OSchip
/
llvm-project
mirror of https://github.com/THU-DSP-LAB/llvm-project.git
13
0
Fork 2
Code Issues Packages Projects Releases Wiki Activity

[lld-macho] Private label aliases to weak symbols should not retain section data 

If we have two files with the same weak symbol like so:

```
ltmp0:
_weak:
  <contents>
```
and

```
ltmp1:
_weak:
  <contents>
```

Linking them together should leave only one copy of `<contents>`, not
two. Previously, we would keep around both copies because of the
private-label `ltmp<N>` symbols (i.e. symbols that start with `l`) -- we
would not coalesce those, so we would treat them as retaining the
contents.

This matters for more than just size -- we are depending upon this
behavior internally for emitting a certain file format. This file
format's header is repeated in each object file, but we want it to
appear just once in our output.

Why can't we not emit those aliases to `_weak`, or reference the
`ltmp<N>` symbols instead of `_weak`? Well, MC actually adds `ltmp<N>`
symbols as part of the assembly-to-binary translation step. So any
codegen at the clang level can't access them.

All that said... this solution is actually kind of hacky. Here, we avoid
creating the private-label symbols at parse time. This is acceptable
since we never emit those symbols in our output. However, in ld64, any
aliasing temporary symbols (ignored or otherwise) won't retain coalesced
data. But implementing this is harder -- we would have to create those
symbols first (so we can emit their names later), but we would have to
ensure the linker correctly shuffles them around when their aliasees get
coalesced.

Additionally, ld64 treats these temporary symbols as functionally
equivalent to the weak symbols themselves -- that is, it will emit weak
binds when those non-weak temporary aliases are referenced. We have
imitated this behavior for private-label symbols, but implementing it for
local aliases in general seems substantially more difficult. I'm not
sure if any programs actually depend on this behavior though, so maybe
it's a moot point.

Finally, ld64 does all this regardless of whether
`.subsections_via_symbols` is specified. We don't. But again, given how
rare the lack of that directive is (I've only seen it from hand-written
assembly inputs), I don't think we need to worry about it.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D139069
This commit is contained in:
Jez Ng 2022-12-01 12:01:07 -05:00
parent ee605b0acc
commit 6736bce6db
3 changed files with 101 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

30
lld/MachO/InputFiles.cpp
View File

@ -622,6 +622,12 @@ void ObjFile::parseRelocations(ArrayRef<SectionHeader> sectionHeaders,
}
}
// Symbols with `l` or `L` as a prefix are linker-private and never appear in
// the output.
static bool isPrivateLabel(StringRef name) {
return name.startswith("l") || name.startswith("L");
}
template <class NList>
static macho::Symbol *createDefined(const NList &sym, StringRef name,
InputSection *isec, uint64_t value,
@ -691,8 +697,7 @@ static macho::Symbol *createDefined(const NList &sym, StringRef name,
}
assert(!isWeakDefCanBeHidden &&
"weak_def_can_be_hidden on already-hidden symbol?");
bool includeInSymtab =
!name.startswith("l") && !name.startswith("L") && !isEhFrameSection(isec);
bool includeInSymtab = !isPrivateLabel(name) && !isEhFrameSection(isec);
return make<Defined>(
name, isec->getFile(), isec, value, size, sym.n_desc & N_WEAK_DEF,
/*isExternal=*/false, /*isPrivateExtern=*/false, includeInSymtab,
@ -825,17 +830,25 @@ void ObjFile::parseSymbols(ArrayRef<typename LP::section> sectionHeaders,
}
sections[i]->doneSplitting = true;
auto getSymName = [strtab](const NList& sym) -> StringRef {
return StringRef(strtab + sym.n_strx);
};
// Calculate symbol sizes and create subsections by splitting the sections
// along symbol boundaries.
// We populate subsections by repeatedly splitting the last (highest
// address) subsection.
llvm::stable_sort(symbolIndices, [&](uint32_t lhs, uint32_t rhs) {
// Put private-label symbols after other symbols at the same address.
if (nList[lhs].n_value == nList[rhs].n_value)
return !isPrivateLabel(getSymName(nList[lhs])) &&
isPrivateLabel(getSymName(nList[rhs]));
return nList[lhs].n_value < nList[rhs].n_value;
});
for (size_t j = 0; j < symbolIndices.size(); ++j) {
const uint32_t symIndex = symbolIndices[j];
const NList &sym = nList[symIndex];
StringRef name = strtab + sym.n_strx;
StringRef name = getSymName(sym);
Subsection &subsec = subsections.back();
InputSection *isec = subsec.isec;
@ -855,8 +868,15 @@ void ObjFile::parseSymbols(ArrayRef<typename LP::section> sectionHeaders,
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);
// If we have an private-label symbol that's an alias, just reuse the
// aliased symbol. Our sorting step above ensures that any such symbols
// will appear after the non-private-label ones. See
// weak-def-alias-ignored.s for the motivation behind this.
if (symbolOffset == 0 && isPrivateLabel(name) && j != 0)
symbols[symIndex] = symbols[symbolIndices[j - 1]];
else
symbols[symIndex] = createDefined(sym, name, isec, symbolOffset,
symbolSize, forceHidden);
continue;
}
auto *concatIsec = cast<ConcatInputSection>(isec);

2
lld/MachO/UnwindInfoSection.cpp
View File

@ -209,7 +209,7 @@ void UnwindInfoSection::addSymbol(const Defined *d) {
// If we have multiple symbols at the same address, only one of them can have
// an associated unwind entry.
if (!p.second && d->unwindEntry) {
assert(!p.first->second->unwindEntry);
assert(p.first->second == d || !p.first->second->unwindEntry);
p.first->second = d;
}
}

75
lld/test/MachO/weak-def-alias-private-label.s Normal file
View File

@ -0,0 +1,75 @@
# REQUIRES: x86
## This test checks that when we coalesce weak definitions, any private-label
## aliases to those weak defs don't cause the coalesced data to be retained.
## This test explicitly creates those private-label symbols, but it was actually
## motivated by MC's aarch64 backend which automatically creates them when
## emitting object files. I've chosen to explicitly create them here since we
## can then reference those symbols for a more complete test.
##
## Not retaining the data matters for more than just size -- we have a use case
## that depends on proper data coalescing to emit a valid file format.
##
## ld64 actually treats all local symbol aliases (not just the private ones) the
## same way. But implementing this is harder -- we would have to create those
## symbols first (so we can emit their names later), but we would have to
## ensure the linker correctly shuffles them around when their aliasees get
## coalesced. Emulating the behavior of weak binds for non-private symbols would
## be even trickier. Let's just deal with private-label symbols for now until we
## find a use case for more general local symbols.
##
## Finally, ld64 does all this regardless of whether .subsections_via_symbols is
## specified. We don't. But again, given how rare the lack of that directive is
## (I've only seen it from hand-written assembly inputs), I don't think we need
## to worry about it.
# RUN: rm -rf %t; split-file %s %t
# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/weak-then-private.s -o %t/weak-then-private.o
# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/private-then-weak.s -o %t/private-then-weak.o
# RUN: %lld -dylib %t/weak-then-private.o %t/private-then-weak.o -o %t/test1
# RUN: %lld -dylib %t/private-then-weak.o %t/weak-then-private.o -o %t/test2
# RUN: llvm-objdump --macho --syms --section="__DATA,__data" --weak-bind %t/test1 | FileCheck %s
# RUN: llvm-objdump --macho --syms --section="__DATA,__data" --weak-bind %t/test2 | FileCheck %s
## Check that we only have one copy of 0x123 in the data, not two.
# CHECK: Contents of (__DATA,__data) section
# CHECK-NEXT: 0000000000001000 23 01 00 00 00 00 00 00 00 10 00 00 00 00 00 00 {{$}}
# CHECK-NEXT: 0000000000001010 00 10 00 00 00 00 00 00 {{$}}
# CHECK-EMPTY:
# CHECK-NEXT: SYMBOL TABLE:
# CHECK-NEXT: 0000000000001008 l O __DATA,__data _ref
# CHECK-NEXT: 0000000000001010 l O __DATA,__data _ref
# CHECK-NEXT: 0000000000001000 w O __DATA,__data _weak
# CHECK-NEXT: 0000000000000000 *UND* dyld_stub_binder
# CHECK-EMPTY:
## Even though the references were to the non-weak `l_ignored` aliases, we
## should still emit weak binds as if they were the `_weak` symbol itself.
# CHECK-NEXT: Weak bind table:
# CHECK-NEXT: segment section address type addend symbol
# CHECK-NEXT: __DATA __data 0x00001008 pointer 0 _weak
# CHECK-NEXT: __DATA __data 0x00001010 pointer 0 _weak
#--- weak-then-private.s
.globl _weak
.weak_definition _weak
.data
_weak:
l_ignored:
.quad 0x123
_ref:
.quad l_ignored
.subsections_via_symbols
#--- private-then-weak.s
.globl _weak
.weak_definition _weak
.data
l_ignored:
_weak:
.quad 0x123
_ref:
.quad l_ignored
.subsections_via_symbols