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Force SHF_MERGE optimizations with -r. 

Some tools (dwarfdump for example) get confused by the current -O0 -r
output since it has multiple copies of .debug_str.

We cannot just merge sections with the same name as they can have
different sh_entsize.

We could have duplicated logic for merging sections based on name and
sh_entsize, but it seems better to just use the existing logic by
enabling optimizations.

llvm-svn: 328640
This commit is contained in:
Rafael Espindola 2018-03-27 17:09:23 +00:00
parent 0a15d24134
commit 35aad41c1b
2 changed files with 61 additions and 3 deletions
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16
lld/ELF/InputFiles.cpp
View File

@ -312,9 +312,19 @@ ObjFile<ELFT>::getShtGroupEntries(const Elf_Shdr &Sec) {
}
template <class ELFT> bool ObjFile<ELFT>::shouldMerge(const Elf_Shdr &Sec) {
// We don't merge sections if -O0 (default is -O1). This makes sometimes
// the linker significantly faster, although the output will be bigger.
if (Config->Optimize == 0)
// On a regular link we don't merge sections if -O0 (default is -O1). This
// sometimes makes the linker significantly faster, although the output will
// be bigger.
//
// Doing the same for -r would create a problem as it would combine sections
// with different sh_entsize. One option would be to just copy every SHF_MERGE
// section as is to the output. While this would produce a valid ELF file with
// usable SHF_MERGE sections, tools like (llvm-)?dwarfdump get confused when
// they see two .debug_str. We could have separate logic for combining
// SHF_MERGE sections based both on their name and sh_entsize, but that seems
// to be more trouble than it is worth. Instead, we just use the regular (-O1)
// logic for -r.
if (Config->Optimize == 0 && !Config->Relocatable)
return false;
// A mergeable section with size 0 is useless because they don't have

48
lld/test/ELF/merge-reloc-O0.s Normal file
View File

@ -0,0 +1,48 @@
# REQUIRES: x86
# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
# RUN: ld.lld %t.o -r -o %t2.o -O0
# RUN: llvm-readobj -s -section-data %t2.o | FileCheck %s
# We combine just the sections with the same name and sh_entsize.
# CHECK: Name: .foo
# CHECK-NEXT: Type: SHT_PROGBITS
# CHECK-NEXT: Flags [
# CHECK-NEXT: SHF_ALLOC
# CHECK-NEXT: SHF_MERGE
# CHECK-NEXT: ]
# CHECK-NEXT: Address:
# CHECK-NEXT: Offset:
# CHECK-NEXT: Size: 16
# CHECK-NEXT: Link:
# CHECK-NEXT: Info:
# CHECK-NEXT: AddressAlignment: 8
# CHECK-NEXT: EntrySize: 8
# CHECK-NEXT: SectionData (
# CHECK-NEXT: 0000: 41000000 00000000 42000000 00000000
# CHECK-NEXT: )
# CHECK: Name: .foo
# CHECK-NEXT: Type: SHT_PROGBITS
# CHECK-NEXT: Flags [
# CHECK-NEXT: SHF_ALLOC
# CHECK-NEXT: SHF_MERGE
# CHECK-NEXT: ]
# CHECK-NEXT: Address:
# CHECK-NEXT: Offset:
# CHECK-NEXT: Size: 8
# CHECK-NEXT: Link:
# CHECK-NEXT: Info:
# CHECK-NEXT: AddressAlignment: 4
# CHECK-NEXT: EntrySize: 4
# CHECK-NEXT: SectionData (
# CHECK-NEXT: 0000: 41000000 42000000
# CHECK-NEXT: )
.section .foo, "aM",@progbits,8,unique,0
.quad 0x41
.section .foo, "aM",@progbits,8,unique,1
.quad 0x42
.section .foo, "aM",@progbits,4,unique,2
.long 0x41
.long 0x42