// fast path: most likely PosBase hasn't changed
	if m.cache.last == b0 {
		return m.cache.base
	}

	b1, ok := m.bases[b0]
	if !ok {
		fn := b0.Filename()
		absfn := trimFilename(b0)

		if b0.IsFileBase() {
			b1 = src.NewFileBase(fn, absfn)
		} else {
			// line directive base
			p0 := b0.Pos()
			p0b := p0.Base()
			if p0b == b0 {
				panic("infinite recursion in makeSrcPosBase")

		}
		if p.file == nil {
			base.ErrorExit()
		}
		lines += p.file.EOF.Line()
	}
	base.Timer.AddEvent(int64(lines), "lines")

	unified(m, noders)
}

// trimFilename returns the "trimmed" filename of b, which is the
// absolute filename after applying -trimpath processing. This
// filename form is suitable for use in object files and export data.
//