On 7/18/21 5:23 PM, Bruno Haible wrote:
Such compiler optimizations would need to be backed by the standards.
Are there initiatives to "outlaw" references to uninitialized storage
in recent C or C++ standards?
No initiatives are needed, at least for C. Using uninitialized storage
is undefined behavior in the current C standard and this has been true
ever since C was standardized. I imagine C++ is similar.
I hope code such as
int x; /* uninitialized */
if (!((x & 1) == 0 || (x & 1) == 1))
abort ();
will never crash. x & 1 can only be 0 or 1. Tertium not datur.
The C standard doesn't guarantee that code will never crash. For
example, the standard allows an implementation that uses two's
complement but where INT_MIN == -INT_MAX and where the bit pattern
0x80000000 is a trap value (i.e., your program aborts if it reads an int
whose machine value is 0x80000000).
GCC does not _know_ that the array
is uninitialized. It's only a "maybe uninitialized".
That's what GCC's diagnostic says, yes. But in cases like these GCC
actually "knows" that variables are uninitialized and it sometimes
optimizes based on this knowledge. For example, for:
_Bool f (void) { char *p; return !p; }
gcc -O2 (GCC 11.1.1 20210531 (Red Hat 11.1.1-3)) "knows" that P is
uninitialized and generates code equivalent to that of:
_Bool f (void) { return 1; }
That is, GCC optimizes away the access to p's value, which GCC can do
because the behavior is undefined.
If GCC ever
infers that it is "certainly uninitialized", we could defeat that
through a use of 'volatile', such as
Yes, some use of volatile should do the trick for GCC (which is what my
patch did). However, one would still have problems with a debugging
implementation, e.g., if GCC ever supports an -fsanitize=uninitialized
option that catches use of uninitialized storage.
This is all low priority of course.
