[ELF] --gdb-index: support --icf={safe,all}

[MaskRay]

Summary

The combination has not been tested before. In the case of ICF,
e.section->getVA(0) equals the start address of the output section.

This can cause incorrect overlapping with the actual function at the
start of the output section and potentially trigger a GDB internal error
in dw2_find_pc_sect_compunit_symtab (presumably because:
if a short address range incorrectly starts at the start address of the
output section, GDB may pick it instead of the correct longer address
range. When mapping an address within the long address range but
out of the scope of the short address range, the routine may find
nothing - while the code asserts that it can find something).

Note that in the case of ICF there may be duplicate address range entries,
but GDB appears to be fine with them.

[grimar]

LGTM

lld/test/ELF/gdb-index-icf.s

29

# -> ##?

[MaskRay]

Thanks!

lld/test/ELF/gdb-index-icf.s

29

I think we should loose the requirement for comment markers starting in the middle of a line.
This is basically clang -S -g produced assembly with a lot of clean-ups. Comments starting in the middle of a line make them immediately stand out, so ## is probably not needed.

lld/ELF/SyntheticSections.cpp

void GdbIndexSection::writeTo(uint8_t *buf) {
   }
 
   // Write the address area.
   hdr->cuTypesOff = buf - start;
   hdr->addressAreaOff = buf - start;
   uint32_t cuOff = 0;
   for (GdbChunk &chunk : chunks) {
     for (AddressEntry &e : chunk.addressAreas) {
-      uint64_t baseAddr = e.section->getVA(0);
+      // In the case of ICF there may be duplicate address range entries.
+      const uint64_t baseAddr = e.section->repl->getVA(0);
       write64le(buf, baseAddr + e.lowAddress);
       write64le(buf + 8, baseAddr + e.highAddress);
       write32le(buf + 16, e.cuIndex + cuOff);
       buf += 20;
     }
     cuOff += chunk.compilationUnits.size();
   }
 

lld/test/ELF/gdb-index-icf.s

+# REQUIRES: x86
+## Test that the address range contributed by an ICF folded function is identical
+## to that of the folded-in function. Not considering ICF may lead to an address
+## range whose low address equals the start address of the output section.
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+# RUN: ld.lld --gdb-index --icf=all -Ttext=0x1000 %t.o -o %t
+# RUN: llvm-dwarfdump --gdb-index %t | FileCheck %s
+
+# CHECK:      Address area offset = 0x38, has 2 entries:
+# CHECK-NEXT:   Low/High address = [0x1001, 0x1002) (Size: 0x1), CU id = 0
+# CHECK-NEXT:   Low/High address = [0x1001, 0x1002) (Size: 0x1), CU id = 1
+
+.text
+nop
+
+.section .text.0,"ax"
+.Lfunc_begin0:
+  ret
+.Lfunc_end0:
+
+.section .text.1,"ax"
+.Lfunc_begin1:
+  ret
+.Lfunc_end1:
+
+.section  .debug_abbrev,"",@progbits
+  .byte  1                         # Abbreviation Code
+  .byte  65                        # DW_TAG_type_unit
+  .byte  0                         # DW_CHILDREN_no
+  .byte  0                         # EOM(1)
+  .byte  0                         # EOM(2)
+
+  .byte  2                         # Abbreviation Code
+  .byte  17                        # DW_TAG_compile_unit
+  .byte  0                         # DW_CHILDREN_no
+  .byte  17                        # DW_AT_low_pc
+  .byte  1                         # DW_FORM_addr
+  .byte  18                        # DW_AT_high_pc
+  .byte  6                         # DW_FORM_data4
+  .byte  0                         # EOM(1)
+  .byte  0                         # EOM(2)
+
+  .byte  0                         # EOM(3)
+
+.section  .debug_info,"",@progbits
+.Lcu_begin0:
+  .long .Lcu_end0-.Lcu_begin0-4    # Length of Unit
+  .short 5                         # DWARF version number
+  .byte  1                         # DWARF Unit Type
+  .byte  8                         # Address Size
+  .long  .debug_abbrev             # Offset Into Abbrev. Section
+  .byte  2                         # Abbrev [2] DW_TAG_compile_unit
+  .quad  .Lfunc_begin0             # DW_AT_low_pc
+  .long  .Lfunc_end0-.Lfunc_begin0 # DW_AT_high_pc
+.Lcu_end0:
+
+.Lcu_begin1:
+  .long .Lcu_end1-.Lcu_begin1-4    # Length of Unit
+  .short 5                         # DWARF version number
+  .byte  1                         # DWARF Unit Type
+  .byte  8                         # Address Size
+  .long  .debug_abbrev             # Offset Into Abbrev. Section
+  .byte  2                         # Abbrev [2] DW_TAG_compile_unit
+  .quad  .Lfunc_begin1             # DW_AT_low_pc
+  .long  .Lfunc_end1-.Lfunc_begin1 # DW_AT_high_pc
+.Lcu_end1: