Hi, On Mon, 22 Feb 2016, Jeff Law wrote:
> > never considers them as candidates. However, we do seem to have proper > > data flow information for them. IMO one of the points of global reg vars, and its long-standing documentation to that effect, is that we do not have proper data flow information ... > temporary in that window seems like a useful thing to do. So I think > the biggest concern would be any kind of implicit use. I'm thinking of > function calls where the callee expects the global register var to have > the right value, perhaps asms as well (someone might reference the > global var in the asm, but not list it in the sets/uses/clobbers because > it wasn't really necessary in the past). ... exactly because of this. Some jit-like code uses global reg vars to reserve registers for the generated code. It would have to add -ffixed-<name> compilation options instead of only the global vars after the patch. I think the advantages of having "good RA" with global reg vars are dubious enough to not warrant breaking backward compatibility. Ciao, Michael.
