[ELF] Implement --force-group-allocation #94704
Merged
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Created using spr 1.3.5-bogner
|
@llvm/pr-subscribers-lld-elf @llvm/pr-subscribers-lld Author: Fangrui Song (MaskRay) ChangesGNU ld's relocatable linking behaviors:
This patch implements --force-group-allocation:
--inhibit-group-allocation restores the default behavior:
GNU ld provides the Full diff: https://github.com/llvm/llvm-project/pull/94704.diff 7 Files Affected:
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..e8442cc993821 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -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) ||
+ args.hasFlag(OPT_force_group_allocation,
+ OPT_inhibit_group_allocation, false);
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..0f176f144d46b 100644
--- a/lld/ELF/InputSection.cpp
+++ b/lld/ELF/InputSection.cpp
@@ -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;
}
diff --git a/lld/ELF/Options.td b/lld/ELF/Options.td
index ff61a566f52f7..23385c76182eb 100644
--- a/lld/ELF/Options.td
+++ b/lld/ELF/Options.td
@@ -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]">;
diff --git a/lld/docs/ld.lld.1 b/lld/docs/ld.lld.1
index da3b926d02a28..82d590a1d084f 100644
--- a/lld/docs/ld.lld.1
+++ b/lld/docs/ld.lld.1
@@ -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
diff --git a/lld/test/ELF/relocatable-comdat.s b/lld/test/ELF/relocatable-comdat.s
index 45ca9fb7a2484..8ca8f107f885e 100644
--- a/lld/test/ELF/relocatable-comdat.s
+++ b/lld/test/ELF/relocatable-comdat.s
@@ -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
|
smithp35
reviewed
Jun 7, 2024
lld/ELF/Driver.cpp
Outdated
| @@ -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) || | |||
There was a problem hiding this comment.
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.
There was a problem hiding this comment.
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.
|
First, thank you so much for doing this. :) I applied this patch to 17.0.6 to compare behavior. The group sections portion appears to work as expected, but I'm confused by one aspect of the relocation section combining: The combined RELA section gets the name This seems uncontrollable by linker script either, for example: |
Np:)
You need commit b8dface and 0930f62 to make the Static relocation sections copied to the output due to -r/--emit-relocs cannot be matched by input section descriptions. BTW, you can remove |
Created using spr 1.3.5-bogner
smithp35
approved these changes
Jun 7, 2024
Created using spr 1.3.5-bogner
MaskRay
deleted the
users/MaskRay/spr/elf-implement-force-group-allocation
branch
nekoshirro
pushed a commit
to nekoshirro/Alchemist-LLVM
that referenced
this pull request
Jun 9, 2024
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: llvm#94704
Signed-off-by: Hafidz Muzakky <[email protected]>
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
GNU ld's relocatable linking behaviors:
SHF_GROUPflag are handled like sections matchedby the
--unique=patternoption. They are processed like orphansections and ignored by input section descriptions.
.group) content is updated as thesection indexes are updated. Section groups can be discarded with
/DISCARD/ : { *(.group) }.-r --force-group-allocationdiscards section groups and allowssections with the
SHF_GROUPflag 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:
addInputSecwill combine relocation sections if their relocated section group
members are combined.
The default behavior is:
addInputSecwill create different OutputDesc copies.
GNU ld provides the
FORCE_GROUP_ALLOCATIONcommand, which is notimplemented.