[lld] 4d9020c - [ELF] Implement --force-group-allocation
via llvm-commits
llvm-commits at lists.llvm.org
Fri Jun 7 14:19:09 PDT 2024
Author: Fangrui Song
Date: 2024-06-07T14:19:06-07:00
New Revision: 4d9020ca0b125a936d5e26b48f07fcfa4bd78004
URL: https://github.com/llvm/llvm-project/commit/4d9020ca0b125a936d5e26b48f07fcfa4bd78004
DIFF: https://github.com/llvm/llvm-project/commit/4d9020ca0b125a936d5e26b48f07fcfa4bd78004.diff
LOG: [ELF] Implement --force-group-allocation
GNU ld's relocatable linking behaviors:
* Sections with the `SHF_GROUP` flag are handled like sections matched
by the `--unique=pattern` option. They are processed like orphan
sections and ignored by input section descriptions.
* Section groups' (usually named `.group`) content is updated as the
section indexes are updated. Section groups can be discarded with
`/DISCARD/ : { *(.group) }`.
`-r --force-group-allocation` discards section groups and allows
sections with the `SHF_GROUP` flag to be matched like normal sections.
If two section group members are placed into the same output section,
their relocation sections (if present) are combined as well.
This behavior can be useful when -r output is used as a pseudo shared
object (e.g., FreeBSD's amd64 kernel modules, CHERIoT compartments).
This patch implements --force-group-allocation:
* Input SHT_GROUP sections are discarded.
* Input sections do not get the SHF_GROUP flag, so `addInputSec`
will combine relocation sections if their relocated section group
members are combined.
The default behavior is:
* Input SHT_GROUP sections are retained.
* Input SHF_GROUP sections can be matched (unlike GNU ld)
* Input SHF_GROUP sections keep the SHF_GROUP flag, so `addInputSec`
will create different OutputDesc copies.
GNU ld provides the `FORCE_GROUP_ALLOCATION` command, which is not
implemented.
Pull Request: https://github.com/llvm/llvm-project/pull/94704
Added:
Modified:
lld/ELF/Config.h
lld/ELF/Driver.cpp
lld/ELF/InputFiles.cpp
lld/ELF/InputSection.cpp
lld/ELF/Options.td
lld/docs/ReleaseNotes.rst
lld/docs/ld.lld.1
lld/test/ELF/comdat.s
lld/test/ELF/relocatable-comdat.s
Removed:
################################################################################
diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
index 883c4a2f84294..0173be396163e 100644
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -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;
diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index ddc574a11314b..ed214ab29d4c6 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -1384,6 +1384,8 @@ 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) || args.hasArg(OPT_force_group_allocation);
if (args.hasArg(OPT_save_temps)) {
// --save-temps implies saving all temps.
diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index d760dddcf5ec5..9021bbd91b5f7 100644
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -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;
diff --git a/lld/ELF/InputSection.cpp b/lld/ELF/InputSection.cpp
index e6c5996c0b392..f4287bc94ee5e 100644
--- a/lld/ELF/InputSection.cpp
+++ b/lld/ELF/InputSection.cpp
@@ -76,12 +76,12 @@ 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 without
+// --force-group-allocation, the SHF_GROUP flag 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;
}
diff --git a/lld/ELF/Options.td b/lld/ELF/Options.td
index ff61a566f52f7..d10174681f5eb 100644
--- a/lld/ELF/Options.td
+++ b/lld/ELF/Options.td
@@ -254,6 +254,9 @@ 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 placed to the same output section, combine their relocations as well">;
+
defm format: Eq<"format", "Change the input format of the inputs following this option">,
MetaVarName<"[default,elf,binary]">;
diff --git a/lld/docs/ReleaseNotes.rst b/lld/docs/ReleaseNotes.rst
index e7a913e025da9..79ddc159a945f 100644
--- a/lld/docs/ReleaseNotes.rst
+++ b/lld/docs/ReleaseNotes.rst
@@ -44,6 +44,10 @@ ELF Improvements
(typical for embedded). It also makes full LTO feasible in such cases, since
IR merging currently prevents the linker script from referring to input
files. (`#90007 <https://github.com/llvm/llvm-project/pull/90007>`_)
+* ``--force-group-allocation`` is implemented to discard ``SHT_GROUP`` sections
+ and combine relocation sections if their relocated section group members are
+ placed to the same output section.
+ (`#94704 <https://github.com/llvm/llvm-project/pull/94704>`_)
Breaking changes
----------------
diff --git a/lld/docs/ld.lld.1 b/lld/docs/ld.lld.1
index da3b926d02a28..bdc35c08ccb8e 100644
--- a/lld/docs/ld.lld.1
+++ b/lld/docs/ld.lld.1
@@ -278,6 +278,8 @@ 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 placed to the same output section, combine their relocations as well.
.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
diff --git a/lld/test/ELF/comdat.s b/lld/test/ELF/comdat.s
index a9a5f5fa3bb62..32dfc91cf5509 100644
--- a/lld/test/ELF/comdat.s
+++ b/lld/test/ELF/comdat.s
@@ -4,6 +4,7 @@
// RUN: ld.lld -shared %t.o %t2.o -o %t
// RUN: llvm-objdump -d %t | FileCheck %s
// RUN: llvm-readelf -S -s %t | FileCheck --check-prefix=READ %s
+// RUN: ld.lld -shared --force-group-allocation %t.o %t2.o -o - | cmp - %t
// Check that we don't crash with --gc-section and that we print a list of
// reclaimed sections on stderr.
diff --git a/lld/test/ELF/relocatable-comdat.s b/lld/test/ELF/relocatable-comdat.s
index 45ca9fb7a2484..c69e4928509d4 100644
--- a/lld/test/ELF/relocatable-comdat.s
+++ b/lld/test/ELF/relocatable-comdat.s
@@ -47,6 +47,16 @@
# 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
+
+# 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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