// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package gc

import (
	"internal/race"
	"math/rand"
	"sort"
	"sync"

	"cmd/compile/internal/base"
	"cmd/compile/internal/ir"
	"cmd/compile/internal/liveness"
	"cmd/compile/internal/ssagen"
	"cmd/compile/internal/typecheck"
	"cmd/compile/internal/types"
	"cmd/compile/internal/walk"
)

// "Portable" code generation.

var (
	compilequeue []*ir.Func // functions waiting to be compiled
)

func enqueueFunc(fn *ir.Func) {
	if ir.CurFunc != nil {
		base.FatalfAt(fn.Pos(), "enqueueFunc %v inside %v", fn, ir.CurFunc)
	}

	if ir.FuncName(fn) == "_" {
		// Skip compiling blank functions.
		// Frontend already reported any spec-mandated errors (#29870).
		return
	}

	if clo := fn.OClosure; clo != nil && !ir.IsTrivialClosure(clo) {
		return // we'll get this as part of its enclosing function
	}

	if len(fn.Body) == 0 {
		// Initialize ABI wrappers if necessary.
		ssagen.InitLSym(fn, false)
		liveness.WriteFuncMap(fn)
		return
	}

	errorsBefore := base.Errors()

	todo := []*ir.Func{fn}
	for len(todo) > 0 {
		next := todo[len(todo)-1]
		todo = todo[:len(todo)-1]

		prepareFunc(next)
		todo = append(todo, next.Closures...)
	}

	if base.Errors() > errorsBefore {
		return
	}

	// Enqueue just fn itself. compileFunctions will handle
	// scheduling compilation of its closures after it's done.
	compilequeue = append(compilequeue, fn)
}

// prepareFunc handles any remaining frontend compilation tasks that
// aren't yet safe to perform concurrently.
func prepareFunc(fn *ir.Func) {
	// Set up the function's LSym early to avoid data races with the assemblers.
	// Do this before walk, as walk needs the LSym to set attributes/relocations
	// (e.g. in MarkTypeUsedInInterface).
	ssagen.InitLSym(fn, true)

	// Calculate parameter offsets.
	types.CalcSize(fn.Type())

	typecheck.DeclContext = ir.PAUTO
	ir.CurFunc = fn
	walk.Walk(fn)
	ir.CurFunc = nil // enforce no further uses of CurFunc
	typecheck.DeclContext = ir.PEXTERN
}

// compileFunctions compiles all functions in compilequeue.
// It fans out nBackendWorkers to do the work
// and waits for them to complete.
func compileFunctions() {
	if len(compilequeue) == 0 {
		return
	}

	if race.Enabled {
		// Randomize compilation order to try to shake out races.
		tmp := make([]*ir.Func, len(compilequeue))
		perm := rand.Perm(len(compilequeue))
		for i, v := range perm {
			tmp[v] = compilequeue[i]
		}
		copy(compilequeue, tmp)
	} else {
		// Compile the longest functions first,
		// since they're most likely to be the slowest.
		// This helps avoid stragglers.
		sort.Slice(compilequeue, func(i, j int) bool {
			return len(compilequeue[i].Body) > len(compilequeue[j].Body)
		})
	}

	// We queue up a goroutine per function that needs to be
	// compiled, but require them to grab an available worker ID
	// before doing any substantial work to limit parallelism.
	workerIDs := make(chan int, base.Flag.LowerC)
	for i := 0; i < base.Flag.LowerC; i++ {
		workerIDs <- i
	}

	var wg sync.WaitGroup
	var asyncCompile func(*ir.Func)
	asyncCompile = func(fn *ir.Func) {
		wg.Add(1)
		go func() {
			worker := <-workerIDs
			ssagen.Compile(fn, worker)
			workerIDs <- worker

			// Done compiling fn. Schedule it's closures for compilation.
			for _, closure := range fn.Closures {
				asyncCompile(closure)
			}
			wg.Done()
		}()
	}

	types.CalcSizeDisabled = true // not safe to calculate sizes concurrently
	base.Ctxt.InParallel = true
	for _, fn := range compilequeue {
		asyncCompile(fn)
	}
	compilequeue = nil
	wg.Wait()
	base.Ctxt.InParallel = false
	types.CalcSizeDisabled = false
}