cmd/compile: add control flow graphs to ssa.html

This CL adds CFGs to ssa.html.
It execs dot to generate SVG,
which then gets inlined into the html.
Some standard naming and javascript hacks
enable integration with the rest of ssa.html.
Clicking on blocks highlights the relevant
part of the CFG, and vice versa.

Sample output and screenshots can be seen in #28177.

CFGs can be turned on with the suffix mask:
:*            - dump CFG for every phase
:lower        - just the lower phase
:lower-layout - lower through layout
:w,x-y        - phases w and x through y

Calling dot after every pass is noticeably slow,
instead use the range of phases.

Dead blocks are not displayed on CFG.

User can zoom and pan individual CFG
when the automatic adjustment has failed.

Dot-related errors are reported
without bringing down the process.

Fixes #28177

Change-Id: Id52c42d86c4559ca737288aa10561b67a119c63d
Reviewed-on: https://go-review.googlesource.com/c/142517
Run-TryBot: Yury Smolsky <yury@smolsky.by>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -438,10 +438,17 @@ func Main(archInit func(*Arch)) {
 	}
 
 	ssaDump = os.Getenv("GOSSAFUNC")
+	if ssaDump != "" {
 		if strings.HasSuffix(ssaDump, "+") {
 			ssaDump = ssaDump[:len(ssaDump)-1]
 			ssaDumpStdout = true
 		}
+		spl := strings.Split(ssaDump, ":")
+		if len(spl) > 1 {
+			ssaDump = spl[0]
+			ssaDumpCFG = spl[1]
+		}
+	}
 
 	trackScopes = flagDWARF
 

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

@@ -26,6 +26,7 @@ var ssaCaches []ssa.Cache
 
 var ssaDump string     // early copy of $GOSSAFUNC; the func name to dump output for
 var ssaDumpStdout bool // whether to dump to stdout
+var ssaDumpCFG string  // generate CFGs for these phases
 const ssaDumpFile = "ssa.html"
 
 // ssaDumpInlined holds all inlined functions when ssaDump contains a function name.
@@ -155,7 +156,7 @@ func buildssa(fn *Node, worker int) *ssa.Func {
 	s.softFloat = s.config.SoftFloat
 
 	if printssa {
-		s.f.HTMLWriter = ssa.NewHTMLWriter(ssaDumpFile, s.f.Frontend(), name)
+		s.f.HTMLWriter = ssa.NewHTMLWriter(ssaDumpFile, s.f.Frontend(), name, ssaDumpCFG)
 		// TODO: generate and print a mapping from nodes to values and blocks
 		dumpSourcesColumn(s.f.HTMLWriter, fn)
 		s.f.HTMLWriter.WriteAST("AST", astBuf)

src/cmd/compile/internal/ssa/func.go

@@ -43,6 +43,7 @@ type Func struct {
 	PrintOrHtmlSSA bool        // true if GOSSAFUNC matches, true even if fe.Log() (spew phase results to stdout) is false.
 
 	scheduled bool // Values in Blocks are in final order
+	laidout   bool // Blocks are ordered
 	NoSplit   bool // true if function is marked as nosplit.  Used by schedule check pass.
 
 	// when register allocation is done, maps value ids to locations

src/cmd/compile/internal/ssa/layout.go

@@ -143,5 +143,7 @@ blockloop:
 			}
 		}
 	}
+	f.laidout = true
 	return order
+	//f.Blocks = order
 }