• Russ Cox's avatar
    cmd/compile: fix liveness computation for heap-escaped parameters · b6dc3e6f
    Russ Cox authored
    The liveness computation of parameters generally was never
    correct, but forcing all parameters to be live throughout the
    function covered up that problem. The new SSA back end is
    too clever: even though it currently keeps the parameter values live
    throughout the function, it may find optimizations that mean
    the current values are not written back to the original parameter
    stack slots immediately or ever (for example if a parameter is set
    to nil, SSA constant propagation may replace all later uses of the
    parameter with a constant nil, eliminating the need to write the nil
    value back to the stack slot), so the liveness code must now
    track the actual operations on the stack slots, exposing these
    problems.
    
    One small problem in the handling of arguments is that nodarg
    can return ONAME PPARAM nodes with adjusted offsets, so that
    there are actually multiple *Node pointers for the same parameter
    in the instruction stream. This might be possible to correct, but
    not in this CL. For now, we fix this by using n.Orig instead of n
    when considering PPARAM and PPARAMOUT nodes.
    
    The major problem in the handling of arguments is general
    confusion in the liveness code about the meaning of PPARAM|PHEAP
    and PPARAMOUT|PHEAP nodes, especially as contrasted with PAUTO|PHEAP.
    The difference between these two is that when a local variable "moves"
    to the heap, it's really just allocated there to start with; in contrast,
    when an argument moves to the heap, the actual data has to be copied
    there from the stack at the beginning of the function, and when a
    result "moves" to the heap the value in the heap has to be copied
    back to the stack when the function returns
    This general confusion is also present in the SSA back end.
    
    The PHEAP bit worked decently when I first introduced it 7 years ago (!)
    in 391425ae. The back end did nothing sophisticated, and in particular
    there was no analysis at all: no escape analysis, no liveness analysis,
    and certainly no SSA back end. But the complications caused in the
    various downstream consumers suggest that this should be a detail
    kept mainly in the front end.
    
    This CL therefore eliminates both the PHEAP bit and even the idea of
    "heap variables" from the back ends.
    
    First, it replaces the PPARAM|PHEAP, PPARAMOUT|PHEAP, and PAUTO|PHEAP
    variable classes with the single PAUTOHEAP, a pseudo-class indicating
    a variable maintained on the heap and available by indirecting a
    local variable kept on the stack (a plain PAUTO).
    
    Second, walkexpr replaces all references to PAUTOHEAP variables
    with indirections of the corresponding PAUTO variable.
    The back ends and the liveness code now just see plain indirected
    variables. This may actually produce better code, but the real goal
    here is to eliminate these little-used and somewhat suspect code
    paths in the back end analyses.
    
    The OPARAM node type goes away too.
    
    A followup CL will do the same to PPARAMREF. I'm not sure that
    the back ends (SSA in particular) are handling those right either,
    and with the framework established in this CL that change is trivial
    and the result clearly more correct.
    
    Fixes #15747.
    
    Change-Id: I2770b1ce3cbc93981bfc7166be66a9da12013d74
    Reviewed-on: https://go-review.googlesource.com/23393Reviewed-by: 's avatarKeith Randall <khr@golang.org>
    Run-TryBot: Russ Cox <rsc@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    b6dc3e6f
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