[lld] bb69a66 - [lld-macho] Coalesce local symbol aliases along with their aliased weak def
Jez Ng via llvm-commits
llvm-commits at lists.llvm.org
Fri Mar 10 22:40:24 PST 2023
Author: Jez Ng
Date: 2023-03-11T01:40:14-05:00
New Revision: bb69a66ced27918f894fb0d5e58e22fda6958d99
URL: https://github.com/llvm/llvm-project/commit/bb69a66ced27918f894fb0d5e58e22fda6958d99
DIFF: https://github.com/llvm/llvm-project/commit/bb69a66ced27918f894fb0d5e58e22fda6958d99.diff
LOG: [lld-macho] Coalesce local symbol aliases along with their aliased weak def
This supersedes {D139069}. In some ways we are now closer to ld64's
behavior: we previously only did this coalescing for private-label
symbols, but now we do it for all locals, just like ld64. However, we no
longer generate weak binds when a local alias to a weak symbol is
referenced. This is merely for implementation simplicity; it's not clear
to me that any real-world programs depend on us emulating this behavior.
The problem with the previous approach is that we ended up with
duplicate references to the same symbol instance in our InputFiles,
which translated into duplicate symbols in our output. While we could
work around that problem by performing a dedup step before emitting the
symbol table, it seems cleaner to not generate duplicate references in
the first place.
Numbers for chromium_framework on my 16 Core Intel Mac Pro:
base diff difference (95% CI)
sys_time 2.243 ± 0.093 2.231 ± 0.066 [ -2.5% .. +1.4%]
user_time 6.529 ± 0.087 6.080 ± 0.050 [ -7.5% .. -6.3%]
wall_time 6.928 ± 0.175 6.474 ± 0.112 [ -7.7% .. -5.4%]
samples 26 31
Yep, that's a massive win... because it turns out that {D140606} and
{D139069} caused a regression (of about the same size.) I just didn't
think to measure them back then. I'm guessing all the extra symbols we
have been emitting did not help perf at all...
Reviewed By: lgrey
Differential Revision: https://reviews.llvm.org/D145455
Added:
lld/test/MachO/local-alias-to-weak.s
Modified:
lld/MachO/InputFiles.cpp
lld/MachO/SymbolTable.cpp
Removed:
lld/test/MachO/private-label-alias.s
################################################################################
diff --git a/lld/MachO/InputFiles.cpp b/lld/MachO/InputFiles.cpp
index ed0d98a2ececd..e0f3e0a4d4591 100644
--- a/lld/MachO/InputFiles.cpp
+++ b/lld/MachO/InputFiles.cpp
@@ -849,15 +849,12 @@ void ObjFile::parseSymbols(ArrayRef<typename LP::section> sectionHeaders,
// 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 that have no flags after other symbols at the
- // same address.
- StringRef lhsName = getSymName(nList[lhs]);
- StringRef rhsName = getSymName(nList[rhs]);
- if (nList[lhs].n_value == nList[rhs].n_value) {
- if (isPrivateLabel(lhsName) && isPrivateLabel(rhsName))
- return nList[lhs].n_desc > nList[rhs].n_desc;
- return !isPrivateLabel(lhsName) && isPrivateLabel(rhsName);
- }
+ // Put extern weak symbols after other symbols at the same address so
+ // that weak symbol coalescing works correctly. See
+ // SymbolTable::addDefined() for details.
+ if (nList[lhs].n_value == nList[rhs].n_value &&
+ nList[lhs].n_type & N_EXT && nList[rhs].n_type & N_EXT)
+ return !(nList[lhs].n_desc & N_WEAK_DEF) && (nList[rhs].n_desc & N_WEAK_DEF);
return nList[lhs].n_value < nList[rhs].n_value;
});
for (size_t j = 0; j < symbolIndices.size(); ++j) {
@@ -883,17 +880,8 @@ 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;
- // If we have an private-label symbol that's an alias, and that alias
- // doesn't have any flags of its own, then we can 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 &&
- sym.n_desc == 0)
- symbols[symIndex] = symbols[symbolIndices[j - 1]];
- else
- symbols[symIndex] = createDefined(sym, name, isec, symbolOffset,
- symbolSize, forceHidden);
+ symbols[symIndex] = createDefined(sym, name, isec, symbolOffset,
+ symbolSize, forceHidden);
continue;
}
auto *concatIsec = cast<ConcatInputSection>(isec);
diff --git a/lld/MachO/SymbolTable.cpp b/lld/MachO/SymbolTable.cpp
index 03631239ccba1..09b3d8a376e60 100644
--- a/lld/MachO/SymbolTable.cpp
+++ b/lld/MachO/SymbolTable.cpp
@@ -61,6 +61,38 @@ struct DuplicateSymbolDiag {
SmallVector<DuplicateSymbolDiag> dupSymDiags;
} // namespace
+// Move symbols at \p fromOff in \p fromIsec into \p toIsec, unless that symbol
+// is \p skip.
+static void transplantSymbolsAtOffset(InputSection *fromIsec,
+ InputSection *toIsec, Defined *skip,
+ uint64_t fromOff, uint64_t toOff) {
+ // Ensure the symbols will still be in address order after our insertions.
+ auto insertIt = llvm::upper_bound(toIsec->symbols, toOff,
+ [](uint64_t off, const Symbol *s) {
+ return cast<Defined>(s)->value < off;
+ });
+ llvm::erase_if(fromIsec->symbols, [&](Symbol *s) {
+ auto *d = cast<Defined>(s);
+ if (d->value != fromOff)
+ return false;
+ if (d != skip) {
+ // This repeated insertion will be quadratic unless insertIt is the end
+ // iterator. However, that is typically the case for files that have
+ // .subsections_via_symbols set.
+ insertIt = toIsec->symbols.insert(insertIt, d);
+ d->isec = toIsec;
+ d->value = toOff;
+ // We don't want to have more than one unwindEntry at a given address, so
+ // drop the redundant ones. We We can safely drop the unwindEntries of
+ // the symbols in fromIsec since we will be adding another unwindEntry as
+ // we finish parsing toIsec's file. (We can assume that toIsec has its
+ // own unwindEntry because of the ODR.)
+ d->unwindEntry = nullptr;
+ }
+ return true;
+ });
+}
+
Defined *SymbolTable::addDefined(StringRef name, InputFile *file,
InputSection *isec, uint64_t value,
uint64_t size, bool isWeakDef,
@@ -82,18 +114,27 @@ Defined *SymbolTable::addDefined(StringRef name, InputFile *file,
defined->referencedDynamically |= isReferencedDynamically;
defined->noDeadStrip |= noDeadStrip;
}
- // FIXME: Handle this for bitcode files.
- if (auto concatIsec = dyn_cast_or_null<ConcatInputSection>(isec))
+ if (auto concatIsec = dyn_cast_or_null<ConcatInputSection>(isec)) {
concatIsec->wasCoalesced = true;
+ // Any local symbols that alias the coalesced symbol should be moved
+ // into the prevailing section. Note that we have sorted the symbols
+ // in ObjFile::parseSymbols() such that extern weak symbols appear
+ // last, so we don't need to worry about subsequent symbols being
+ // added to an already-coalesced section.
+ if (defined->isec)
+ transplantSymbolsAtOffset(concatIsec, defined->isec,
+ /*skip=*/nullptr, value, defined->value);
+ }
return defined;
}
if (defined->isWeakDef()) {
- // FIXME: Handle this for bitcode files.
if (auto concatIsec =
dyn_cast_or_null<ConcatInputSection>(defined->isec)) {
concatIsec->wasCoalesced = true;
- concatIsec->symbols.erase(llvm::find(concatIsec->symbols, defined));
+ if (isec)
+ transplantSymbolsAtOffset(concatIsec, isec, defined, defined->value,
+ value);
}
} else {
std::string srcLoc1 = defined->getSourceLocation();
@@ -382,7 +423,7 @@ struct UndefinedDiag {
};
MapVector<const Undefined *, UndefinedDiag> undefs;
-}
+} // namespace
void macho::reportPendingDuplicateSymbols() {
for (const auto &duplicate : dupSymDiags) {
diff --git a/lld/test/MachO/local-alias-to-weak.s b/lld/test/MachO/local-alias-to-weak.s
new file mode 100644
index 0000000000000..feb4c0a2e4296
--- /dev/null
+++ b/lld/test/MachO/local-alias-to-weak.s
@@ -0,0 +1,149 @@
+# REQUIRES: x86
+## This test checks that when we coalesce weak definitions, their local symbol
+## aliases defs don't cause the coalesced data to be retained. This was
+## motivated by MC's aarch64 backend which automatically creates `ltmp<N>`
+## symbols at the start of each .text section. These symbols are frequently
+## aliases of other symbols created by clang or other inputs to MC. 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. We also
+## need this behavior to properly deduplicate the __objc_protolist section;
+## failure to do this can result in dyld crashing on iOS 13.
+##
+## 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-local.s -o %t/weak-then-local.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/local-then-weak.s -o %t/local-then-weak.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/no-subsections.s -o %t/no-subsections.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/no-dead-strip.s -o %t/no-dead-strip.o
+
+# RUN: %lld -lSystem -dylib %t/weak-then-local.o %t/local-then-weak.o -o %t/test1
+# RUN: llvm-objdump --macho --syms --section="__DATA,__data" --weak-bind %t/test1 | FileCheck %s
+# RUN: %lld -lSystem -dylib %t/local-then-weak.o %t/weak-then-local.o -o %t/test2
+# 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: 0000000000001000 l O __DATA,__data _alias
+# CHECK-NEXT: 0000000000001008 l O __DATA,__data _ref
+# CHECK-NEXT: 0000000000001000 l O __DATA,__data _alias
+# 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 `_alias` symbols, ld64 still
+## emits weak binds as if they were the `_weak` symbol itself. We do not. I
+## don't know of any programs that rely on this behavior, so I'm just
+## documenting it here.
+# CHECK-NEXT: Weak bind table:
+# CHECK-NEXT: segment section address type addend symbol
+# CHECK-EMPTY:
+
+# RUN: %lld -lSystem -dylib %t/local-then-weak.o %t/no-subsections.o -o %t/sub-nosub
+# RUN: llvm-objdump --macho --syms --section="__DATA,__data" %t/sub-nosub | FileCheck %s --check-prefix SUB-NOSUB
+
+## This test case demonstrates a shortcoming of LLD: If .subsections_via_symbols
+## isn't enabled, we don't elide the contents of coalesced weak symbols if they
+## are part of a section that has other non-coalesced symbols. In contrast, LD64
+## does elide the contents.
+# SUB-NOSUB: Contents of (__DATA,__data) section
+# SUB-NOSUB-NEXT: 0000000000001000 23 01 00 00 00 00 00 00 00 10 00 00 00 00 00 00
+# SUB-NOSUB-NEXT: 0000000000001010 00 00 00 00 00 00 00 00 23 01 00 00 00 00 00 00
+# SUB-NOSUB-EMPTY:
+# SUB-NOSUB-NEXT: SYMBOL TABLE:
+# SUB-NOSUB-NEXT: 0000000000001000 l O __DATA,__data _alias
+# SUB-NOSUB-NEXT: 0000000000001008 l O __DATA,__data _ref
+# SUB-NOSUB-NEXT: 0000000000001010 l O __DATA,__data _zeros
+# SUB-NOSUB-NEXT: 0000000000001000 l O __DATA,__data _alias
+# SUB-NOSUB-NEXT: 0000000000001000 w O __DATA,__data _weak
+# SUB-NOSUB-NEXT: 0000000000000000 *UND* dyld_stub_binder
+
+# RUN: %lld -lSystem -dylib %t/no-subsections.o %t/local-then-weak.o -o %t/nosub-sub
+# RUN: llvm-objdump --macho --syms --section="__DATA,__data" %t/nosub-sub | FileCheck %s --check-prefix NOSUB-SUB
+
+# NOSUB-SUB: Contents of (__DATA,__data) section
+# NOSUB-SUB-NEXT: 0000000000001000 00 00 00 00 00 00 00 00 23 01 00 00 00 00 00 00
+# NOSUB-SUB-NEXT: 0000000000001010 08 10 00 00 00 00 00 00 {{$}}
+# NOSUB-SUB-EMPTY:
+# NOSUB-SUB-NEXT: SYMBOL TABLE:
+# NOSUB-SUB-NEXT: 0000000000001000 l O __DATA,__data _zeros
+# NOSUB-SUB-NEXT: 0000000000001008 l O __DATA,__data _alias
+# NOSUB-SUB-NEXT: 0000000000001008 l O __DATA,__data _alias
+# NOSUB-SUB-NEXT: 0000000000001010 l O __DATA,__data _ref
+# NOSUB-SUB-NEXT: 0000000000001008 w O __DATA,__data _weak
+# NOSUB-SUB-NEXT: 0000000000000000 *UND* dyld_stub_binder
+
+## Verify that we don't drop any flags that the aliases have (such as
+## .no_dead_strip). This is a regression test. We previously had subsections
+## that were mistakenly stripped.
+
+# RUN: %lld -lSystem -dead_strip %t/no-dead-strip.o -o %t/no-dead-strip
+# RUN: llvm-objdump --macho --section-headers %t/no-dead-strip | FileCheck %s \
+# RUN: --check-prefix=NO-DEAD-STRIP
+# NO-DEAD-STRIP: __data 00000010
+
+#--- weak-then-local.s
+.globl _weak
+.weak_definition _weak
+.data
+_weak:
+_alias:
+ .quad 0x123
+
+_ref:
+ .quad _alias
+
+.subsections_via_symbols
+
+#--- local-then-weak.s
+.globl _weak
+.weak_definition _weak
+.data
+_alias:
+_weak:
+ .quad 0x123
+
+_ref:
+ .quad _alias
+
+.subsections_via_symbols
+
+#--- no-subsections.s
+.globl _weak
+.weak_definition _weak
+.data
+_zeros:
+.space 8
+
+_weak:
+_alias:
+ .quad 0x123
+
+#--- no-dead-strip.s
+.globl _main
+
+_main:
+ ret
+
+.data
+.no_dead_strip l_foo, l_bar
+
+_foo:
+l_foo:
+ .quad 0x123
+
+l_bar:
+_bar:
+ .quad 0x123
+
+.subsections_via_symbols
diff --git a/lld/test/MachO/private-label-alias.s b/lld/test/MachO/private-label-alias.s
deleted file mode 100644
index c5eb6277206c8..0000000000000
--- a/lld/test/MachO/private-label-alias.s
+++ /dev/null
@@ -1,105 +0,0 @@
-# REQUIRES: x86
-## This test checks that when we coalesce weak definitions, any flag-less
-## 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: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/no-dead-strip.s -o %t/no-dead-strip.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: %lld -dead_strip %t/no-dead-strip.o -o %t/no-dead-strip
-
-# 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
-
-## Verify that we don't drop any flags that private-label aliases have (such as
-## .no_dead_strip). This is a regression test. We previously had subsections
-## that were mistakenly stripped.
-
-# RUN: llvm-objdump --macho --section-headers %t/no-dead-strip | FileCheck %s \
-# RUN: --check-prefix=NO-DEAD-STRIP
-# NO-DEAD-STRIP: __data 00000010
-
-#--- 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
-
-#--- no-dead-strip.s
-.globl _main
-
-_main:
- ret
-
-.data
-.no_dead_strip l_foo, l_bar
-
-_foo:
-l_foo:
- .quad 0x123
-
-l_bar:
-_bar:
- .quad 0x123
-
-.subsections_via_symbols
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