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Add support for generating uninitialized sections #306

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merged 1 commit into from
Nov 8, 2022
Merged

Add support for generating uninitialized sections #306

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30 changes: 22 additions & 8 deletions llvm/tools/objwriter/objwriter.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -243,11 +243,16 @@ MCSection *ObjectWriter::GetSpecificSection(const char *SectionName,
const char *ComdatName) {
Triple TheTriple(TripleName);
MCSection *Section = nullptr;
SectionKind Kind = (attributes & CustomSectionAttributes_Executable)
? SectionKind::getText()
: (attributes & CustomSectionAttributes_Writeable)
? SectionKind::getData()
: SectionKind::getReadOnly();
SectionKind Kind;
if (attributes & CustomSectionAttributes_Executable)
Kind = SectionKind::getText();
else if (attributes & CustomSectionAttributes_Uninitialized)
Kind = SectionKind::getBSS();
else if (attributes & CustomSectionAttributes_Writeable)
Kind = SectionKind::getData();
else
Kind = SectionKind::getReadOnly();

switch (TheTriple.getObjectFormat()) {
case Triple::MachO: {
unsigned typeAndAttributes = 0;
Expand All @@ -260,6 +265,9 @@ MCSection *ObjectWriter::GetSpecificSection(const char *SectionName,
// boundaries.
typeAndAttributes |= MachO::S_ATTR_PURE_INSTRUCTIONS;
}
if (attributes & CustomSectionAttributes_Uninitialized) {
typeAndAttributes |= MachO::S_ZEROFILL;
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Is zerofill needed for correctness or just MachO wants that for uninitialized data?

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@MichalStrehovsky MichalStrehovsky Nov 7, 2022 •
edited
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Without that we were generating a lot of zeros in the object file I generated.

It does the same thing as COFF::IMAGE_SCN_CNT_UNINITIALIZED_DATA for PE and ELF::SHT_NOBITS for ELF below. But yeah, I don't know why LLVM has both these format specific enums and SectionKind above.

I haven't actually tried end-to-end on mac, but the object file looks about right now in llvm-objdump.

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I see. I am not familiar with MachO at this level. I guess it needs to fill the space with zeroes one way or another. So it is either zeroes in the obj file or a fill it with zeroes at load/demand time. The flag could be selecting that.
Makes sense.

}
Section = OutContext->getMachOSection(
(attributes & CustomSectionAttributes_Executable) ? "__TEXT" : "__DATA",
SectionName, typeAndAttributes, Kind);
Expand All @@ -271,8 +279,11 @@ MCSection *ObjectWriter::GetSpecificSection(const char *SectionName,
if (attributes & CustomSectionAttributes_Executable) {
Characteristics |= COFF::IMAGE_SCN_CNT_CODE | COFF::IMAGE_SCN_MEM_EXECUTE;
} else if (attributes & CustomSectionAttributes_Writeable) {
Characteristics |=
COFF::IMAGE_SCN_CNT_INITIALIZED_DATA | COFF::IMAGE_SCN_MEM_WRITE;
Characteristics |= COFF::IMAGE_SCN_MEM_WRITE;
if (attributes & CustomSectionAttributes_Uninitialized)
Characteristics |= COFF::IMAGE_SCN_CNT_UNINITIALIZED_DATA;
else
Characteristics |= COFF::IMAGE_SCN_CNT_INITIALIZED_DATA;
} else {
Characteristics |= COFF::IMAGE_SCN_CNT_INITIALIZED_DATA;
}
Expand All @@ -299,8 +310,11 @@ MCSection *ObjectWriter::GetSpecificSection(const char *SectionName,
} else if (attributes & CustomSectionAttributes_Writeable) {
Flags |= ELF::SHF_WRITE;
}
unsigned SectionType = (attributes & CustomSectionAttributes_Uninitialized)
? ELF::SHT_NOBITS
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for BSS section the SHT_NOBITS should be combined with SHF_ALLOC, to be present in memory, which we get from line 301 above and SHF_WRITE, which we get because we set the writeable attribute in object writer at:

              case SectionType.Uninitialized:
                    attributes |= CustomSectionAttributes.Uninitialized | CustomSectionAttributes.Writeable;
                    break;

Looks right to me.

: ELF::SHT_PROGBITS;
Section =
OutContext->getELFSection(SectionName, ELF::SHT_PROGBITS, Flags, 0,
OutContext->getELFSection(SectionName, SectionType, Flags, 0,
ComdatName != nullptr ? ComdatName : "");
break;
}
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1 change: 1 addition & 0 deletions llvm/tools/objwriter/objwriter.h
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Original file line number Diff line number Diff line change
Expand Up @@ -44,6 +44,7 @@ enum CustomSectionAttributes : int32_t {
CustomSectionAttributes_ReadOnly = 0x0000,
CustomSectionAttributes_Writeable = 0x0001,
CustomSectionAttributes_Executable = 0x0002,
CustomSectionAttributes_Uninitialized = 0x0004,
CustomSectionAttributes_MachO_Init_Func_Pointers = 0x0100,
};

Expand Down