runtime: add execution tracer v2 behind GOEXPERIMENT=exectracer2

This change mostly implements the design described in #60773 and
includes a new scalable parser for the new trace format, available in
internal/trace/v2. I'll leave this commit message short because this is
clearly an enormous CL with a lot of detail.

This change does not hook up the new tracer into cmd/trace yet. A
follow-up CL will handle that.

For #60773.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-linux-amd64-longtest-race
Change-Id: I5d2aca2cc07580ed3c76a9813ac48ec96b157de0
Reviewed-on: https://go-review.googlesource.com/c/go/+/494187
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 145 insertions, 0 deletions

diff --git a/src/runtime/trace2map.go b/src/runtime/trace2map.go
new file mode 100644
index 0000000000..4a5a7ecba4
--- /dev/null
+++ b/src/runtime/trace2map.go
@@ -0,0 +1,145 @@
+// Copyright 2023 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.
+
+//go:build goexperiment.exectracer2
+
+// Simple hash table for tracing. Provides a mapping
+// between variable-length data and a unique ID. Subsequent
+// puts of the same data will return the same ID.
+//
+// Uses a region-based allocation scheme and assumes that the
+// table doesn't ever grow very big.
+//
+// This is definitely not a general-purpose hash table! It avoids
+// doing any high-level Go operations so it's safe to use even in
+// sensitive contexts.
+
+package runtime
+
+import (
+	"runtime/internal/atomic"
+	"runtime/internal/sys"
+	"unsafe"
+)
+
+type traceMap struct {
+	lock mutex // Must be acquired on the system stack
+	seq  atomic.Uint64
+	mem  traceRegionAlloc
+	tab  [1 << 13]atomic.UnsafePointer // *traceMapNode (can't use generics because it's notinheap)
+}
+
+type traceMapNode struct {
+	_    sys.NotInHeap
+	link atomic.UnsafePointer // *traceMapNode (can't use generics because it's notinheap)
+	hash uintptr
+	id   uint64
+	data []byte
+}
+
+// next is a type-safe wrapper around link.
+func (n *traceMapNode) next() *traceMapNode {
+	return (*traceMapNode)(n.link.Load())
+}
+
+// stealID steals an ID from the table, ensuring that it will not
+// appear in the table anymore.
+func (tab *traceMap) stealID() uint64 {
+	return tab.seq.Add(1)
+}
+
+// put inserts the data into the table.
+//
+// It's always safe to noescape data because its bytes are always copied.
+//
+// Returns a unique ID for the data and whether this is the first time
+// the data has been added to the map.
+func (tab *traceMap) put(data unsafe.Pointer, size uintptr) (uint64, bool) {
+	if size == 0 {
+		return 0, false
+	}
+	hash := memhash(data, 0, size)
+	// First, search the hashtable w/o the mutex.
+	if id := tab.find(data, size, hash); id != 0 {
+		return id, false
+	}
+	// Now, double check under the mutex.
+	// Switch to the system stack so we can acquire tab.lock
+	var id uint64
+	var added bool
+	systemstack(func() {
+		lock(&tab.lock)
+		if id = tab.find(data, size, hash); id != 0 {
+			unlock(&tab.lock)
+			return
+		}
+		// Create new record.
+		id = tab.seq.Add(1)
+		vd := tab.newTraceMapNode(data, size, hash, id)
+
+		// Insert it into the table.
+		//
+		// Update the link first, since the node isn't published yet.
+		// Then, store the node in the table as the new first node
+		// for the bucket.
+		part := int(hash % uintptr(len(tab.tab)))
+		vd.link.StoreNoWB(tab.tab[part].Load())
+		tab.tab[part].StoreNoWB(unsafe.Pointer(vd))
+		unlock(&tab.lock)
+
+		added = true
+	})
+	return id, added
+}
+
+// find looks up data in the table, assuming hash is a hash of data.
+//
+// Returns 0 if the data is not found, and the unique ID for it if it is.
+func (tab *traceMap) find(data unsafe.Pointer, size, hash uintptr) uint64 {
+	part := int(hash % uintptr(len(tab.tab)))
+	for vd := tab.bucket(part); vd != nil; vd = vd.next() {
+		// Synchronization not necessary. Once published to the table, these
+		// values are immutable.
+		if vd.hash == hash && uintptr(len(vd.data)) == size {
+			if memequal(unsafe.Pointer(&vd.data[0]), data, size) {
+				return vd.id
+			}
+		}
+	}
+	return 0
+}
+
+// bucket is a type-safe wrapper for looking up a value in tab.tab.
+func (tab *traceMap) bucket(part int) *traceMapNode {
+	return (*traceMapNode)(tab.tab[part].Load())
+}
+
+func (tab *traceMap) newTraceMapNode(data unsafe.Pointer, size, hash uintptr, id uint64) *traceMapNode {
+	// Create data array.
+	sl := notInHeapSlice{
+		array: tab.mem.alloc(size),
+		len:   int(size),
+		cap:   int(size),
+	}
+	memmove(unsafe.Pointer(sl.array), data, size)
+
+	// Create metadata structure.
+	meta := (*traceMapNode)(unsafe.Pointer(tab.mem.alloc(unsafe.Sizeof(traceMapNode{}))))
+	*(*notInHeapSlice)(unsafe.Pointer(&meta.data)) = sl
+	meta.id = id
+	meta.hash = hash
+	return meta
+}
+
+// reset drops all allocated memory from the table and resets it.
+//
+// tab.lock must be held. Must run on the system stack because of this.
+//
+//go:systemstack
+func (tab *traceMap) reset() {
+	assertLockHeld(&tab.lock)
+	tab.mem.drop()
+	tab.seq.Store(0)
+	tab.tab = [1 << 13]atomic.UnsafePointer{}
+}