We set up static symbols during walk that we later make copies of to initialize local variables. It is difficult to ascertain at that time exactly when copying a symbol is profitable vs locally initializing an autotmp. During SSA, we are much better placed to optimize. This change recognizes when we are copying from a global readonly all-zero symbol and replaces it with direct zeroing. This often allows the all-zero symbol to be deadcode eliminated at link time. This is not ideal--it makes for large object files, and longer link times--but it is the cleanest fix I could find. This makes the final binary for the program in golang#38554 shrink from >500mb to ~2.2mb. It also shrinks the standard binaries: file before after Δ % addr2line 4412496 4404304 -8192 -0.186% buildid 2893816 2889720 -4096 -0.142% cgo 4841048 4832856 -8192 -0.169% compile 19926480 1992243 -4048 -0.020% cover 5281816 5277720 -4096 -0.078% link 6734648 6730552 -4096 -0.061% nm 4366240 4358048 -8192 -0.188% objdump 4755968 4747776 -8192 -0.172% pprof 14653060 14612100 -40960 -0.280% trace 11805940 1177726 -28672 -0.243% vet 7185560 7181416 -4144 -0.058% total 113588440 113465560 -122880 -0.108% And not just by removing unnecessary symbols; the program text shrinks a bit as well. Fixes golang#38554 Change-Id: I8381ae6084ae145a5e0cd9410c451e52c0dc51c8 Reviewed-on: https://go-review.googlesource.com/c/go/+/229704 Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com> Reviewed-by: Keith Randall <khr@golang.org>

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Commits on Apr 24, 2020

cmd/compile: optimize Move with all-zero ro sym src to Zero