cmd/compile: add initial backend concurrency support

This CL adds initial support for concurrent backend compilation.

CAVEATS

I suspect it's going to end up on Twitter.
If you're coming here from the internet:

* Don't believe the hype.
* If you're going to try it out, please also try with
  the race detector enabled and report an issue
  at golang.org/issue/new if you see a race report.
  Just run 'go install -race cmd/compile' and
  'go build -a yourpackages'. And then run make.bash
  again to return a reasonable compiler.

BACKGROUND

The compiler currently consists (very roughly) of the following phases:

1. Initialization.
2. Lexing and parsing into the cmd/compile/internal/syntax AST.
3. Translation into the cmd/compile/internal/gc AST.
4. Some gc AST passes: typechecking, escape analysis, inlining,
   closure handling, expression evaluation ordering (order.go),
   and some lowering and optimization (walk.go).
5. Translation into the cmd/compile/internal/ssa SSA form.
6. Optimization and lowering of SSA form.
7. Translation from SSA form to assembler instructions.
8. Translation from assembler instructions to machine code.
9. Writing lots of output: machine code, DWARF symbols,
   type and reflection info, export data.

Phase 2 was already concurrent as of Go 1.8.

Phase 3 is planned for eventual removal;
we hope to go straight from syntax AST to SSA.

Phases 5–8 are per-function; this CL adds support for
processing multiple functions concurrently.
The slowest phases in the compiler are 5 and 6,
so this offers the opportunity for some good speed-ups.

Unfortunately, it's not quite that straightforward.
In the current compiler, the latter parts of phase 4
(order, walk) are done function-at-a-time as needed.
Making order and walk concurrency-safe proved hard,
and they're not particularly slow, so there wasn't much reward.
To enable phases 5–8 to be done concurrently,
when concurrent backend compilation is requested,
we complete phase 4 for all functions
before starting later phases for any functions.

Also, in reality, we automatically generate new
functions in phase 9, such as method wrappers
and equality and has routines.
Those new functions then go through phases 4–8.
This CL disables concurrent backend compilation
after the first, big, user-provided batch of
functions has been compiled.
This is done to keep things simple,
and because the autogenerated functions
tend to be small, few, simple, and fast to compile.

USAGE

Concurrent backend compilation still defaults to off.
To set the number of functions that may be backend-compiled
concurrently, use the compiler flag -c.
In future work, cmd/go will automatically set -c.

Furthermore, this CL has been intentionally written
so that the c=1 path has no backend concurrency whatsoever,
not even spawning any goroutines.
This helps ensure that, should problems arise
let in the development cycle,
we can simply have cmd/go set -c=1 always,
and revert to the original compiler behavior.

MUTEXES

Most of the work required to make concurrent backend
compilation safe has occurred over the past month.
This CL adds a handful of mutexes to get the rest of the way there;
they are the mutexes that I didn't see a clean way to avoid.
Some of them may still be eliminable in future work.

In no particular order:

* gc.funcsymsmu. The global funcsyms slice is populated
  lazily when we need function symbols for closures.
  This occurs during gc AST to SSA translation.
  The function funcsym also does a package lookup,
  which is a source of races on types.Pkg.Syms;
  funcsymsmu also covers that package lookup.
  This mutex is low priority: it adds a single global,
  it is in an infrequently used code path, and it is low contention.
  It requires additional sorting to preserve reproducible builds.

* gc.largeStackFramesMu. We don't discover until after SSA compilation
  that a function's stack frame is gigantic.
  Recording that error happens basically never,
  but it does happen concurrently.
  Fix with a low priority mutex and sorting.

* obj.Link.hashmu. ctxt.hash stores the mapping from
  types.Syms (compiler symbols) to obj.LSyms (linker symbols).
  It is accessed fairly heavily through all the phases.
  This is easily the most heavily contended mutex.
  I hope that the syncmap proposed in golang#18177 may
  provide some free speed-ups here.
  Some lookups may also be removable.

* gc.signatlistmu. The global signatlist map is
  populated with types through several of the concurrent phases,
  including notably via ngotype during DWARF generation.
  It is low priority for removal, aside from some mild
  awkwardness to avoid deadlocks due to recursive calls.

* gc.typepkgmu. Looking up symbols in the types package
  happens a fair amount during backend compilation
  and DWARF generation, particularly via ngotype.
  This mutex helps us to avoid a broader mutex on types.Pkg.Syms.
  It has low-to-moderate contention.

* types.internedStringsmu. gc AST to SSA conversion and
  some SSA work introduce new autotmps.
  Those autotmps have their names interned to reduce allocations.
  That interning requires protecting types.internedStrings.
  The autotmp names are heavily re-used, and the mutex
  overhead and contention here are low, so it is probably
  a worthwhile performance optimization to keep this mutex.

* types.Sym.Lsymmu. Syms keep a cache of their associated LSym,
  to reduce lookups in ctxt.Hash. This cache itself
  needs concurrency protection. This mutex adds to the size
  of a moderately important data structure, but the alloc benchmarks
  below show that this doesn't hurt much in practice.
  It is moderately contended, mostly because when lookups fail,
  the lock is held while vying for the contended ctxt.Hash mutex.
  The fact that it keeps load off the ctxt.Hash mutex, though,
  makes this mutex worth keeping.
  Also, a fair number of calls to Linksym could be avoided
  by judicious addition of a local variable.

TESTING

I have been testing this code locally by running
'go install -race cmd/compile'
and then doing
'go build -a -gcflags=-c=128 std cmd'
for all architectures and a variety of compiler flags.
This obviously needs to be made part of the builders,
but it is too expensive to make part of all.bash.
I have filed golang#19962 for this.

REPRODUCIBLE BUILDS

This version of the compiler generates reproducible builds.
Testing reproducible builds also needs automation, however,
and is also too expensive for all.bash.
This is golang#19961.

Also of note is that some of the compiler flags used by 'toolstash -cmp'
are currently incompatible with concurrent backend compilation.
They still work fine with c=1.
Time will tell whether this is a problem.

NEXT STEPS

* Continue to find and fix races and bugs,
  using a combination of code inspection, fuzzing,
  and hopefully some community experimentation.
  I do not know of any outstanding races,
  but there probably are some.
* Improve testing.
* Improve performance, for many values of c.
* Integrate with cmd/go and fine tune.
* Support concurrent compilation with the -race flag.
  It is a sad irony that it does not yet work.
* Minor code cleanup that has been deferred during
  the last month due to uncertainty about the
  ultimate shape of this CL.

PERFORMANCE

Here's the buried lede, at last. :)

All benchmarks are from my 8 core 2.9 GHz Intel Core i7 darwin/amd64 laptop.

First, going from tip to this CL with c=1
costs about 1-2% CPU and has almost no memory impact.

name       old time/op       new time/op       delta
Template         193ms ± 4%        195ms ± 5%  +0.99%  (p=0.007 n=50+50)
Unicode         82.0ms ± 3%       84.7ms ± 4%  +3.26%  (p=0.000 n=47+48)
GoTypes          539ms ± 4%        549ms ± 3%  +1.90%  (p=0.000 n=46+46)
SSA              5.92s ± 2%        5.95s ± 3%  +0.49%  (p=0.018 n=42+48)
Flate            121ms ± 4%        122ms ± 3%    ~     (p=0.783 n=48+46)
GoParser         143ms ± 6%        144ms ± 3%  +0.90%  (p=0.001 n=49+47)
Reflect          342ms ± 4%        347ms ± 3%  +1.47%  (p=0.000 n=48+49)
Tar              104ms ± 5%        105ms ± 4%  +1.06%  (p=0.003 n=47+47)
XML              197ms ± 4%        199ms ± 5%  +0.85%  (p=0.014 n=48+49)

name       old user-time/op  new user-time/op  delta
Template         238ms ±10%        241ms ±10%    ~     (p=0.066 n=50+50)
Unicode          104ms ± 4%        106ms ± 5%  +2.23%  (p=0.000 n=46+49)
GoTypes          706ms ± 3%        714ms ± 5%  +1.15%  (p=0.002 n=45+49)
SSA              8.21s ± 3%        8.27s ± 2%  +0.78%  (p=0.002 n=43+47)
Flate            144ms ± 7%        145ms ± 5%    ~     (p=0.098 n=49+49)
GoParser         175ms ± 4%        177ms ± 4%  +1.35%  (p=0.003 n=47+50)
Reflect          435ms ± 6%        433ms ± 7%    ~     (p=0.852 n=50+50)
Tar              121ms ± 6%        122ms ± 7%    ~     (p=0.143 n=48+50)
XML              240ms ± 4%        241ms ± 4%    ~     (p=0.231 n=50+49)

name       old alloc/op      new alloc/op      delta
Template        38.7MB ± 0%       38.7MB ± 0%    ~     (p=0.690 n=5+5)
Unicode         29.8MB ± 0%       29.8MB ± 0%    ~     (p=0.222 n=5+5)
GoTypes          113MB ± 0%        113MB ± 0%    ~     (p=0.310 n=5+5)
SSA             1.24GB ± 0%       1.24GB ± 0%  +0.04%  (p=0.008 n=5+5)
Flate           25.2MB ± 0%       25.2MB ± 0%    ~     (p=0.841 n=5+5)
GoParser        31.7MB ± 0%       31.7MB ± 0%    ~     (p=0.151 n=5+5)
Reflect         77.5MB ± 0%       77.5MB ± 0%    ~     (p=0.690 n=5+5)
Tar             26.4MB ± 0%       26.4MB ± 0%    ~     (p=0.690 n=5+5)
XML             42.0MB ± 0%       42.0MB ± 0%    ~     (p=0.690 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          378k ± 0%         377k ± 1%    ~     (p=0.841 n=5+5)
Unicode           321k ± 0%         322k ± 0%    ~     (p=0.310 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%    ~     (p=0.310 n=5+5)
SSA              9.67M ± 0%        9.69M ± 0%  +0.20%  (p=0.008 n=5+5)
Flate             233k ± 0%         233k ± 1%    ~     (p=1.000 n=5+5)
GoParser          315k ± 0%         314k ± 1%    ~     (p=0.151 n=5+5)
Reflect           971k ± 0%         970k ± 0%    ~     (p=0.690 n=5+5)
Tar               249k ± 0%         249k ± 0%    ~     (p=0.310 n=5+5)
XML               391k ± 0%         390k ± 0%    ~     (p=0.841 n=5+5)

Comparing this CL to itself, from c=1 to c=2
improves real times 20-30%, costs 5-10% more CPU time,
and adds about 2% alloc.
The allocation increase comes from allocating more ssa.Caches.

name       old time/op       new time/op       delta
Template         202ms ± 3%        149ms ± 3%  -26.15%  (p=0.000 n=49+49)
Unicode         87.4ms ± 4%       84.2ms ± 3%   -3.68%  (p=0.000 n=48+48)
GoTypes          560ms ± 2%        398ms ± 2%  -28.96%  (p=0.000 n=49+49)
Compiler         2.46s ± 3%        1.76s ± 2%  -28.61%  (p=0.000 n=48+46)
SSA              6.17s ± 2%        4.04s ± 1%  -34.52%  (p=0.000 n=49+49)
Flate            126ms ± 3%         92ms ± 2%  -26.81%  (p=0.000 n=49+48)
GoParser         148ms ± 4%        107ms ± 2%  -27.78%  (p=0.000 n=49+48)
Reflect          361ms ± 3%        281ms ± 3%  -22.10%  (p=0.000 n=49+49)
Tar              109ms ± 4%         86ms ± 3%  -20.81%  (p=0.000 n=49+47)
XML              204ms ± 3%        144ms ± 2%  -29.53%  (p=0.000 n=48+45)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        246ms ± 4%     ~     (p=0.401 n=50+48)
Unicode          109ms ± 4%        111ms ± 4%   +1.47%  (p=0.000 n=44+50)
GoTypes          728ms ± 3%        765ms ± 3%   +5.04%  (p=0.000 n=46+50)
Compiler         3.33s ± 3%        3.41s ± 2%   +2.31%  (p=0.000 n=49+48)
SSA              8.52s ± 2%        9.11s ± 2%   +6.93%  (p=0.000 n=49+47)
Flate            149ms ± 4%        161ms ± 3%   +8.13%  (p=0.000 n=50+47)
GoParser         181ms ± 5%        192ms ± 2%   +6.40%  (p=0.000 n=49+46)
Reflect          452ms ± 9%        474ms ± 2%   +4.99%  (p=0.000 n=50+48)
Tar              126ms ± 6%        136ms ± 4%   +7.95%  (p=0.000 n=50+49)
XML              247ms ± 5%        264ms ± 3%   +6.94%  (p=0.000 n=48+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       39.3MB ± 0%   +1.48%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.2MB ± 0%   +1.19%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        114MB ± 0%   +0.69%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        447MB ± 0%   +0.95%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.26GB ± 0%   +0.89%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       25.9MB ± 1%   +2.35%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       32.2MB ± 0%   +1.59%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       78.9MB ± 0%   +0.91%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.0MB ± 0%   +1.80%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       43.4MB ± 0%   +2.35%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          379k ± 0%         378k ± 0%     ~     (p=0.421 n=5+5)
Unicode           322k ± 0%         321k ± 0%     ~     (p=0.222 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.548 n=5+5)
Compiler         4.12M ± 0%        4.11M ± 0%   -0.14%  (p=0.032 n=5+5)
SSA              9.72M ± 0%        9.72M ± 0%     ~     (p=0.421 n=5+5)
Flate             234k ± 1%         234k ± 0%     ~     (p=0.421 n=5+5)
GoParser          316k ± 1%         315k ± 0%     ~     (p=0.222 n=5+5)
Reflect           980k ± 0%         979k ± 0%     ~     (p=0.095 n=5+5)
Tar               249k ± 1%         249k ± 1%     ~     (p=0.841 n=5+5)
XML               392k ± 0%         391k ± 0%     ~     (p=0.095 n=5+5)

From c=1 to c=4, real time is down ~40%, CPU usage up 10-20%, alloc up ~5%:

name       old time/op       new time/op       delta
Template         203ms ± 3%        131ms ± 5%  -35.45%  (p=0.000 n=50+50)
Unicode         87.2ms ± 4%       84.1ms ± 2%   -3.61%  (p=0.000 n=48+47)
GoTypes          560ms ± 4%        310ms ± 2%  -44.65%  (p=0.000 n=50+49)
Compiler         2.47s ± 3%        1.41s ± 2%  -43.10%  (p=0.000 n=50+46)
SSA              6.17s ± 2%        3.20s ± 2%  -48.06%  (p=0.000 n=49+49)
Flate            126ms ± 4%         74ms ± 2%  -41.06%  (p=0.000 n=49+48)
GoParser         148ms ± 4%         89ms ± 3%  -39.97%  (p=0.000 n=49+50)
Reflect          360ms ± 3%        242ms ± 3%  -32.81%  (p=0.000 n=49+49)
Tar              108ms ± 4%         73ms ± 4%  -32.48%  (p=0.000 n=50+49)
XML              203ms ± 3%        119ms ± 3%  -41.56%  (p=0.000 n=49+48)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        287ms ± 9%  +16.98%  (p=0.000 n=50+50)
Unicode          109ms ± 4%        118ms ± 5%   +7.56%  (p=0.000 n=46+50)
GoTypes          735ms ± 4%        806ms ± 2%   +9.62%  (p=0.000 n=50+50)
Compiler         3.34s ± 4%        3.56s ± 2%   +6.78%  (p=0.000 n=49+49)
SSA              8.54s ± 3%       10.04s ± 3%  +17.55%  (p=0.000 n=50+50)
Flate            149ms ± 6%        176ms ± 3%  +17.82%  (p=0.000 n=50+48)
GoParser         181ms ± 5%        213ms ± 3%  +17.47%  (p=0.000 n=50+50)
Reflect          453ms ± 6%        499ms ± 2%  +10.11%  (p=0.000 n=50+48)
Tar              126ms ± 5%        149ms ±11%  +18.76%  (p=0.000 n=50+50)
XML              246ms ± 5%        287ms ± 4%  +16.53%  (p=0.000 n=49+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       40.4MB ± 0%   +4.21%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.9MB ± 0%   +3.68%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        116MB ± 0%   +2.71%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        455MB ± 0%   +2.75%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.27GB ± 0%   +1.84%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       26.9MB ± 1%   +6.31%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       33.2MB ± 0%   +4.61%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       80.2MB ± 0%   +2.53%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.9MB ± 0%   +5.19%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       44.6MB ± 0%   +5.20%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          380k ± 0%         379k ± 0%   -0.39%  (p=0.032 n=5+5)
Unicode           321k ± 0%         321k ± 0%     ~     (p=0.841 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.421 n=5+5)
Compiler         4.12M ± 0%        4.14M ± 0%   +0.52%  (p=0.008 n=5+5)
SSA              9.72M ± 0%        9.76M ± 0%   +0.37%  (p=0.008 n=5+5)
Flate             234k ± 1%         234k ± 1%     ~     (p=0.690 n=5+5)
GoParser          316k ± 0%         317k ± 1%     ~     (p=0.841 n=5+5)
Reflect           981k ± 0%         981k ± 0%     ~     (p=1.000 n=5+5)
Tar               250k ± 0%         249k ± 1%     ~     (p=0.151 n=5+5)
XML               393k ± 0%         392k ± 0%     ~     (p=0.056 n=5+5)

Going beyond c=4 on my machine tends to increase CPU time and allocs
without impacting real time.

The CPU time numbers matter, because when there are many concurrent
compilation processes, that will impact the overall throughput.

The numbers above are in many ways the best case scenario;
we can take full advantage of all cores.
Fortunately, the most common compilation scenario is incremental
re-compilation of a single package during a build/test cycle.

Updates golang#15756

Change-Id: I6725558ca2069edec0ac5b0d1683105a9fff6bea

Author: josharian

src/cmd/compile/internal/gc/dcl.go

@@ -1063,6 +1063,8 @@ func funcsymname(s *types.Sym) string {
 
 // funcsym returns s·f.
 func funcsym(s *types.Sym) *types.Sym {
+	// Lock funcsymsmu immediately, so that it can also guard the package lookup.
+	funcsymsmu.Lock()
 	sf, existed := s.Pkg.LookupOK(funcsymname(s))
 	// Don't export s·f when compiling for dynamic linking.
 	// When dynamically linking, the necessary function
@@ -1071,6 +1073,7 @@ func funcsym(s *types.Sym) *types.Sym {
 	if !Ctxt.Flag_dynlink && !existed {
 		funcsyms = append(funcsyms, s)
 	}
+	funcsymsmu.Unlock()
 	return sf
 }
 
@@ -1096,7 +1099,9 @@ func makefuncsym(s *types.Sym) {
 		return
 	}
 	if _, existed := s.Pkg.LookupOK(funcsymname(s)); !existed {
+		funcsymsmu.Lock()
 		funcsyms = append(funcsyms, s)
+		funcsymsmu.Unlock()
 	}
 }
 

src/cmd/compile/internal/gc/go.go

@@ -10,6 +10,7 @@ import (
 	"cmd/internal/bio"
 	"cmd/internal/obj"
 	"cmd/internal/src"
+	"sync"
 )
 
 const (
@@ -171,7 +172,10 @@ var exportlist []*Node
 
 var importlist []*Node // imported functions and methods with inlinable bodies
 
-var funcsyms []*types.Sym
+var (
+	funcsymsmu sync.Mutex // protects funcsyms and associated package lookups (see func funcsym)
+	funcsyms   []*types.Sym
+)
 
 var dclcontext Class // PEXTERN/PAUTO
 

src/cmd/compile/internal/gc/gsubr.go

@@ -55,10 +55,12 @@ type Progs struct {
 }
 
 // newProgs returns a new Progs for fn.
-func newProgs(fn *Node) *Progs {
+// worker indicates which of the backend workers will use the Progs.
+func newProgs(fn *Node, worker int) *Progs {
 	pp := new(Progs)
 	if Ctxt.CanReuseProgs() {
-		pp.progcache = sharedProgArray[:]
+		sz := len(sharedProgArray) / nBackendWorkers
+		pp.progcache = sharedProgArray[sz*worker : sz*(worker+1)]
 	}
 	pp.curfn = fn
 

src/cmd/compile/internal/gc/main.go

@@ -172,6 +172,7 @@ func Main(archInit func(*Arch)) {
 	objabi.Flagcount("W", "debug parse tree after type checking", &Debug['W'])
 	flag.StringVar(&asmhdr, "asmhdr", "", "write assembly header to `file`")
 	flag.StringVar(&buildid, "buildid", "", "record `id` as the build id in the export metadata")
+	flag.IntVar(&nBackendWorkers, "c", 1, "number of concurrent backend compilations")
 	flag.BoolVar(&pure_go, "complete", false, "compiling complete package (no C or assembly)")
 	flag.StringVar(&debugstr, "d", "", "print debug information about items in `list`")
 	flag.BoolVar(&flagDWARF, "dwarf", true, "generate DWARF symbols")
@@ -267,6 +268,12 @@ func Main(archInit func(*Arch)) {
 	if compiling_runtime && Debug['N'] != 0 {
 		log.Fatal("cannot disable optimizations while compiling runtime")
 	}
+	if nBackendWorkers < 1 {
+		log.Fatalf("-c must be at least 1, got %d", nBackendWorkers)
+	}
+	if nBackendWorkers > 1 && !concurrentBackendAllowed() {
+		log.Fatalf("cannot use concurrent backend compilation with provided flags; invoked as %v", os.Args)
+	}
 
 	// parse -d argument
 	if debugstr != "" {
@@ -540,13 +547,31 @@ func Main(archInit func(*Arch)) {
 		}
 		timings.AddEvent(fcount, "funcs")
 
+		if nBackendWorkers > 1 {
+			for _, fn := range needscompile {
+				compilec <- fn
+			}
+			close(compilec)
+			needscompile = nil
+			compilewg.Wait()
+		}
+		// We autogenerate and compile some small functions
+		// such as method wrappers and equality/hash routines
+		// while exporting code.
+		// Disable concurrent compilation from here on,
+		// at least until this convoluted structure has been unwound.
+		nBackendWorkers = 1
+
 		if nsavederrors+nerrors == 0 {
 			fninit(xtop)
 		}
 
 		if compiling_runtime {
 			checknowritebarrierrec()
 		}
+		obj.SortSlice(largeStackFrames, func(i, j int) bool {
+			return largeStackFrames[i].Before(largeStackFrames[j])
+		})
 		for _, largePos := range largeStackFrames {
 			yyerrorl(largePos, "stack frame too large (>2GB)")
 		}
@@ -1019,3 +1044,37 @@ func clearImports() {
 func IsAlias(sym *types.Sym) bool {
 	return sym.Def != nil && asNode(sym.Def).Sym != sym
 }
+
+// By default, assume any debug flags are incompatible with concurrent compilation.
+// A few are safe and potentially in common use for normal compiles, though; mark them as such here.
+var concurrentFlagOK = [256]bool{
+	'B': true, // disabled bounds checking
+	'C': true, // disable printing of columns in error messages
+	'I': true, // add `directory` to import search path
+	'N': true, // disable optimizations
+	'l': true, // disable inlining
+}
+
+func concurrentBackendAllowed() bool {
+	for i, x := range Debug {
+		if x != 0 && !concurrentFlagOK[i] {
+			return false
+		}
+	}
+	// Debug_asm by itself is ok, because all printing occurs
+	// while writing the object file, and that is non-concurrent.
+	// Adding Debug_vlog, however, causes Debug_asm to also print
+	// while flushing the plist, which happens concurrently.
+	if Debug_vlog || debugstr != "" || debuglive > 0 {
+		return false
+	}
+	// TODO: test and add builders for GOEXPERIMENT values, and enable
+	if os.Getenv("GOEXPERIMENT") != "" {
+		return false
+	}
+	// TODO: fix races and enable the following flags
+	if Ctxt.Flag_shared || Ctxt.Flag_dynlink || flag_race {
+		return false
+	}
+	return true
+}

src/cmd/compile/internal/gc/obj.go

@@ -220,6 +220,11 @@ func dumpglobls() {
 		ggloblnod(n)
 	}
 
+	funcsymsmu.Lock()
+	defer funcsymsmu.Unlock()
+	obj.SortSlice(funcsyms, func(i, j int) bool {
+		return linksymname(funcsyms[i]) < linksymname(funcsyms[j])
+	})
 	for _, s := range funcsyms {
 		sf := s.Pkg.Lookup(funcsymname(s))
 		dsymptr(sf, 0, s, 0)
@@ -265,9 +270,11 @@ func Linksym(s *types.Sym) *obj.LSym {
 	if s == nil {
 		return nil
 	}
+	s.Lsymmu.Lock()
 	if s.Lsym == nil {
 		s.Lsym = Ctxt.Lookup(linksymname(s), 0)
 	}
+	s.Lsymmu.Unlock()
 	return s.Lsym
 }
 

src/cmd/compile/internal/gc/pgen.go

@@ -14,10 +14,18 @@ import (
 	"cmd/internal/sys"
 	"fmt"
 	"sort"
+	"sync"
 )
 
 // "Portable" code generation.
 
+var (
+	nBackendWorkers int            // the number of concurrent backend workers, set by a compiler flag
+	needscompile    []*Node        // slice of functions waiting to be compiled
+	compilewg       sync.WaitGroup // wait for all backend compilers to complete
+	compilec        chan *Node     // channel of functions for backend compilers to drain
+)
+
 func emitptrargsmap() {
 	if Curfn.Func.Nname.Sym.Name == "_" {
 		return
@@ -207,14 +215,32 @@ func compile(fn *Node) {
 	// Set up the function's LSym early to avoid data races with the assemblers.
 	fn.Func.initLSym()
 
-	// Build an SSA backend function.
-	ssafn := buildssa(fn)
-	pp := newProgs(fn)
+	if compilenow() {
+		compileSSA(fn, 0)
+	} else {
+		needscompile = append(needscompile, fn)
+	}
+}
+
+// compilenow reports whether to compile immediately or enqueue in needscompile.
+func compilenow() bool {
+	return nBackendWorkers == 1
+}
+
+// compileSSA builds an SSA backend function,
+// uses it to generate a plist,
+// and flushes that plist to machine code.
+// worker indicates which of the backend workers is doing the processing.
+func compileSSA(fn *Node, worker int) {
+	ssafn := buildssa(fn, worker)
+	pp := newProgs(fn, worker)
 	genssa(ssafn, pp)
 	if pp.Text.To.Offset < 1<<31 {
 		pp.Flush()
 	} else {
+		largeStackFramesMu.Lock()
 		largeStackFrames = append(largeStackFrames, fn.Pos)
+		largeStackFramesMu.Unlock()
 	}
 	// fieldtrack must be called after pp.Flush. See issue 20014.
 	fieldtrack(pp.Text.From.Sym, fn.Func.FieldTrack)

src/cmd/compile/internal/gc/reflect.go

@@ -14,6 +14,7 @@ import (
 	"os"
 	"sort"
 	"strings"
+	"sync"
 )
 
 type itabEntry struct {
@@ -36,9 +37,13 @@ type ptabEntry struct {
 }
 
 // runtime interface and reflection data structures
-var signatlist = make(map[*types.Type]bool)
-var itabs []itabEntry
-var ptabs []ptabEntry
+var (
+	signatlistmu sync.Mutex // protects signatlist
+	signatlist   = make(map[*types.Type]bool)
+
+	itabs []itabEntry
+	ptabs []ptabEntry
+)
 
 type Sig struct {
 	name   string
@@ -907,8 +912,18 @@ func typesymname(t *types.Type) string {
 	return name
 }
 
+// typepkgmu guards typepkg lookups.
+var typepkgmu sync.Mutex
+
+func typeLookup(name string) *types.Sym {
+	typepkgmu.Lock()
+	s := typepkg.Lookup(name)
+	typepkgmu.Unlock()
+	return s
+}
+
 func typesym(t *types.Type) *types.Sym {
-	return typepkg.Lookup(typesymname(t))
+	return typeLookup(typesymname(t))
 }
 
 // tracksym returns the symbol for tracking use of field/method f, assumed
@@ -919,7 +934,7 @@ func tracksym(t *types.Type, f *types.Field) *types.Sym {
 
 func typesymprefix(prefix string, t *types.Type) *types.Sym {
 	p := prefix + "." + t.ShortString()
-	s := typepkg.Lookup(p)
+	s := typeLookup(p)
 
 	//print("algsym: %s -> %+S\n", p, s);
 
@@ -931,7 +946,9 @@ func typenamesym(t *types.Type) *types.Sym {
 		Fatalf("typenamesym %v", t)
 	}
 	s := typesym(t)
+	signatlistmu.Lock()
 	addsignat(t)
+	signatlistmu.Unlock()
 	return s
 }
 
@@ -1421,14 +1438,17 @@ func addsignat(t *types.Type) {
 
 func dumptypestructs() {
 	// copy types from externdcl list to signatlist
+	signatlistmu.Lock()
 	for _, n := range externdcl {
 		if n.Op == OTYPE {
 			addsignat(n.Type)
 		}
 	}
+	signatlistmu.Unlock()
 
 	// Process signatlist. Use a loop, as dtypesym adds
 	// entries to signatlist while it is being processed.
+	signatlistmu.Lock()
 	signats := make([]typeAndStr, len(signatlist))
 	for len(signatlist) > 0 {
 		signats = signats[:0]
@@ -1437,6 +1457,9 @@ func dumptypestructs() {
 			signats = append(signats, typeAndStr{t: t, s: typesymname(t)})
 			delete(signatlist, t)
 		}
+		// Don't hold signatlistmu while processing signats,
+		// since signats can generate new entries for signatlist.
+		signatlistmu.Unlock()
 		sort.Sort(typesByString(signats))
 		for _, ts := range signats {
 			t := ts.t
@@ -1445,7 +1468,9 @@ func dumptypestructs() {
 				dtypesym(types.NewPtr(t))
 			}
 		}
+		signatlistmu.Lock()
 	}
+	signatlistmu.Unlock()
 
 	// process itabs
 	for _, i := range itabs {
@@ -1560,7 +1585,7 @@ func dalgsym(t *types.Type) *types.Sym {
 		// we use one algorithm table for all AMEM types of a given size
 		p := fmt.Sprintf(".alg%d", t.Width)
 
-		s = typepkg.Lookup(p)
+		s = typeLookup(p)
 
 		if s.AlgGen() {
 			return s
@@ -1570,7 +1595,7 @@ func dalgsym(t *types.Type) *types.Sym {
 		// make hash closure
 		p = fmt.Sprintf(".hashfunc%d", t.Width)
 
-		hashfunc = typepkg.Lookup(p)
+		hashfunc = typeLookup(p)
 
 		ot := 0
 		ot = dsymptr(hashfunc, ot, Runtimepkg.Lookup("memhash_varlen"), 0)
@@ -1580,7 +1605,7 @@ func dalgsym(t *types.Type) *types.Sym {
 		// make equality closure
 		p = fmt.Sprintf(".eqfunc%d", t.Width)
 
-		eqfunc = typepkg.Lookup(p)
+		eqfunc = typeLookup(p)
 
 		ot = 0
 		ot = dsymptr(eqfunc, ot, Runtimepkg.Lookup("memequal_varlen"), 0)