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[lld-macho] Don't fold subsections with symbols at nonzero offsets 

Symbols occur at non-zero offsets in a subsection if they are
`.alt_entry` symbols, or if `.subsections_via_symbols` is omitted.

It doesn't seem like ld64 supports folding those subsections either.
Moreover, supporting this it makes `foldIdentical` a lot more
complicated to implement. The existing implementation has some
questionable behavior around STABS omission -- if a section with an
non-zero offset symbol was folded into one without, we would omit the
STABS entry for the non-zero offset symbol.

I will be following up with a diff that makes `foldIdentical` zero out
the symbol sizes for folded symbols. Again, this is much easier to
implement if we don't have to worry about non-zero offsets.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D136000
This commit is contained in:
Jez Ng 2022-10-18 17:21:39 -04:00
parent 181b04d276
commit 16d784159f
5 changed files with 18 additions and 30 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
lld/MachO/ICF.cpp
View File

@ -425,8 +425,8 @@ void macho::foldIdenticalSections(bool onlyCfStrings) {
bool isFoldable = (!onlyCfStrings || isCfStringSection(isec)) &&
(isCodeSection(isec) || isFoldableWithAddendsRemoved ||
isGccExceptTabSection(isec)) &&
!isec->keepUnique && !isec->shouldOmitFromOutput() &&
hasFoldableFlags;
!isec->keepUnique && !isec->hasAltEntry &&
!isec->shouldOmitFromOutput() && hasFoldableFlags;
if (isFoldable) {
foldable.push_back(isec);
for (Defined *d : isec->symbols)

1
lld/MachO/InputFiles.cpp
View File

@ -854,6 +854,7 @@ void ObjFile::parseSymbols(ArrayRef<typename LP::section> sectionHeaders,
// 4. If we have a literal section (e.g. __cstring and __literal4).
if (!subsectionsViaSymbols || symbolOffset == 0 ||
sym.n_desc & N_ALT_ENTRY || !isa<ConcatInputSection>(isec)) {
isec->hasAltEntry = symbolOffset != 0;
symbols[symIndex] = createDefined(sym, name, isec, symbolOffset,
symbolSize, forceHidden);
continue;

24
lld/MachO/InputSection.cpp
View File

@ -141,31 +141,15 @@ void ConcatInputSection::foldIdentical(ConcatInputSection *copy) {
for (auto &copySym : copy->symbols)
copySym->wasIdenticalCodeFolded = true;
// Merge the sorted vectors of symbols together.
auto it = symbols.begin();
for (auto copyIt = copy->symbols.begin(); copyIt != copy->symbols.end();) {
if (it == symbols.end()) {
symbols.push_back(*copyIt++);
it = symbols.end();
} else if ((*it)->value > (*copyIt)->value) {
std::swap(*it++, *copyIt);
} else {
++it;
}
}
symbols.insert(symbols.end(), copy->symbols.begin(), copy->symbols.end());
copy->symbols.clear();
// Remove duplicate compact unwind info for symbols at the same address.
if (symbols.empty())
return;
it = symbols.begin();
uint64_t v = (*it)->value;
for (++it; it != symbols.end(); ++it) {
Defined *d = *it;
if (d->value == v)
d->unwindEntry = nullptr;
else
v = d->value;
for (auto it = symbols.begin() + 1; it != symbols.end(); ++it) {
assert((*it)->value == 0);
(*it)->unwindEntry = nullptr;
}
}

12
lld/MachO/InputSection.h
View File

@ -64,11 +64,12 @@ public:
protected:
InputSection(Kind kind, const Section &section, ArrayRef<uint8_t> data,
uint32_t align)
: sectionKind(kind), align(align), data(data), section(section) {}
: sectionKind(kind), keepUnique(false), hasAltEntry(false), align(align),
data(data), section(section) {}
InputSection(const InputSection &rhs)
: sectionKind(rhs.sectionKind), align(rhs.align), data(rhs.data),
section(rhs.section) {}
: sectionKind(rhs.sectionKind), keepUnique(false), hasAltEntry(false),
align(rhs.align), data(rhs.data), section(rhs.section) {}
Kind sectionKind;
@ -77,7 +78,10 @@ public:
bool isFinal = false;
// keep the address of the symbol(s) in this section unique in the final
// binary ?
bool keepUnique = false;
bool keepUnique : 1;
// Does this section have symbols at offsets other than zero? (NOTE: only
// applies to ConcatInputSections.)
bool hasAltEntry : 1;
uint32_t align = 1;
OutputSection *parent = nullptr;

7
lld/test/MachO/icf.s
View File

@ -25,7 +25,7 @@
# CHECK: [[#%x,DYLIB_REF_4:]] l F __TEXT,__text _dylib_ref_4
# CHECK: [[#%x,ALT:]] l F __TEXT,__text _alt
# CHECK: [[#%x,WITH_ALT_ENTRY:]] l F __TEXT,__text _with_alt_entry
# CHECK: [[#%x,WITH_ALT_ENTRY]] l F __TEXT,__text _no_alt_entry
# CHECK: [[#%x,NO_ALT_ENTRY:]] l F __TEXT,__text _no_alt_entry
# CHECK: [[#%x,DEFINED_REF_WITH_ADDEND_2:]] l F __TEXT,__text _defined_ref_with_addend_1
# CHECK: [[#%x,DEFINED_REF_WITH_ADDEND_2]] l F __TEXT,__text _defined_ref_with_addend_2
# CHECK: [[#%x,DEFINED_REF_WITH_ADDEND_3:]] l F __TEXT,__text _defined_ref_with_addend_3
@ -74,7 +74,7 @@
# CHECK: callq 0x[[#%x,DYLIB_REF_4]] <_dylib_ref_4>
# CHECK: callq 0x[[#%x,ALT]] <_alt>
# CHECK: callq 0x[[#%x,WITH_ALT_ENTRY]] <_with_alt_entry>
# CHECK: callq 0x[[#%x,WITH_ALT_ENTRY]] <_with_alt_entry>
# CHECK: callq 0x[[#%x,NO_ALT_ENTRY]] <_no_alt_entry>
# CHECK: callq 0x[[#%x,DEFINED_REF_WITH_ADDEND_2]] <_defined_ref_with_addend_2>
# CHECK: callq 0x[[#%x,DEFINED_REF_WITH_ADDEND_2]] <_defined_ref_with_addend_2>
# CHECK: callq 0x[[#%x,DEFINED_REF_WITH_ADDEND_3]] <_defined_ref_with_addend_3>
@ -161,8 +161,7 @@ _dylib_ref_4:
mov ___nan + 1@GOTPCREL(%rip), %rax
callq ___inf + 1
## We can merge two sections even if one of them has an alt entry. Just make
## sure we don't merge the alt entry symbol with a regular symbol.
## Sections with alt entries cannot be merged.
.alt_entry _alt
_with_alt_entry:
movq $3132, %rax