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cmd/compile: eliminate write barriers when writing non-heap ptrs
We don't need a write barrier if: 1) The location we're writing to doesn't hold a heap pointer, and 2) The value we're writing isn't a heap pointer. The freshly returned value from runtime.newobject satisfies (1). Pointers to globals, and the contents of the read-only data section satisfy (2). This is particularly helpful for code like: p := []string{"abc", "def", "ghi"} Where the compiler generates: a := new([3]string) move(a, statictmp_) // eliminates write barriers here p := a[:] For big slice literals, this makes the code a smaller and faster to compile. Update #13554. Reduces the compile time by ~10% and RSS by ~30%. Change-Id: Icab81db7591c8777f68e5d528abd48c7e44c87eb Reviewed-on: https://go-review.googlesource.com/c/151498 Run-TryBot: Keith Randall <khr@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Austin Clements <austin@google.com> Reviewed-by: Cherry Zhang <cherryyz@google.com>
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