https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93301
--- Comment #12 from Alexander Cherepanov <ch3root at openwall dot com> --- (In reply to rguent...@suse.de from comment #9) > This example simply shows that we're propagating the equivalence > y == x - 1 in the way that the controlled x - 1 becomes y (that's > possibly a QOI issue in this particular case where we _do_ see > that y is not initialized - but as said in general we cannot know) > and that we then optimize condition ? uninit : 5 as 5 (we're > optimistically treating uninit equal to 5 here which is OK). Right, nothing new here, just a simpler example. (Building something upon my first example was not fun. This one proved to be much nicer.) > Note the (void)&y doesn't do anything but it really looks like > it is required to make the testcase conforming (otherwise you > wouldn't use it?). This is just a ritual dance dedicated to Itanium. C11, 6.3.2.1p2, has the following text added for DR 338[1]: "If the lvalue designates an object of automatic storage duration that could have been declared with the register storage class (never had its address taken), and that object is uninitialized (not declared with an initializer and no assignment to it has been performed prior to use), the behavior is undefined." OTOH maybe it's worth to spent a minute on this DR given we are discussing speculative execution. [2] has a nice explanation of the topic. IIUC you can try to hoist a dereference of a pointer from a loop -- you will just get a NaT (Not-a-Thing) value in a register instead of a crash if the pointer is bad. But if your speculatively executed hoisted code have to write to memory a value from a register that corresponds to an uninitialized variable and this register contains a NaT, you get your crash back:-) According to [3] the whole thing is to be EOLed the next year, so not sure how relevant this is. [1] http://open-std.org/jtc1/sc22/wg14/www/docs/dr_338.htm [2] https://devblogs.microsoft.com/oldnewthing/?p=41003 [3] https://en.wikipedia.org/wiki/IA-64#End_of_life:_2021
