Here's a variant that works even at -O3, the trick seems to be to combine __builtin_choose_expr with __builtin_constant_p https://godbolt.org/g/pQeRvE #include <stdlib.h> void assertFailure( void) __attribute__((warning("Compile time assertion failure"))); int z(void); int main() { int j; for( j=0;j<4;j++) { if( z()) break; } __builtin_choose_expr( __builtin_constant_p(!(j < 4)), ((!(j < 4)) ? assertFailure() : (void)0), ((!(j < 4)) ? abort() : (void)0)); return 0; } On Fri, May 11, 2018 at 10:37 AM, Marc Glisse <marc.glisse@xxxxxxxx> wrote: > On Fri, 11 May 2018, John Carter wrote: > > Here is an example where gcc's optimizers go, ahhh, strange.... >> > > There is nothing strange about it. You are trying to abuse > __builtin_constant_p for warnings when it is meant for optimization. Linux > kernel developers had the same issue. > > Try play with the optimizations settings on godbolt. >> https://godbolt.org/#g:!((g:!((g:!((h:codeEditor,i:(j:1,lang >> :___c,source:'%23include+%3Cstdlib.h%3E%0A%0Avoid+assertFail >> ure(+void)+__attribute__((warning(%22Compile+time+ >> assertion+failure%22)))%3B%0A%0Aint+z(void)%3B%0A%0A%0Aint+ >> main()%0A%7B%0A+++int+j%3B%0A%0A+++for(++j%3D0%3Bj%3C4%3Bj% >> 2B%2B)+%7B%0A++++++if(+z())+break%3B%0A+++%7D%0A+++%0A+++ >> if(+__builtin_constant_p(!!(j+%3C+4)))%0A+++%7B%0A++++++if(! >> !(j+%3C+4))%0A+++++++++assertFailure()%3B%0A+++%7D%0A+++ >> else%0A++++++if(+!!(j+%3C+4))%0A+++++++++abort()%3B%0A+++% >> 0A+++return+0%3B%0A%7D%0A'),l:'5',n:'0',o:'C+source+%231',t: >> '0')),k:50,l:'4',n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:( >> compiler:cg81,filters:(b:'0',binary:'1',commentOnly:'0', >> demangle:'0',directives:'0',execute:'1',intel:'0',trim:'0' >> ),lang:___c,libs:!(),options:'-Os+-Wall',source:1),l:'5',n:' >> 0',o:'x86-64+gcc+8.1+(Editor+%231,+Compiler+%231)+C',t:'0')) >> ,k:50,l:'4',n:'0',o:'',s:0,t:'0')),l:'2',n:'0',o:'',t:'0')),version:4 >> >> On -Os it behaves sane, on -O2 or higher it calls assertFailure. >> >> #include <stdlib.h> >> >> void assertFailure( void) __attribute__((warning("Compile time assertion >> failure"))); >> >> int z(void); >> >> >> int main() >> { >> int j; >> >> for( j=0;j<4;j++) { >> if( z()) break; >> } >> >> if( __builtin_constant_p(!(j < 4))) >> > > gcc duplicates this piece of code, one version if we came through "break" > and one version if the loop completed. For each version, the value of j<4 > is known at compile-time and optimized as such. For optimization purposes, > this is perfect. > > > { >> if(!(j < 4)) >> assertFailure(); >> } >> else >> if( !(j < 4)) >> abort(); >> >> return 0; >> } >> > > -- > Marc Glisse > -- John Carter Phone : (64)(3) 358 6639 Tait Electronics PO Box 1645 Christchurch New Zealand -- This Communication is Confidential. We only send and receive email on the basis of the terms set out at www.taitradio.com/email_disclaimer <http://www.taitradio.com/email_disclaimer>
