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Replace assert(false) with an unreachable() macro.

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Taking a leaf out of the LLVM code base: this macro still includes an
assert(false) so that the message will show up in a typical build, but
it follows it up with a call to a function explicitly marked as no-
return.

So this ought to do a better job of convincing compilers that once a
code path hits this function it _really doesn't_ have to still faff
about with making up a bogus return value or filling in a variable
that 'might be used uninitialised' in the following code that won't be
reached anyway.

I've gone through the existing code looking for the assert(false) /
assert(0) idiom and replaced all the ones I found with the new macro,
which also meant I could remove a few pointless return statements and
variable initialisations that I'd already had to put in to placate
compiler front ends.
This commit is contained in:
Simon Tatham
2019-01-03 08:12:19 +00:00
parent 61dec9a07a
commit 0112936ef7
27 changed files with 89 additions and 75 deletions
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18
misc.h
View File

@ -11,8 +11,10 @@
#include <stdio.h> /* for FILE * */
#include <stdarg.h> /* for va_list */
#include <stdlib.h> /* for abort */
#include <time.h> /* for struct tm */
#include <limits.h> /* for INT_MAX/MIN */
#include <assert.h> /* for assert (obviously) */
unsigned long parse_blocksize(const char *bs);
char ctrlparse(char *s, char **next);
@ -169,6 +171,22 @@ bool smemeq(const void *av, const void *bv, size_t len);
char *buildinfo(const char *newline);
/*
* A function you can put at points in the code where execution should
* never reach in the first place. Better than assert(false), or even
* assert(false && "some explanatory message"), because some compilers
* don't interpret assert(false) as a declaration of unreachability,
* so they may still warn about pointless things like some variable
* not being initialised on the unreachable code path.
*
* I follow the assertion with a call to abort() just in case someone
* compiles with -DNDEBUG, and I wrap that abort inside my own
* function labelled NORETURN just in case some unusual kind of system
* header wasn't foresighted enough to label abort() itself that way.
*/
static inline NORETURN void unreachable_internal(void) { abort(); }
#define unreachable(msg) (assert(false && msg), unreachable_internal())
/*
* Debugging functions.
*