cmd/compile: use sparse algorithm for phis in large program
This adds a sparse method for locating nearest ancestors
in a dominator tree, and checks blocks with more than one
predecessor for differences and inserts phi functions where
there are.
Uses reversed post order to cut number of passes, running
it from first def to last use ("last use" for paramout and
mem is end-of-program; last use for a phi input from a
backedge is the source of the back edge)
Includes a cutover from old algorithm to new to avoid paying
large constant factor for small programs. This keeps normal
builds running at about the same time, while not running
over-long on large machine-generated inputs.
Add "phase" flags for ssa/build -- ssa/build/stats prints
number of blocks, values (before and after linking references
and inserting phis, so expansion can be measured), and their
product; the product governs the cutover, where a good value
seems to be somewhere between 1 and 5 million.
Among the files compiled by make.bash, this is the shape of
the tail of the distribution for #blocks, #vars, and their
product:
#blocks #vars product
max 6171 28180 173,898,780
99.9% 1641 6548 10,401,878
99% 463 1909 873,721
95% 152 639 95,235
90% 84 359 30,021
The old algorithm is indeed usually fastest, for 99%ile
values of usually.
The fix to LookupVarOutgoing
( https://go-review.googlesource.com/#/c/22790/ )
deals with some of the same problems addressed by this CL,
but on at least one bug ( #15537 ) this change is still
a significant help.
With this CL:
/tmp/gopath$ rm -rf pkg bin
/tmp/gopath$ time go get -v -gcflags -memprofile=y.mprof \
github.com/gogo/protobuf/test/theproto3/combos/...
...
real 4m35.200s
user 13m16.644s
sys 0m36.712s
and pprof reports 3.4GB allocated in one of the larger profiles
With tip:
/tmp/gopath$ rm -rf pkg bin
/tmp/gopath$ time go get -v -gcflags -memprofile=y.mprof \
github.com/gogo/protobuf/test/theproto3/combos/...
...
real 10m36.569s
user 25m52.286s
sys 4m3.696s
and pprof reports 8.3GB allocated in the same larger profile
With this CL, most of the compilation time on the benchmarked
input is spent in register/stack allocation (cumulative 53%)
and in the sparse lookup algorithm itself (cumulative 20%).
Fixes #15537.
Change-Id: Ia0299dda6a291534d8b08e5f9883216ded677a00
Reviewed-on: https://go-review.googlesource.com/22342
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/src/cmd/compile/internal/gc/ssa.go b/src/cmd/compile/internal/gc/ssa.go
index 19af921..fdf040d 100644
--- a/src/cmd/compile/internal/gc/ssa.go
+++ b/src/cmd/compile/internal/gc/ssa.go
@@ -218,8 +218,16 @@
return nil
}
+ prelinkNumvars := s.f.NumValues()
+ sparseDefState := s.locatePotentialPhiFunctions(fn)
+
// Link up variable uses to variable definitions
- s.linkForwardReferences()
+ s.linkForwardReferences(sparseDefState)
+
+ if ssa.BuildStats > 0 {
+ s.f.LogStat("build", s.f.NumBlocks(), "blocks", prelinkNumvars, "vars_before",
+ s.f.NumValues(), "vars_after", prelinkNumvars*s.f.NumBlocks(), "ssa_phi_loc_cutoff_score")
+ }
// Don't carry reference this around longer than necessary
s.exitCode = Nodes{}
@@ -3741,7 +3749,8 @@
return s.variable(&memVar, ssa.TypeMem)
}
-func (s *state) linkForwardReferences() {
+func (s *state) linkForwardReferences(dm *sparseDefState) {
+
// Build SSA graph. Each variable on its first use in a basic block
// leaves a FwdRef in that block representing the incoming value
// of that variable. This function links that ref up with possible definitions,
@@ -3756,13 +3765,13 @@
for len(s.fwdRefs) > 0 {
v := s.fwdRefs[len(s.fwdRefs)-1]
s.fwdRefs = s.fwdRefs[:len(s.fwdRefs)-1]
- s.resolveFwdRef(v)
+ s.resolveFwdRef(v, dm)
}
}
// resolveFwdRef modifies v to be the variable's value at the start of its block.
// v must be a FwdRef op.
-func (s *state) resolveFwdRef(v *ssa.Value) {
+func (s *state) resolveFwdRef(v *ssa.Value, dm *sparseDefState) {
b := v.Block
name := v.Aux.(*Node)
v.Aux = nil
@@ -3801,6 +3810,7 @@
args := argstore[:0]
for _, e := range b.Preds {
p := e.Block()
+ p = dm.FindBetterDefiningBlock(name, p) // try sparse improvement on p
args = append(args, s.lookupVarOutgoing(p, v.Type, name, v.Line))
}
