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@ -2500,15 +2500,16 @@ const gchar * g_type_name_from_class (GTypeClass *g_class);
/* --- implementation bits --- */
#ifndef G_DISABLE_CAST_CHECKS
#if (defined(G_DISABLE_CAST_CHECKS) && G_DISABLE_CAST_CHECKS != 0) || (!defined(G_DISABLE_CAST_CHECKS) && defined(__OPTIMIZE__))
# define _G_TYPE_CIC(ip, gt, ct) ((ct*) ip)
# define _G_TYPE_CCC(cp, gt, ct) ((ct*) cp)
#else
# define _G_TYPE_CIC(ip, gt, ct) \
((ct*) (void *) g_type_check_instance_cast ((GTypeInstance*) ip, gt))
# define _G_TYPE_CCC(cp, gt, ct) \
((ct*) (void *) g_type_check_class_cast ((GTypeClass*) cp, gt))
#else /* G_DISABLE_CAST_CHECKS */
# define _G_TYPE_CIC(ip, gt, ct) ((ct*) ip)
# define _G_TYPE_CCC(cp, gt, ct) ((ct*) cp)
#endif /* G_DISABLE_CAST_CHECKS */
#endif
#define _G_TYPE_CHI(ip) (g_type_check_instance ((GTypeInstance*) ip))
#define _G_TYPE_CHV(vl) (g_type_check_value ((GValue*) vl))
#define _G_TYPE_IGC(ip, gt, ct) ((ct*) (((GTypeInstance*) ip)->g_class))