305a0ac123
entry: x = MOVQconst [7] ... b1: goto b2 b2: v = Phi(x, y, z) Transform that program to: entry: ... b1: x = MOVQconst [7] goto b2 b2: v = Phi(x, y, z) This CL moves constant-generating instructions used by a phi to the appropriate immediate predecessor of the phi's block. We used to put all constants in the entry block. Unfortunately, in large functions we have lots of constants at the start of the function, all of which are used by lots of phis throughout the function. This leads to the constants being live through most of the function (especially if there is an outer loop). That's an O(n^2) problem. Note that most of the non-phi uses of constants have already been folded into instructions (ADDQconst, MOVQstoreconst, etc.). This CL may be generally useful for other instances of compiler slowness, I'll have to check. It may cause some programs to run slower, but probably not by much, as rematerializeable values like these constants are allocated late (not at their originally scheduled location) anyway. This CL is definitely a minimal change that can be considered for 1.7. We probably want to do a better job in the tighten pass generally, not just for phi args. Leaving that for 1.8. Update #16407 Change-Id: If112a8883b4ef172b2f37dea13e44bda9346c342 Reviewed-on: https://go-review.googlesource.com/25046 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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