cmd/compile: don't use CMOV ops to compute load addresses
We want to issue loads as soon as possible, especially when they are going to miss in the cache. Using a conditional move (CMOV) here: i := ... if cond { i++ } ... = a[i] means that we have to wait for cond to be computed before the load is issued. Without a CMOV, if the branch is predicted correctly the load can be issued in parallel with computing cond. Even if the branch is predicted incorrectly, maybe the speculative load is close to the real load, and we get a prefetch for free. In the worst case, when the prediction is wrong and the address is way off, we only lose by the time difference between the CMOV latency (~2 cycles) and the mispredict restart latency (~15 cycles). We only squash CMOVs that affect load addresses. Results of CMOVs that are used for other things (store addresses, store values) we use as before. Fixes #26306 Change-Id: I82ca14b664bf05e1d45e58de8c4d9c775a127ca1 Reviewed-on: https://go-review.googlesource.com/c/145717 Run-TryBot: Keith Randall <khr@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by:Josh Bleecher Snyder <josharian@gmail.com>
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