On Wed, May 06, 2020 at 10:58:55AM +1000, Michael Ellerman wrote: > >> The "m<>" here is breaking GCC 4.6.3, which we allegedly still support. > > > > [ You shouldn't use 4.6.3, there has been 4.6.4 since a while. And 4.6 > > is nine years old now. Most projects do not support < 4.8 anymore, on > > any architecture. ] > > Moving up to 4.6.4 wouldn't actually help with this though would it?
Nope. But 4.6.4 is a bug-fix release, 91 bugs fixed since 4.6.3, so you should switch to it if you can :-) > Also I have 4.6.3 compilers already built, I don't really have time to > rebuild them for 4.6.4. > > The kernel has a top-level minimum version, which I'm not in charge of, see: > > https://www.kernel.org/doc/html/latest/process/changes.html?highlight=gcc Yes, I know. And it is much preferred not to have stricter requirements for Power, I know that too. Something has to give though :-/ > There were discussions about making 4.8 the minimum, but I'm not sure > where they got to. Yeah, just petered out I think? All significant distros come with a 4.8 as system compiler. > >> Plain "m" works, how much does the "<>" affect code gen in practice? > >> > >> A quick diff here shows no difference from removing "<>". > > > > It will make it impossible to use update-form instructions here. That > > probably does not matter much at all, in this case. > > > > If you remove the "<>" constraints, also remove the "%Un" output modifier? > > So like this? > > diff --git a/arch/powerpc/include/asm/uaccess.h > b/arch/powerpc/include/asm/uaccess.h > index 62cc8d7640ec..ca847aed8e45 100644 > --- a/arch/powerpc/include/asm/uaccess.h > +++ b/arch/powerpc/include/asm/uaccess.h > @@ -207,10 +207,10 @@ do { > \ > > #define __put_user_asm_goto(x, addr, label, op) \ > asm volatile goto( \ > - "1: " op "%U1%X1 %0,%1 # put_user\n" \ > + "1: " op "%X1 %0,%1 # put_user\n" \ > EX_TABLE(1b, %l2) \ > : \ > - : "r" (x), "m<>" (*addr) \ > + : "r" (x), "m" (*addr) \ > : \ > : label) Like that. But you will have to do that to *all* places we use the "<>" constraints, or wait for more stuff to fail? And, there probably are places we *do* want update form insns used (they do help in some loops, for example)? Segher
