| Message ID | 20150918121919.fa20552946703ae772d636e9@linux-foundation.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Fri, Sep 18, 2015 at 12:19:19PM -0700, Andrew Morton wrote: > On Wed, 16 Sep 2015 04:55:46 +0200 Andi Kleen <andi@firstfloor.org> wrote: > > > > Under what circumstances will the compiler (or linker?) do this? > > > > Compiler. > > > > > LTO enabled? > > > > Yes it's for LTO. The optimization allows the compiler to drop unused > > functions, which is very popular with users (a lot use it to get smaller > > kernel images) > > > > Does this look truthful and complete? > > > --- a/include/linux/compiler-gcc.h~a > +++ a/include/linux/compiler-gcc.h > @@ -205,7 +205,10 @@ > > #if GCC_VERSION >= 40600 > /* > - * Tell the optimizer that something else uses this function or variable. > + * When used with Link Time Optimization, gcc can optimize away C functions or > + * variables which are referenced only from assembly code. __visible tells the > + * optimizer that something else uses this function or variable, thus preventing > + * this. Yes, In a few cases I also used it to work around LTO bugs in older gcc releases. I don't think any of those fixes made it into mainline though, and they are not needed anymore with 5.x -Andi
--- a/include/linux/compiler-gcc.h~a +++ a/include/linux/compiler-gcc.h @@ -205,7 +205,10 @@ #if GCC_VERSION >= 40600 /* - * Tell the optimizer that something else uses this function or variable. + * When used with Link Time Optimization, gcc can optimize away C functions or + * variables which are referenced only from assembly code. __visible tells the + * optimizer that something else uses this function or variable, thus preventing + * this. */ #define __visible __attribute__((externally_visible)) #endif