[dev.link] cmd/link: add PPC64 debugging option to encourage text section splits

Add a new debugging command line option (-debugppc64textsize=N) that
forces the start of a new text section after ".text" hits N bytes as
opposed to the architected limit of 2^26. This is intended to enable
testing of the linker code paths that handle multiple .text sections
on PPC64 without resorting to building giant applications.

Updates #20492.

Change-Id: I74ab7fd1e412e9124de5bd0d8d248c5e73225ae3
Reviewed-on: https://go-review.googlesource.com/c/go/+/241073
Run-TryBot: Than McIntosh <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Jeremy Faller <[email protected]>

Author: thanm

src/cmd/link/internal/ld/data.go

@@ -2247,37 +2247,52 @@ func assignAddress(ctxt *Link, sect *sym.Section, n int, s loader.Sym, va uint64
 
 	// Only break at outermost syms.
 
-	if ctxt.Arch.InFamily(sys.PPC64) && ldr.OuterSym(s) == 0 && ctxt.IsExternal() && va-sect.Vaddr+funcsize+maxSizeTrampolinesPPC64(ldr, s, isTramp) > 0x1c00000 {
-		// Set the length for the previous text section
-		sect.Length = va - sect.Vaddr
-
-		// Create new section, set the starting Vaddr
-		sect = addsection(ctxt.loader, ctxt.Arch, &Segtext, ".text", 05)
-		sect.Vaddr = va
-		ldr.SetSymSect(s, sect)
-
-		// Create a symbol for the start of the secondary text sections
-		ntext := ldr.CreateSymForUpdate(fmt.Sprintf("runtime.text.%d", n), 0)
-		ntext.SetSect(sect)
-		if ctxt.IsAIX() {
-			// runtime.text.X must be a real symbol on AIX.
-			// Assign its address directly in order to be the
-			// first symbol of this new section.
-			ntext.SetType(sym.STEXT)
-			ntext.SetSize(int64(MINFUNC))
-			ntext.SetOnList(true)
-			ctxt.tramps = append(ctxt.tramps, ntext.Sym())
-
-			ntext.SetValue(int64(va))
-			va += uint64(ntext.Size())
-
-			if align := ldr.SymAlign(s); align != 0 {
-				va = uint64(Rnd(int64(va), int64(align)))
-			} else {
-				va = uint64(Rnd(int64(va), int64(Funcalign)))
+	// For debugging purposes, allow text size limit to be cranked down,
+	// so as to stress test the code that handles multiple text sections.
+	var textSizelimit uint64 = 0x1c00000
+	if *FlagDebugTextSize != 0 {
+		textSizelimit = uint64(*FlagDebugTextSize)
+	}
+
+	if ctxt.Arch.InFamily(sys.PPC64) && ldr.OuterSym(s) == 0 && ctxt.IsExternal() {
+		// Sanity check: make sure the limit is larger than any
+		// individual text symbol.
+		if funcsize > textSizelimit {
+			panic(fmt.Sprintf("error: ppc64 text size limit %d less than text symbol %s size of %d", textSizelimit, ldr.SymName(s), funcsize))
+		}
+
+		if va-sect.Vaddr+funcsize+maxSizeTrampolinesPPC64(ldr, s, isTramp) > textSizelimit {
+			// Set the length for the previous text section
+			sect.Length = va - sect.Vaddr
+
+			// Create new section, set the starting Vaddr
+			sect = addsection(ctxt.loader, ctxt.Arch, &Segtext, ".text", 05)
+			sect.Vaddr = va
+			ldr.SetSymSect(s, sect)
+
+			// Create a symbol for the start of the secondary text sections
+			ntext := ldr.CreateSymForUpdate(fmt.Sprintf("runtime.text.%d", n), 0)
+			ntext.SetSect(sect)
+			if ctxt.IsAIX() {
+				// runtime.text.X must be a real symbol on AIX.
+				// Assign its address directly in order to be the
+				// first symbol of this new section.
+				ntext.SetType(sym.STEXT)
+				ntext.SetSize(int64(MINFUNC))
+				ntext.SetOnList(true)
+				ctxt.tramps = append(ctxt.tramps, ntext.Sym())
+
+				ntext.SetValue(int64(va))
+				va += uint64(ntext.Size())
+
+				if align := ldr.SymAlign(s); align != 0 {
+					va = uint64(Rnd(int64(va), int64(align)))
+				} else {
+					va = uint64(Rnd(int64(va), int64(Funcalign)))
+				}
 			}
+			n++
 		}
-		n++
 	}
 
 	ldr.SetSymValue(s, 0)

src/cmd/link/internal/ld/main.go

@@ -74,22 +74,23 @@ var (
 	flagExtldflags = flag.String("extldflags", "", "pass `flags` to external linker")
 	flagExtar      = flag.String("extar", "", "archive program for buildmode=c-archive")
 
-	flagA           = flag.Bool("a", false, "no-op (deprecated)")
-	FlagC           = flag.Bool("c", false, "dump call graph")
-	FlagD           = flag.Bool("d", false, "disable dynamic executable")
-	flagF           = flag.Bool("f", false, "ignore version mismatch")
-	flagG           = flag.Bool("g", false, "disable go package data checks")
-	flagH           = flag.Bool("h", false, "halt on error")
-	flagN           = flag.Bool("n", false, "dump symbol table")
-	FlagS           = flag.Bool("s", false, "disable symbol table")
-	FlagW           = flag.Bool("w", false, "disable DWARF generation")
-	flag8           bool // use 64-bit addresses in symbol table
-	flagInterpreter = flag.String("I", "", "use `linker` as ELF dynamic linker")
-	FlagDebugTramp  = flag.Int("debugtramp", 0, "debug trampolines")
-	FlagStrictDups  = flag.Int("strictdups", 0, "sanity check duplicate symbol contents during object file reading (1=warn 2=err).")
-	FlagRound       = flag.Int("R", -1, "set address rounding `quantum`")
-	FlagTextAddr    = flag.Int64("T", -1, "set text segment `address`")
-	flagEntrySymbol = flag.String("E", "", "set `entry` symbol name")
+	flagA             = flag.Bool("a", false, "no-op (deprecated)")
+	FlagC             = flag.Bool("c", false, "dump call graph")
+	FlagD             = flag.Bool("d", false, "disable dynamic executable")
+	flagF             = flag.Bool("f", false, "ignore version mismatch")
+	flagG             = flag.Bool("g", false, "disable go package data checks")
+	flagH             = flag.Bool("h", false, "halt on error")
+	flagN             = flag.Bool("n", false, "dump symbol table")
+	FlagS             = flag.Bool("s", false, "disable symbol table")
+	FlagW             = flag.Bool("w", false, "disable DWARF generation")
+	flag8             bool // use 64-bit addresses in symbol table
+	flagInterpreter   = flag.String("I", "", "use `linker` as ELF dynamic linker")
+	FlagDebugTramp    = flag.Int("debugtramp", 0, "debug trampolines")
+	FlagDebugTextSize = flag.Int("debugppc64textsize", 0, "debug PPC64 text section max")
+	FlagStrictDups    = flag.Int("strictdups", 0, "sanity check duplicate symbol contents during object file reading (1=warn 2=err).")
+	FlagRound         = flag.Int("R", -1, "set address rounding `quantum`")
+	FlagTextAddr      = flag.Int64("T", -1, "set text segment `address`")
+	flagEntrySymbol   = flag.String("E", "", "set `entry` symbol name")
 
 	cpuprofile     = flag.String("cpuprofile", "", "write cpu profile to `file`")
 	memprofile     = flag.String("memprofile", "", "write memory profile to `file`")