Andrew Pinski <pins...@gmail.com> writes: > On Mon, Dec 19, 2011 at 2:06 PM, Richard Sandiford > <rdsandif...@googlemail.com> wrote: >> forward_propagate_subreg has code to optimise sequences like: >> >> (set (reg:DI X) (*_extend:DI (reg:SI Y))) >> (... (subreg:SI (reg:DI X)) ...) >> >> by replacing (subreg:SI (reg:DI X)) with (reg:SI Y). However, there is >> a special case to stop this happening if Y is loaded from memory and the >> extension matches LOAD_EXTEND_OP. In that case the transformation isn't >> profitable, because we'll be converting a single load into a load followed >> by an extension. >> >> The same problem applies to any SI->DI sign extension on 64-bit MIPS, >> because SI registers are stored in sign-extended form. The comment >> below explains why in a bit more detail. >> >> This optimisation was causing a failure in gcc.target/mips/octeon-bbit-2.c >> for -mabi=64. We reused the input to a sign_extend instruction, such that >> the inputs and outputs were simultaneously live and could no longer be tied. > > This is PR 42839.
Doh. I really should to search for this stuff in bugzilla first. I've just added a PR reference to the ChangeLog entry. I don't think it's the kind of thing that should be backported to branches, but I'm never too sure whether that means the bug should be closed as fixed or not. Richard
