https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892
--- Comment #37 from David Brown <david at westcontrol dot com> --- (In reply to Martin Sebor from comment #35) > Here are the proposed changes: > > Pointer Provenance: > http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2219.htm#proposed-technical- > corrigendum > > Trap Representations: > http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2220.htm#proposed-technical- > corrigendum > > Unspecified Values: > http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2221.htm#proposed-technical- > corrigendum I am a little unsure of the suggestions for unspecified values here. Can I give some examples, to see if my interpretation is correct? Let's use a new gcc builtin "__builtin_unspecified()" that returns an unspecified value of int type, with no possible traps (no gcc target has trap representations for int types, AFAIK). int x = __builtin_unspecified(); int y = __builtin_unspecified(); if (x == y) doThis(); // The compiler can skip doThis() if (x != y) doThat(); // The compiler can skip doThat() too if (x == y) doThis(); else doThat(); // The compiler can choose to doThis() or doThat(), // but must do one or the other if (x == x) doThis(); // This compiler must doThis() if (x != x) doThat(); // The compiler cannot doThat() if (x == 3) doThis(); // The compiler can choose to doThis() // if the compiler does choose to doThis() the it fixes the value of x as 3 if (x & 0x01) doThis(); else doThat(); // The compiler can choose do doThis() or doThat(), // but that choice fixes the LSB of x This could allow for a range of possible optimisations, especially if there is a nice way to make unspecified values like __builtin_unspecified(). (Unspecified values of other types could be made by casts.) For example: struct opt_int { bool valid; int value; }; struct opt_int safe_sqrt(struct opt_int x) { opt_int y; if (!x.valid || x.value < 0) { y.valid = false; y.value = __builtin_unspecified(); } else { y.valid = true; y.value = unsafe_sqrt(x.value); } return y; } This kind of structure would mean minimal effort when you only need part of a struct to contain specified values.
