• Keith Randall's avatar
    cmd/compile,runtime: redo how map assignments work · 442de98c
    Keith Randall authored
    To compile:
      m[k] = v
    instead of:
      mapassign(maptype, m, &k, &v), do
    do:
      *mapassign(maptype, m, &k) = v
    
    mapassign returns a pointer to the value slot in the map.  It is just
    like mapaccess except that it will allocate a new slot if k is not
    already present in the map.
    
    This makes map accesses faster but potentially larger (codewise).
    
    It is faster because the write into the map is done when the compiler
    knows the concrete type, so it can be done with a few store
    instructions instead of calling typedmemmove.  We also potentially
    avoid stack temporaries to hold v.
    
    The code can be larger when the map has pointers in its value type,
    since there is a write barrier call in addition to the mapassign call.
    That makes the code at the callsite a bit bigger (go binary is 0.3%
    bigger).
    
    This CL is in preparation for doing operations like m[k] += v with
    only a single runtime call.  That will roughly double the speed of
    such operations.
    
    Update #17133
    Update #5147
    
    Change-Id: Ia435f032090a2ed905dac9234e693972fe8c2dc5
    Reviewed-on: https://go-review.googlesource.com/30815
    Run-TryBot: Keith Randall <khr@golang.org>
    Reviewed-by: default avatarMatthew Dempsky <mdempsky@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    442de98c
runtime.go 6.11 KB