[PATCH] D34800: [ELF] Remove unused synthetic sections from script commands

[Rafael Avila de Espindola via llvm-commits]

Is anything blocking committing this?

Cheers,
Rafael

Rafael Avila de Espindola <rafael.espindola at gmail.com> writes:

> LGTM, thanks!
>
> Cheers,
> Rafael
>
> Petr Hosek via Phabricator via llvm-commits
> <llvm-commits at lists.llvm.org> writes:
>
>> phosek created this revision.
>> phosek added a project: lld.
>> Herald added a subscriber: emaste.
>>
>> Script commands are processed before unused synthetic sections are
>> removed. Therefore, if a linker script matches one of these sections
>> it'll get emitted as an empty output section because the logic for
>> removing unused synthetic sections ignores script commands which
>> could have already matched and captured one of these sections. This
>> patch fixes that by also removing the unused synthetic sections from
>> the script commands.
>>
>>
>> Repository:
>>   rL LLVM
>>
>> https://reviews.llvm.org/D34800
>>
>> Files:
>>   ELF/Writer.cpp
>>   test/ELF/linkerscript/unused-synthetic.s
>>
>>
>> Index: test/ELF/linkerscript/unused-synthetic.s
>> ===================================================================
>> --- /dev/null
>> +++ test/ELF/linkerscript/unused-synthetic.s
>> @@ -0,0 +1,18 @@
>> +# REQUIRES: x86
>> +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
>> +# RUN: echo "SECTIONS { \
>> +# RUN:    .got  : { *(.got) } \
>> +# RUN:    .plt  : { *(.plt) } \
>> +# RUN:    .text : { *(.text) } \
>> +# RUN:  }" > %t.script
>> +# RUN: ld.lld -shared -o %t.so --script %t.script %t.o
>> +
>> +# RUN: llvm-objdump -section-headers %t.so | FileCheck %s
>> +# CHECK-NOT:  .got
>> +# CHECK-NOT:  .plt
>> +# CHECK:      .text
>> +# CHECK-NEXT: .dynsym
>> +
>> +.global _start
>> +_start:
>> +  nop
>> Index: ELF/Writer.cpp
>> ===================================================================
>> --- ELF/Writer.cpp
>> +++ ELF/Writer.cpp
>> @@ -1149,8 +1149,17 @@
>>      SS->Live = false;
>>      // If there are no other sections in the output section, remove it from the
>>      // output.
>> -    if (OS->Sections.empty())
>> +    if (OS->Sections.empty()) {
>>        V.erase(std::find(V.begin(), V.end(), OS));
>> +      // Also remove script commands matching the output section.
>> +      auto &Cmds = Script->Opt.Commands;
>> +      auto I = std::remove_if(Cmds.begin(), Cmds.end(), [&](BaseCommand *Cmd) {
>> +        if (auto *OSCmd = dyn_cast<OutputSectionCommand>(Cmd))
>> +          return OSCmd->Sec == OS;
>> +        return false;
>> +      });
>> +      Cmds.erase(I, Cmds.end());
>> +    }
>>    }
>>  }
>>  
>>
>>
>> Index: test/ELF/linkerscript/unused-synthetic.s
>> ===================================================================
>> --- /dev/null
>> +++ test/ELF/linkerscript/unused-synthetic.s
>> @@ -0,0 +1,18 @@
>> +# REQUIRES: x86
>> +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
>> +# RUN: echo "SECTIONS { \
>> +# RUN:    .got  : { *(.got) } \
>> +# RUN:    .plt  : { *(.plt) } \
>> +# RUN:    .text : { *(.text) } \
>> +# RUN:  }" > %t.script
>> +# RUN: ld.lld -shared -o %t.so --script %t.script %t.o
>> +
>> +# RUN: llvm-objdump -section-headers %t.so | FileCheck %s
>> +# CHECK-NOT:  .got
>> +# CHECK-NOT:  .plt
>> +# CHECK:      .text
>> +# CHECK-NEXT: .dynsym
>> +
>> +.global _start
>> +_start:
>> +  nop
>> Index: ELF/Writer.cpp
>> ===================================================================
>> --- ELF/Writer.cpp
>> +++ ELF/Writer.cpp
>> @@ -1149,8 +1149,17 @@
>>      SS->Live = false;
>>      // If there are no other sections in the output section, remove it from the
>>      // output.
>> -    if (OS->Sections.empty())
>> +    if (OS->Sections.empty()) {
>>        V.erase(std::find(V.begin(), V.end(), OS));
>> +      // Also remove script commands matching the output section.
>> +      auto &Cmds = Script->Opt.Commands;
>> +      auto I = std::remove_if(Cmds.begin(), Cmds.end(), [&](BaseCommand *Cmd) {
>> +        if (auto *OSCmd = dyn_cast<OutputSectionCommand>(Cmd))
>> +          return OSCmd->Sec == OS;
>> +        return false;
>> +      });
>> +      Cmds.erase(I, Cmds.end());
>> +    }
>>    }
>>  }
>>  
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