https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118141
--- Comment #11 from Andrew Pinski <pinskia at gcc dot gnu.org> --- (In reply to Richard Yao from comment #10) > I had thought the strict aliasing rule only applied within function scope, > although that appears to be a misunderstanding upon checking this: > > https://gist.github.com/shafik/848ae25ee209f698763cffee272a58f8 > > This code compiles correctly: > > https://godbolt.org/z/3Tdnac8TE > > For my own education, is this a strict aliasing rule violation too or am I > okay because I used a union type before the void pointer? Techincally the above is undefined behavior because one field of an union is active at a time and you can only access via that one but GCC has an documented extension about this being well defined. See https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/Optimize-Options.html#index-fstrict-aliasing The practice of reading from a different union member than the one most recently written to (called “type-punning”) is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So, the code above works as expected.
