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cmd/compile: better escape analysis tag encoding #31504

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mdempsky opened this issue Apr 16, 2019 · 0 comments

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commented Apr 16, 2019

Currently escape.go mostly reuses esc.go's tagging scheme for simplicity, but that encoding scheme is unnecessarily lossy.

In particular, escape tags can only report that a parameter flows to the heap (EscHeap), or that it's dereferenced once before flowing to the heap (EscContentEscapes). However, there are hundreds of parameters in std cmd that are always dereferenced at least twice before flowing to the heap.

A quick experiment (tracking the shortest param path to heap and then logging when it's >=2) shows:

derefs    count
2         686
3         165
4         53
5         12
6         1
7         2

And this is probably an underestimate, since any functions that call other tagged functions will be analyzed based on the latter's lossy tags.

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