• Russ Cox's avatar
    cmd/gc, runtime: make GODEBUG=gcdead=1 mode work with liveness · 28f1868f
    Russ Cox authored
    Trying to make GODEBUG=gcdead=1 work with liveness
    and in particular ambiguously live variables.
    
    1. In the liveness computation, mark all ambiguously live
    variables as live for the entire function, except the entry.
    They are zeroed directly after entry, and we need them not
    to be poisoned thereafter.
    
    2. In the liveness computation, compute liveness (and deadness)
    for all parameters, not just pointer-containing parameters.
    Otherwise gcdead poisons untracked scalar parameters and results.
    
    3. Fix liveness debugging print for -live=2 to use correct bitmaps.
    (Was not updated for compaction during compaction CL.)
    
    4. Correct varkill during map literal initialization.
    Was killing the map itself instead of the inserted value temp.
    
    5. Disable aggressive varkill cleanup for call arguments if
    the call appears in a defer or go statement.
    
    6. In the garbage collector, avoid bug scanning empty
    strings. An empty string is two zeros. The multiword
    code only looked at the first zero and then interpreted
    the next two bits in the bitmap as an ordinary word bitmap.
    For a string the bits are 11 00, so if a live string was zero
    length with a 0 base pointer, the poisoning code treated
    the length as an ordinary word with code 00, meaning it
    needed poisoning, turning the string into a poison-length
    string with base pointer 0. By the same logic I believe that
    a live nil slice (bits 11 01 00) will have its cap poisoned.
    Always scan full multiword struct.
    
    7. In the runtime, treat both poison words (PoisonGC and
    PoisonStack) as invalid pointers that warrant crashes.
    
    Manual testing as follows:
    
    - Create a script called gcdead on your PATH containing:
    
            #!/bin/bash
            GODEBUG=gcdead=1 GOGC=10 GOTRACEBACK=2 exec "$@"
    - Now you can build a test and then run 'gcdead ./foo.test'.
    - More importantly, you can run 'go test -short -exec gcdead std'
       to run all the tests.
    
    Fixes #7676.
    
    While here, enable the precise scanning of slices, since that was
    disabled due to bugs like these. That now works, both with and
    without gcdead.
    
    Fixes #7549.
    
    LGTM=khr
    R=khr
    CC=golang-codereviews
    https://golang.org/cl/83410044
    28f1868f
malloc.h 22 KB