• Heschi Kreinick's avatar
    cmd/compile: reimplement location list generation · 2075a932
    Heschi Kreinick authored
    Completely redesign and reimplement location list generation to be more
    efficient, and hopefully not too hard to understand.
    
    RegKills are gone. Instead of using the regalloc's liveness
    calculations, redo them using the Ops' clobber information. Besides
    saving a lot of Values, this avoids adding RegKills to blocks that would
    be empty otherwise, which was messing up optimizations. This does mean
    that it's much harder to tell whether the generation process is buggy
    (there's nothing to cross-check it with), and there may be disagreements
    with GC liveness. But the performance gain is significant, and it's nice
    not to be messing with earlier compiler phases.
    
    The intermediate representations are gone. Instead of producing
    ssa.BlockDebugs, then dwarf.LocationLists, and then finally real
    location lists, go directly from the SSA to a (mostly) real location
    list. Because the SSA analysis happens before assembly, it stores
    encoded block/value IDs where PCs would normally go. It would be easier
    to do the SSA analysis after assembly, but I didn't want to retain the
    SSA just for that.
    
    Generation proceeds in two phases: first, it traverses the function in
    CFG order, storing the state of the block at the beginning and end. End
    states are used to produce the start states of the successor blocks. In
    the second phase, it traverses in program text order and produces the
    location lists. The processing in the second phase is redundant, but
    much cheaper than storing the intermediate representation. It might be
    possible to combine the two phases somewhat to take advantage of cases
    where the CFG matches the block layout, but I haven't tried.
    
    Location lists are finalized by adding a base address selection entry,
    translating each encoded block/value ID to a real PC, and adding the
    terminating zero entry. This probably won't work on OSX, where dsymutil
    will choke on the base address selection. I tried emitting CU-relative
    relocations for each address, and it was *very* bad for performance --
    it uses more memory storing all the relocations than it does for the
    actual location list bytes. I think I'm going to end up synthesizing the
    relocations in the linker only on OSX, but TBD.
    
    TestNexting needs updating: with more optimizations working, the
    debugger doesn't stop on the continue (line 88) any more, and the test's
    duplicate suppression kicks in. Also, dx and dy live a little longer
    now, but they have the correct values.
    
    Change-Id: Ie772dfe23a4e389ca573624fac4d05401ae32307
    Reviewed-on: https://go-review.googlesource.com/89356
    Run-TryBot: Heschi Kreinick <heschi@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: 's avatarDavid Chase <drchase@google.com>
    2075a932
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