caio/clang-p2996
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[BOLT] Ignore special symbols as function aliases in updateELFSymbolTable

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Exempt special symbols (hot text/data and _end symbol) from normal
handling. We only need to set their value and make them absolute.

If these symbols are handled as normal symbols and if they alias
functions we may create non-sensical symbols, e.g. __hot_start.cold.

Test Plan: updated hot-end-symbol.s

Reviewers: maksfb, rafaelauler, ayermolo, dcci

Reviewed By: dcci, maksfb

Pull Request: https://github.com/llvm/llvm-project/pull/92713
This commit is contained in:
Amir Ayupov
2024-05-20 16:55:11 -07:00
committed by GitHub
parent 888e087b09
commit bb627b0a0c
2 changed files with 37 additions and 28 deletions
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62
bolt/lib/Rewrite/RewriteInstance.cpp
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@@ -4808,6 +4808,40 @@ void RewriteInstance::updateELFSymbolTable(
// Create a new symbol based on the existing symbol.
ELFSymTy NewSymbol = Symbol;
// Handle special symbols based on their name.
Expected<StringRef> SymbolName = Symbol.getName(StringSection);
assert(SymbolName && "cannot get symbol name");
auto updateSymbolValue = [&](const StringRef Name,
std::optional<uint64_t> Value = std::nullopt) {
NewSymbol.st_value = Value ? *Value : getNewValueForSymbol(Name);
NewSymbol.st_shndx = ELF::SHN_ABS;
BC->outs() << "BOLT-INFO: setting " << Name << " to 0x"
<< Twine::utohexstr(NewSymbol.st_value) << '\n';
};
if (*SymbolName == "__hot_start" || *SymbolName == "__hot_end") {
if (opts::HotText) {
updateSymbolValue(*SymbolName);
++NumHotTextSymsUpdated;
}
goto registerSymbol;
}
if (*SymbolName == "__hot_data_start" || *SymbolName == "__hot_data_end") {
if (opts::HotData) {
updateSymbolValue(*SymbolName);
++NumHotDataSymsUpdated;
}
goto registerSymbol;
}
if (*SymbolName == "_end") {
if (NextAvailableAddress > Symbol.st_value)
updateSymbolValue(*SymbolName, NextAvailableAddress);
goto registerSymbol;
}
if (Function) {
// If the symbol matched a function that was not emitted, update the
// corresponding section index but otherwise leave it unchanged.
@@ -4904,33 +4938,7 @@ void RewriteInstance::updateELFSymbolTable(
}
}
// Handle special symbols based on their name.
Expected<StringRef> SymbolName = Symbol.getName(StringSection);
assert(SymbolName && "cannot get symbol name");
auto updateSymbolValue = [&](const StringRef Name,
std::optional<uint64_t> Value = std::nullopt) {
NewSymbol.st_value = Value ? *Value : getNewValueForSymbol(Name);
NewSymbol.st_shndx = ELF::SHN_ABS;
BC->outs() << "BOLT-INFO: setting " << Name << " to 0x"
<< Twine::utohexstr(NewSymbol.st_value) << '\n';
};
if (opts::HotText &&
(*SymbolName == "__hot_start" || *SymbolName == "__hot_end")) {
updateSymbolValue(*SymbolName);
++NumHotTextSymsUpdated;
}
if (opts::HotData && (*SymbolName == "__hot_data_start" ||
*SymbolName == "__hot_data_end")) {
updateSymbolValue(*SymbolName);
++NumHotDataSymsUpdated;
}
if (*SymbolName == "_end" && NextAvailableAddress > Symbol.st_value)
updateSymbolValue(*SymbolName, NextAvailableAddress);
registerSymbol:
if (IsDynSym)
Write((&Symbol - cantFail(Obj.symbols(&SymTabSection)).begin()) *
sizeof(ELFSymTy),