This is an archive of the discontinued LLVM Phabricator instance.

Differential D33647

[ELF] - Linkerscript: implement NOLOAD section type.
ClosedPublic

Authored by grimar on May 29 2017, 6:45 AM.

Details

Reviewers
ruiu
• rafael
Commits
rGfbb0463f3988: [ELF] - Linkerscript: implement NOLOAD section type.
rLLD304925: [ELF] - Linkerscript: implement NOLOAD section type.
rL304925: [ELF] - Linkerscript: implement NOLOAD section type.
Summary

This is PR32351

Each output section may have a type. The type is a keyword in parentheses. The following types are defined:
NOLOAD
The section should be marked as not loadable, so that it will not be loaded into memory when the program is run.
(https://sourceware.org/binutils/docs/ld/Output-Section-Type.html#Output-Section-Type)

Parch makes such sections to be SHT_NOBITS. That looks consistent with what BFD do. Gold do something different now,
what looks like a wrong implementation simply.

Use case is next (taken from https://mcuoneclipse.com/2014/04/19/gnu-linker-can-you-not-initialize-my-variable/):

  1. We want to place variable at absolute address and use next code:
static unsigned char Var[4096] __attribute__((section (".data_noload_a")));
  1. Our script looks like:
.data_noload_a 0x2000000 : { *(.data_noload_a) }

Problem is that Var will stop be SHT_NOBITS and became SHT_PROGBITS.
(if attribute used with section, the variable is treated like an initialized variable, see article above)
We may want to save space, and do not write 4096 of zeroes to output file. NOLOAD is a solution.

NOLOAD forces output section to be SHT_NOBITS.

Diff Detail

Repository
rL LLVM

Event Timeline

grimar created this revision.May 29 2017, 6:45 AM
Herald added a subscriber: emaste. · View Herald TranscriptMay 29 2017, 6:45 AM
grimar added a parent revision: D33646: [ELF] - Do not crash when linkerscript applies fill to .bss..May 29 2017, 6:45 AM
grimar updated this revision to Diff 100666.May 29 2017, 11:24 PM
  • Addressed review comments.
grimar edited the summary of this revision. (Show Details)May 29 2017, 11:27 PM
ruiu added inline comments.Jun 5 2017, 3:11 PM
ELF/LinkerScript.h
138 ↗(On Diff #100666)

Noload is probably a better name.

ELF/ScriptParser.cpp
575 ↗(On Diff #100666)

Read a section type. Currently, only NOLOAD is supported.

577 ↗(On Diff #100666)

You can move this above if (peek() != ":"), no?

grimar updated this revision to Diff 101516.Jun 6 2017, 12:06 AM
  • Addressed review comments.
ELF/LinkerScript.h
138 ↗(On Diff #100666)

Done.

ELF/ScriptParser.cpp
575 ↗(On Diff #100666)

Done.

577 ↗(On Diff #100666)

Do you mean something like this (new diff) ?

ruiu added inline comments.Jun 6 2017, 9:07 AM
ELF/ScriptParser.cpp
574 ↗(On Diff #101516)

Why can't you do peek() != "("?

grimar updated this revision to Diff 101744.Jun 7 2017, 8:33 AM
  • Addressed review comments.
ELF/ScriptParser.cpp
574 ↗(On Diff #101516)

My mistype/mistake =\ Sure we can do that.

ruiu accepted this revision.Jun 7 2017, 9:15 AM

LGTM

ELF/LinkerScript.cpp
436–437 ↗(On Diff #101744)

Move this before SecToCmd[Sec] = Cmd; so that assignments to Sec->{SectionIndex,Type} are next to each other in the source code.

This revision is now accepted and ready to land.Jun 7 2017, 9:15 AM
Closed by commit rL304925: [ELF] - Linkerscript: implement NOLOAD section type. (authored by grimar). · Explain WhyJun 7 2017, 9:31 AM
This revision was automatically updated to reflect the committed changes.

Revision Contents

PathSize
lld/
trunk/
ELF/
1 line
2 lines
18 lines
test/
ELF/
linkerscript/
46 lines

Diff 101757

lld/trunk/ELF/LinkerScript.h

Show First 20 Lines • Show All 128 Lines • ▼ Show 20 Linesstruct OutputSectionCommand : BaseCommand {
Expr LMAExpr; Expr LMAExpr;
Expr SubalignExpr; Expr SubalignExpr;
std::vector<BaseCommand *> Commands; std::vector<BaseCommand *> Commands;
std::vector<StringRef> Phdrs; std::vector<StringRef> Phdrs;
llvm::Optional<uint32_t> Filler; llvm::Optional<uint32_t> Filler;
ConstraintKind Constraint = ConstraintKind::NoConstraint; ConstraintKind Constraint = ConstraintKind::NoConstraint;
std::string Location; std::string Location;
std::string MemoryRegionName; std::string MemoryRegionName;
bool Noload = false;
template <class ELFT> void finalize(); template <class ELFT> void finalize();
template <class ELFT> void writeTo(uint8_t *Buf); template <class ELFT> void writeTo(uint8_t *Buf);
template <class ELFT> void maybeCompress(); template <class ELFT> void maybeCompress();
uint32_t getFiller(); uint32_t getFiller();
}; };
// This struct represents one section match pattern in SECTIONS() command. // This struct represents one section match pattern in SECTIONS() command.
▲ Show 20 Lines • Show All 155 Lines • Show Last 20 Lines

lld/trunk/ELF/LinkerScript.cpp

Show First 20 Lines • Show All 425 Lines • ▼ Show 20 Lines if (auto *Cmd = dyn_cast<OutputSectionCommand>(Opt.Commands[I])) {
} }
// Add input sections to an output section. // Add input sections to an output section.
for (InputSectionBase *S : V) for (InputSectionBase *S : V)
Factory.addInputSec(S, Cmd->Name, Cmd->Sec); Factory.addInputSec(S, Cmd->Name, Cmd->Sec);
if (OutputSection *Sec = Cmd->Sec) { if (OutputSection *Sec = Cmd->Sec) {
assert(Sec->SectionIndex == INT_MAX); assert(Sec->SectionIndex == INT_MAX);
Sec->SectionIndex = I; Sec->SectionIndex = I;
if (Cmd->Noload)
Sec->Type = SHT_NOBITS;
SecToCommand[Sec] = Cmd; SecToCommand[Sec] = Cmd;
} }
} }
} }
CurOutSec = nullptr; CurOutSec = nullptr;
} }
void LinkerScript::fabricateDefaultCommands() { void LinkerScript::fabricateDefaultCommands() {
▲ Show 20 Lines • Show All 797 Lines • Show Last 20 Lines

lld/trunk/ELF/ScriptParser.cpp

Show First 20 Lines • Show All 562 Lines • ▼ Show 20 Linesuint32_t ScriptParser::readFill() {
return V; return V;
} }
OutputSectionCommand * OutputSectionCommand *
ScriptParser::readOutputSectionDescription(StringRef OutSec) { ScriptParser::readOutputSectionDescription(StringRef OutSec) {
OutputSectionCommand *Cmd = OutputSectionCommand *Cmd =
Script->createOutputSectionCommand(OutSec, getCurrentLocation()); Script->createOutputSectionCommand(OutSec, getCurrentLocation());
if (peek() != ":") {
// Read an address expression. // Read an address expression.
// https://sourceware.org/binutils/docs/ld/Output-Section-Address.html // https://sourceware.org/binutils/docs/ld/Output-Section-Address.html
if (peek() != ":") if (peek() != "(")
Cmd->AddrExpr = readExpr(); Cmd->AddrExpr = readExpr();
// Read a section type. Currently, only NOLOAD is supported.
// https://sourceware.org/binutils/docs/ld/Output-Section-Type.html
if (consume("(")) {
expect("NOLOAD");
expect(")");
Cmd->Noload = true;
}
}
expect(":"); expect(":");
if (consume("AT")) if (consume("AT"))
Cmd->LMAExpr = readParenExpr(); Cmd->LMAExpr = readParenExpr();
if (consume("ALIGN")) if (consume("ALIGN"))
Cmd->AlignExpr = readParenExpr(); Cmd->AlignExpr = readParenExpr();
if (consume("SUBALIGN")) if (consume("SUBALIGN"))
Cmd->SubalignExpr = readParenExpr(); Cmd->SubalignExpr = readParenExpr();
▲ Show 20 Lines • Show All 628 Lines • Show Last 20 Lines

lld/trunk/test/ELF/linkerscript/noload.s

# REQUIRES: x86
# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
# RUN: echo "SECTIONS { \
# RUN: .data_noload_a (NOLOAD) : { *(.data_noload_a) } \
# RUN: .data_noload_b 0x10000 (NOLOAD) : { *(.data_noload_b) } };" > %t.script
# RUN: ld.lld -o %t --script %t.script %t.o
# RUN: llvm-readobj --symbols -sections %t
# CHECK: Section {
# CHECK-NEXT: Index: 2
# CHECK-NEXT: Name: .data_noload_a
# CHECK-NEXT: Type: SHT_NOBITS
# CHECK-NEXT: Flags [
# CHECK-NEXT: SHF_ALLOC
# CHECK-NEXT: SHF_WRITE
# CHECK-NEXT: ]
# CHECK-NEXT: Address: 0x0
# CHECK-NEXT: Offset: 0x1000
# CHECK-NEXT: Size: 4096
# CHECK-NEXT: Link: 0
# CHECK-NEXT: Info: 0
# CHECK-NEXT: AddressAlignment: 1
# CHECK-NEXT: EntrySize: 0
# CHECK-NEXT: }
# CHECK-NEXT: Section {
# CHECK-NEXT: Index: 3
# CHECK-NEXT: Name: .data_noload_b
# CHECK-NEXT: Type: SHT_NOBITS
# CHECK-NEXT: Flags [
# CHECK-NEXT: SHF_ALLOC
# CHECK-NEXT: SHF_WRITE
# CHECK-NEXT: ]
# CHECK-NEXT: Address: 0x10000
# CHECK-NEXT: Offset: 0x1000
# CHECK-NEXT: Size: 4096
# CHECK-NEXT: Link: 0
# CHECK-NEXT: Info: 0
# CHECK-NEXT: AddressAlignment: 1
# CHECK-NEXT: EntrySize: 0
# CHECK-NEXT: }
.section .data_noload_a,"aw",@progbits
.zero 4096
.section .data_noload_b,"aw",@progbits
.zero 4096