cmd/link: enable DWARF combining on macOS ARM64

It appears the machoCalcStart function is meant to align the
segment, but it doesn't. Replace it with an actual alignment
calculation. Also, use the alignment from the configuration,
instead of hardcode.

With this fix we could enable DWARF combining on macOS ARM64.

Change-Id: I19ec771b77d752b83a54c53b6ee65af78a31b8ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/253558
Reviewed-by: Than McIntosh <[email protected]>

Author: cherrymui

src/cmd/link/internal/arm64/obj.go

@@ -105,7 +105,7 @@ func archinit(ctxt *ld.Link) {
 			*ld.FlagTextAddr = 4096 + int64(ld.HEADR)
 		}
 		if *ld.FlagRound == -1 {
-			*ld.FlagRound = 4096
+			*ld.FlagRound = 16384 // 16K page alignment
 		}
 	}
 }

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

@@ -1240,11 +1240,11 @@ func (ctxt *Link) hostlink() {
 
 	// On darwin, whether to combine DWARF into executable.
 	// Only macOS supports unmapped segments such as our __DWARF segment.
-	combineDwarf := ctxt.IsDarwin() && !*FlagS && !*FlagW && !debug_s && machoPlatform == PLATFORM_MACOS && ctxt.IsAMD64()
+	combineDwarf := ctxt.IsDarwin() && !*FlagS && !*FlagW && !debug_s && machoPlatform == PLATFORM_MACOS
 
 	switch ctxt.HeadType {
 	case objabi.Hdarwin:
-		if machoPlatform == PLATFORM_MACOS && ctxt.IsAMD64() {
+		if combineDwarf {
 			// Leave room for DWARF combining.
 			// -headerpad is incompatible with -fembed-bitcode.
 			argv = append(argv, "-Wl,-headerpad,1144")

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

@@ -16,10 +16,6 @@ import (
 	"unsafe"
 )
 
-const (
-	pageAlign = 12 // 4096 = 1 << 12
-)
-
 type loadCmd struct {
 	Cmd macho.LoadCmd
 	Len uint32
@@ -138,7 +134,7 @@ func machoCombineDwarf(ctxt *Link, exef *os.File, exem *macho.File, dsym, outexe
 	// Now copy the dwarf data into the output.
 	// Kernel requires all loaded segments to be page-aligned in the file,
 	// even though we mark this one as being 0 bytes of virtual address space.
-	dwarfstart := machoCalcStart(realdwarf.Offset, linkseg.Offset, pageAlign)
+	dwarfstart := Rnd(int64(linkseg.Offset), int64(*FlagRound))
 	if _, err := outf.Seek(dwarfstart, 0); err != nil {
 		return err
 	}
@@ -166,7 +162,7 @@ func machoCombineDwarf(ctxt *Link, exef *os.File, exem *macho.File, dsym, outexe
 	if _, err := exef.Seek(int64(linkseg.Offset), 0); err != nil {
 		return err
 	}
-	linkstart := machoCalcStart(linkseg.Offset, uint64(dwarfstart)+dwarfsize, pageAlign)
+	linkstart := Rnd(dwarfstart+int64(dwarfsize), int64(*FlagRound))
 	if _, err := outf.Seek(linkstart, 0); err != nil {
 		return err
 	}
@@ -432,12 +428,3 @@ func machoUpdateLoadCommand(r loadCmdReader, linkseg *macho.Segment, linkoffset
 	}
 	return nil
 }
-
-func machoCalcStart(origAddr, newAddr uint64, alignExp uint32) int64 {
-	align := uint64(1 << alignExp)
-	origMod, newMod := origAddr%align, newAddr%align
-	if origMod == newMod {
-		return int64(newAddr)
-	}
-	return int64(newAddr + align + origMod - newMod)
-}