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[ELF] Implement --force-group-allocation #94704

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Merged
merged 3 commits into from
Jun 7, 2024
Merged

[ELF] Implement --force-group-allocation #94704

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880b593
[𝘀𝗽𝗿] initial version
MaskRay Jun 7, 2024
c178000
remove --inhibit-group-allocation. improve test
MaskRay Jun 7, 2024
8927236
add URL to release note
MaskRay Jun 7, 2024
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1 change: 1 addition & 0 deletions lld/ELF/Config.h
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Expand Up @@ -286,6 +286,7 @@ struct Config {
bool relax;
bool relaxGP;
bool relocatable;
bool resolveGroups;
bool relrGlibc = false;
bool relrPackDynRelocs = false;
llvm::DenseSet<llvm::StringRef> saveTempsArgs;
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3 changes: 3 additions & 0 deletions lld/ELF/Driver.cpp
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Expand Up @@ -1384,6 +1384,9 @@ static void readConfigs(opt::InputArgList &args) {
config->relaxGP = args.hasFlag(OPT_relax_gp, OPT_no_relax_gp, false);
config->rpath = getRpath(args);
config->relocatable = args.hasArg(OPT_relocatable);
config->resolveGroups = !args.hasArg(OPT_relocatable) ||
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I'm not sure I've understood the benefit of --inhibit-group-allocation.

  • For a non relocatable link it is ignored.
  • For a relocatable link its behaviour is the default state, so there is no need to explicitly use it.

The only reason I can think to use it if there is an existing linker command-line that with --force-group-allocation that I want to disable. I don't think that is likely though.

Unless I've missed something, would it be better to keep the command-line interface simple with

config->resolveGroups = !args.hasArg(OPT_relocatable) ||
args.hasFlag(OPT_force_group_allocation, false);

I couldn't find it in the GNU ld documentation.

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I was thinking that --inhibit-group-allocation could be kept to document the default behavior. I agree that it's not useful and I will remove it.

args.hasFlag(OPT_force_group_allocation,
OPT_inhibit_group_allocation, false);

if (args.hasArg(OPT_save_temps)) {
// --save-temps implies saving all temps.
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2 changes: 1 addition & 1 deletion lld/ELF/InputFiles.cpp
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Expand Up @@ -676,7 +676,7 @@ template <class ELFT> void ObjFile<ELFT>::parse(bool ignoreComdats) {
symtab.comdatGroups.try_emplace(CachedHashStringRef(signature), this)
.second;
if (keepGroup) {
if (config->relocatable)
if (!config->resolveGroups)
this->sections[i] = createInputSection(
i, sec, check(obj.getSectionName(sec, shstrtab)));
continue;
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9 changes: 5 additions & 4 deletions lld/ELF/InputSection.cpp
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Expand Up @@ -76,12 +76,13 @@ InputSectionBase::InputSectionBase(InputFile *file, uint64_t flags,
invokeELFT(parseCompressedHeader,);
}

// Drop SHF_GROUP bit unless we are producing a re-linkable object file.
// SHF_GROUP is a marker that a section belongs to some comdat group.
// That flag doesn't make sense in an executable.
// SHF_INFO_LINK and SHF_GROUP are normally resolved and not copied to the
// output section. However, for relocatable linking with the default
// --inhibit-group-allocation, the SHF_GROUP marker and section groups are
// retained.
static uint64_t getFlags(uint64_t flags) {
flags &= ~(uint64_t)SHF_INFO_LINK;
if (!config->relocatable)
if (config->resolveGroups)
flags &= ~(uint64_t)SHF_GROUP;
return flags;
}
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5 changes: 5 additions & 0 deletions lld/ELF/Options.td
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Expand Up @@ -254,6 +254,11 @@ def fix_cortex_a53_843419: F<"fix-cortex-a53-843419">,
def fix_cortex_a8: F<"fix-cortex-a8">,
HelpText<"Apply fixes for ARM Cortex-A8 erratum 657417">;

def force_group_allocation: FF<"force-group-allocation">,
HelpText<"Only meaningful for -r. Section groups are discarded. If two section group members are combined into the same output section, combine their relocations as well">;
def inhibit_group_allocation: FF<"inhibit-group-allocation">,
HelpText<"This is the default for -r. Section groups are retained. Section group members' relocations are not combined">;

defm format: Eq<"format", "Change the input format of the inputs following this option">,
MetaVarName<"[default,elf,binary]">;

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4 changes: 4 additions & 0 deletions lld/docs/ld.lld.1
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Expand Up @@ -278,6 +278,10 @@ Set the
field to the specified value.
.It Fl -fini Ns = Ns Ar symbol
Specify a finalizer function.
.It Fl -force-group-allocation
Only meaningful for -r. Section groups are discarded. If two section group members are combined into the same output section, combine their relocations as well.
.It Fl -inhibit-group-allocation
This is the default for -r. Section groups are retained. Section group members' relocations are not combined.
.It Fl -format Ns = Ns Ar input-format , Fl b Ar input-format
Specify the format of the inputs following this option.
.Ar input-format
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12 changes: 12 additions & 0 deletions lld/test/ELF/relocatable-comdat.s
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Expand Up @@ -47,6 +47,18 @@
# COMBINE-NEXT: .rela.text
# COMBINE-NEXT: .rela.text

## If --force-group-allocation is specified, discard .group and combine .rela.* if their relocated sections are combined.
# RUN: ld.lld -r -T combine.lds a.o a.o --force-group-allocation -o combine-a.ro
# RUN: llvm-readelf -g -S combine-a.ro | FileCheck %s --check-prefix=COMBINE-A
## --inhibit-group-allocation restores the default behavior.
# RUN: ld.lld -r -T combine.lds a.o a.o --force-group-allocation --inhibit-group-allocation -o - | cmp - combine.ro

# COMBINE-A: Name Type Address Off Size ES Flg Lk Inf Al
# COMBINE-A: .rodata PROGBITS 0000000000000000 {{.*}} 000002 00 A 0 0 1
# COMBINE-A-NEXT: .text PROGBITS 0000000000000000 {{.*}} 000010 00 AX 0 0 4
# COMBINE-A-NEXT: .rela.text RELA 0000000000000000 {{.*}} 000030 18 I [[#]] [[#]] 8
# COMBINE-A-NEXT: .note.GNU-stack

# RUN: echo 'SECTIONS { /DISCARD/ : {*(.rodata.*)} }' > discard-rodata.lds
# RUN: ld.lld -r -T discard-rodata.lds a.o a.o -o discard-rodata.ro
# RUN: llvm-readelf -g -S discard-rodata.ro | FileCheck %s --check-prefix=NO-RODATA
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