• Jason Ekstrand's avatar
    intel/fs: Add an UNDEF instruction to avoid excess live ranges · f4ef34f2
    Jason Ekstrand authored
    With 8 and 16-bit types and anything where we have to use non-trivial
    strides registersto deal with restrictions, we end up with things that
    look like partial writes even though we don't care about any values in
    the register except those written by that instruction.  This is
    particularly important when dealing with loops because liveness sees
    is_partial_write and the fact that an old version from a previous loop
    iteration may be valid at that point and extends all purely partially
    written values to the entire loop.
    
    This commit adds a new UNDEF instruction which does nothing (the
    generator doesn't emit anything) but which does a fake write to the
    register.  This informs liveness that we don't care about any values
    before that point so it won't consider those registers to be falsely
    live.  We can safely emit UNDEF instructions for all SSA values that
    come in from NIR and nearly all temporaries generated by various stages
    of the compiler.  In particular, we need to insert UNDEF instructions
    when we handle region restrictions because the newly allocated registers
    are almost guaranteed to be partially written.
    
    No shader-db changes.
    
    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110432Reviewed-by: 's avatarMatt Turner <mattst88@gmail.com>
    f4ef34f2