In http://reviews.llvm.org/D6638#103013, @dsanders wrote:
> > > On Thu Dec 18 2014 at 1:21:53 PM Daniel Sanders > > > <daniel.sand...@imgtec.com<mailto:daniel.sand...@imgtec.com>> wrote: > > > > > For the record, gcc does use $at for code generation, take a look at > > mips_print_operand for the @ symbol and then look at all of the cases it's > > being used. > > > > That's correct. However, LLVM currently doesn't use $at for codegen and > > quite a bit of effort has been put in to making that the case. I'd like to > > retain this advantage over GCC if I can. > > > That's not what the original mail said and there's no real reason to avoid > > using $at. I'm not sure I understand this rationale. > > > I see where we're confusing you, Toma and I aren't being precise enough. In > GCC, $at is a reserved register. As you say, it does use $at in generated > code but as far as I know, it only does so as a very short lived temporary. > In LLVM, $at is an allocatable register and is freely used in all code > generation. Yes... > In GCC inline assembly, $at is still a reserved register and users have taken > advantage of this. In LLVM, $at needs to be clobbered by inline assembly so > that the users usage of $at doesn't break the compiler generated code. > > > The point I was making is that the rationale for this patch is completely > > mistaken. i.e. gcc does use $at for code > > > generation so unless gcc is adding an implicit clobber of $at in inline > > asm then it's not going to be correct. > > > GCC goes further than an implicit clobber. It has promised not to keep any > values in $at. The idea behind the gcc usage is that anything that needs an explicit relocation or was a macro instruction before the demacroifying (yay, made up words) uses $at. We can definitely avoid using $at for anything in llvm, just pull it out of the register allocation pool? I'm not sure where you're going with this? -eric http://reviews.llvm.org/D6638 EMAIL PREFERENCES http://reviews.llvm.org/settings/panel/emailpreferences/ _______________________________________________ cfe-commits mailing list cfe-commits@cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
