gasm-sdk/asm/elf_dwarf_sections.go

// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: BSD-3-Clause

package asm

// dwarfELFSections holds the laid-out DWARF sections ready for inclusion
// in an ELF file.
type dwarfELFSections struct {
	abbrevOff, abbrevSize   int
	infoOff, infoSize       int
	lineOff, lineSize       int
	lineStrOff, lineStrSize int
	frameOff, frameSize     int
	// Relocations for .debug_info address references.
	infoRelocs []elfDwarfReloc
	// Relocations for .debug_line address references.
	lineRelocs []elfDwarfReloc
}

type elfDwarfReloc struct {
	off    uint64
	sym    int // symbol index in .symtab
	addend int64
}

// appendDWARFSections generates and appends DWARF5 debug sections to the ELF
// output.  It returns the section offsets/sizes and relocations for the caller
// to emit section headers and relocation records.
//
// symIdx maps function names to their .symtab indices (needed for relocations
// against .text symbols).  The map uses objectName format (pkg.name); the
// DWARF code uses bare function names, so we build a reverse lookup.
func appendDWARFSections(out *[]byte, img *Image, srcFile string, symIdx map[string]int, align func(int)) *dwarfELFSections {
	// Build a lookup from bare function name to symbol index.
	nameToIdx := make(map[string]int, len(symIdx))
	for name, idx := range symIdx {
		// Strip package prefix: "pkg.name" → "name".
		if i := len(name) - 1; i >= 0 {
			for j := len(name) - 1; j >= 0; j-- {
				if name[j] == '.' {
					nameToIdx[name[j+1:]] = idx
					break
				}
			}
		}
		nameToIdx[name] = idx
	}
	ds := emitDWARF(img, srcFile)
	if ds == nil || len(ds.debugAbbrev) == 0 {
		return nil
	}

	result := &dwarfELFSections{}

	// .debug_abbrev
	align(1)
	result.abbrevOff = len(*out)
	result.abbrevSize = len(ds.debugAbbrev)
	*out = append(*out, ds.debugAbbrev...)

	// .debug_line_str
	align(1)
	result.lineStrOff = len(*out)
	result.lineStrSize = len(ds.debugLineStr)
	*out = append(*out, ds.debugLineStr...)

	// .debug_line
	align(1)
	result.lineOff = len(*out)
	result.lineSize = len(ds.debugLine)
	lineBase := len(*out)
	*out = append(*out, ds.debugLine...)

	// Patch .debug_line relocations: replace placeholder addresses with
	// actual .text offsets via symbol lookup.
	for _, dr := range ds.lineRelocs {
		if idx, ok := nameToIdx[dr.name]; ok {
			result.lineRelocs = append(result.lineRelocs, elfDwarfReloc{
				off:    uint64(lineBase) + dr.off,
				sym:    idx,
				addend: dr.addend,
			})
		}
	}

	// .debug_info
	align(1)
	result.infoOff = len(*out)
	result.infoSize = len(ds.debugInfo)
	infoBase := len(*out)
	*out = append(*out, ds.debugInfo...)

	// .debug_frame
	if len(ds.debugFrame) > 0 {
		align(1)
		result.frameOff = len(*out)
		result.frameSize = len(ds.debugFrame)
		*out = append(*out, ds.debugFrame...)
	}

	// Patch .debug_info relocations.
	for _, dr := range ds.infoRelocs {
		if idx, ok := nameToIdx[dr.name]; ok {
			result.infoRelocs = append(result.infoRelocs, elfDwarfReloc{
				off:    uint64(infoBase) + dr.off,
				sym:    idx,
				addend: dr.addend,
			})
		}
	}

	return result
}

// dwarfSectionNames returns the DWARF section names for the string table.
var dwarfSectionNames = []string{
	".debug_abbrev", ".debug_info", ".debug_line", ".debug_line_str",
	".debug_frame", ".rela.debug_info", ".rela.debug_line",
}