• Keith Randall's avatar
    cmd/compile: keep value use counts in SSA · 56e0ecc5
    Keith Randall authored
    Keep track of how many uses each Value has.  Each appearance in
    Value.Args and in Block.Control counts once.
    
    The number of uses of a value is generically useful to
    constrain rewrite rules.  For instance, we might want to
    prevent merging index operations into loads if the same
    index expression is used lots of times.
    
    But I have one use in particular for which the use count is required.
    We must make sure we don't combine ops with loads if the load has
    more than one use.  Otherwise, we may split a single load
    into multiple loads and that breaks perceived behavior in
    the presence of races.  In particular, the load of m.state
    in sync/mutex.go:Lock can't be done twice.  (I have a separate
    CL which triggers the mutex failure.  This CL has a test which
    demonstrates a similar failure.)
    
    Change-Id: Icaafa479239f48632a069d0c3f624e6ebc6b1f0e
    Reviewed-on: https://go-review.googlesource.com/20790
    Run-TryBot: Keith Randall <khr@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: 's avatarTodd Neal <todd@tneal.org>
    56e0ecc5
atomicload.go 798 Bytes