• Austin Clements's avatar
    cmd/compile: add go:notinheap type pragma · 77527a31
    Austin Clements authored
    This adds a //go:notinheap pragma for declarations of types that must
    not be heap allocated. We ensure these rules by disallowing new(T),
    make([]T), append([]T), or implicit allocation of T, by disallowing
    conversions to notinheap types, and by propagating notinheap to any
    struct or array that contains notinheap elements.
    
    The utility of this pragma is that we can eliminate write barriers for
    writes to pointers to go:notinheap types, since the write barrier is
    guaranteed to be a no-op. This will let us mark several scheduler and
    memory allocator structures as go:notinheap, which will let us
    disallow write barriers in the scheduler and memory allocator much
    more thoroughly and also eliminate some problematic hybrid write
    barriers.
    
    This also makes go:nowritebarrierrec and go:yeswritebarrierrec much
    more powerful. Currently we use go:nowritebarrier all over the place,
    but it's almost never what you actually want: when write barriers are
    illegal, they're typically illegal for a whole dynamic scope. Partly
    this is because go:nowritebarrier has been around longer, but it's
    also because go:nowritebarrierrec couldn't be used in situations that
    had no-op write barriers or where some nested scope did allow write
    barriers. go:notinheap eliminates many no-op write barriers and
    go:yeswritebarrierrec makes it possible to opt back in to write
    barriers, so these two changes will let us use go:nowritebarrierrec
    far more liberally.
    
    This updates #13386, which is about controlling pointers from non-GC'd
    memory to GC'd memory. That would require some additional pragma (or
    pragmas), but could build on this pragma.
    
    Change-Id: I6314f8f4181535dd166887c9ec239977b54940bd
    Reviewed-on: https://go-review.googlesource.com/30939Reviewed-by: 's avatarKeith Randall <khr@golang.org>
    Reviewed-by: 's avatarMatthew Dempsky <mdempsky@google.com>
    77527a31
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