• David Lazar's avatar
    cmd/compile,link: generate PC-value tables with inlining information · 699175a1
    David Lazar authored
    In order to generate accurate tracebacks, the runtime needs to know the
    inlined call stack for a given PC. This creates two tables per function
    for this purpose. The first table is the inlining tree (stored in the
    function's funcdata), which has a node containing the file, line, and
    function name for every inlined call. The second table is a PC-value
    table that maps each PC to a node in the inlining tree (or -1 if the PC
    is not the result of inlining).
    
    To give the appearance that inlining hasn't happened, the runtime also
    needs the original source position information of inlined AST nodes.
    Previously the compiler plastered over the line numbers of inlined AST
    nodes with the line number of the call. This meant that the PC-line
    table mapped each PC to line number of the outermost call in its inlined
    call stack, with no way to access the innermost line number.
    
    Now the compiler retains line numbers of inlined AST nodes and writes
    the innermost source position information to the PC-line and PC-file
    tables. Some tools and tests expect to see outermost line numbers, so we
    provide the OutermostLine function for displaying line info.
    
    To keep track of the inlined call stack for an AST node, we extend the
    src.PosBase type with an index into a global inlining tree. Every time
    the compiler inlines a call, it creates a node in the global inlining
    tree for the call, and writes its index to the PosBase of every inlined
    AST node. The parent of this node is the inlining tree index of the
    call. -1 signifies no parent.
    
    For each function, the compiler creates a local inlining tree and a
    PC-value table mapping each PC to an index in the local tree.  These are
    written to an object file, which is read by the linker.  The linker
    re-encodes these tables compactly by deduplicating function names and
    file names.
    
    This change increases the size of binaries by 4-5%. For example, this is
    how the go1 benchmark binary is impacted by this change:
    
    section             old bytes   new bytes   delta
    .text               3.49M ± 0%  3.49M ± 0%   +0.06%
    .rodata             1.12M ± 0%  1.21M ± 0%   +8.21%
    .gopclntab          1.50M ± 0%  1.68M ± 0%  +11.89%
    .debug_line          338k ± 0%   435k ± 0%  +28.78%
    Total               9.21M ± 0%  9.58M ± 0%   +4.01%
    
    Updates #19348.
    
    Change-Id: Ic4f180c3b516018138236b0c35e0218270d957d3
    Reviewed-on: https://go-review.googlesource.com/37231
    Run-TryBot: David Lazar <lazard@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: 's avatarAustin Clements <austin@google.com>
    699175a1
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