https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114441
--- Comment #5 from Yang Wang <njuwy at smail dot nju.edu.cn> --- (In reply to Xi Ruoyao from comment #4) > (In reply to Yang Wang from comment #3) > > (In reply to Andrew Pinski from comment #1) > > > This is not a GCC bug, > > > > > > You need to use -mcmodel=large if you have huge statically allocated > > > arrays. > > > > > > The default -mcmodel=medium does not support more than 2GB size arrays > > > > > > See > > > https://gcc.gnu.org/onlinedocs/gcc-13.2.0/gcc/x86-Options.html#index- > > > mcmodel_003dlarge-3 > > > > Thanks for your reply! Indeed, the compilation was successful when either > > the -mcmodel=medium or -mcmodel=large was enabled. However, I'm still > > curious why it can be compiled successfully at the -O3 optimization level > > when the default -mcmodel=small is enabled? As far as I know, the -O3 > > optimization level is the highest. > > Because the entire `c` function is optimized to empty. Note that `c` always > invokes undefined behavior: > > t.cc: In function 'void c()': > t.cc:10:30: warning: iteration 1 invokes undefined behavior > [-Waggressive-loop-optimizations] > 10 | a[d][e][f][g][i] = 2; > | ~~~~~~~~~~~~~~~~~^~~ > t.cc:9:32: note: within this loop > 9 | for (size_t i = 0; i < 16; ++i) > | ~~^~~~ > > So the compiler is allowed to optimize it into nothing. > > A test case without undefined behavior: > > #include <stdio.h> > int a[1][1][1][1][1]; > short b[6268435456]; > void c() { > a[0][0][0][0][0] = 114514; > } > int main() {} > > fails at -O3 too. > > Also note that -O levels are for optimizing the *valid* programs, not for > debugging the compiler. It's wrong to report bugs solely because of > "different behaviors with different -O levels" (without analysing the > validity of the test case). Thanks for your reply!I'll pay more attention.
