On Tue, Oct 2, 2018 at 10:57 AM Bart Van Assche <bvanassche@xxxxxxx> wrote: > > On Tue, 2018-10-02 at 10:24 -0700, Nick Desaulniers wrote: > > On Mon, Oct 1, 2018 at 6:16 PM Bart Van Assche <bvanassche@xxxxxxx> wrote: > > > Additionally, zero initializers should be left out to minimize > > > the size of object files. > > > > Sorry, my understanding was that global variables either occupy the > > .bss section or the .data section, depending on whether they were > > zero-initialized vs initialized to non-zero, respectively (where > > non-initialized are treated as zero initialized). Looks like without > > the explicit zero initialization, compilers will put the symbols in a > > "common" section, which `man 1 nm` says is also unitialized data. I > > didn't think .bss sections occupied space in an object file or binary; > > the kernel's loader would set up the mappings at execution? Can you > > clarify? > > Explicitly initialized global and static variables end up in the .data > section and need space in that section. Unless the initial value is zero. https://godbolt.org/z/curRoO So you don't wind up with an increase in binary size simply by having global variables initialized to zero, right? Instead the kernel knows to create a zero'd out mapping for bss. You don't need a run of zeros in the binary. So I disagree when you said earlier "zero initializers should be left out to minimize the size of object files." I assert they don't affect the size of the binary. If you had many global variables all initialized to zero, why would you encode that many zeros in a binary, when you can just set a size on the bss section and have the kernel create the appropriate sized and zero'd mapping? > That is not the case if the > initializer is left out and these variables end up in the .bss section. >From my above link, gcc will put globals without initializers into "common." > From https://en.wikipedia.org/wiki/.bss: > > "In computer programming, the name .bss or bss is used by many compilers > and linkers for the portion of an object file or executable containing > statically-allocated variables that are not explicitly initialized to any > value. It is often referred to as the "bss section" or "bss segment". > > Typically only the length of the bss section, but no data, is stored in > the object file." > > This is why checkpatch warns if a global or static variable is initialized > explicitly to zero. From scripts/checkpatch.pl: > > our $zero_initializer = qr{(?:(?:0[xX])?0+$Int_type?|NULL|false)\b}; > > # check for global initialisers. > > if ($line =~ /^\+$Type\s*$Ident(?:\s+$Modifier)*\s*=\s*($zero_initializer)\s*;/) { > if (ERROR("GLOBAL_INITIALISERS", > "do not initialise globals to $1\n" . $herecurr) && $fix) { > $fixed[$fixlinenr] =~ s/(^.$Type\s*$Ident(?:\s+$Modifier)*)\s*=\s*$zero_initializer\s*;/$1;/; > } > } > > # check for static initialisers. > > if ($line =~ /^\+.*\bstatic\s.*=\s*($zero_initializer)\s*;/) { > if (ERROR("INITIALISED_STATIC", > "do not initialise statics to $1\n" . $herecurr) && $fix) { > $fixed[$fixlinenr] =~ s/(\bstatic\s.*?)\s*=\s*$zero_initializer\s*;/$1;/; > } > } > > Bart. -- Thanks, ~Nick Desaulniers
