[llvm-dev] Unable to evaluate offset when using aliases

[Jason Holajter via llvm-dev]

We have encountered a problem when using multiple aliases in IR and would like feedback as to whether this use is expected to work properly.

The following IR is causing a failure in llc when lowering to an object file:

; RUN: llc double-alias.ll -filetype=obj -o double-alias.o
@bar = global i32 0
@foo2 = alias i32, i32* @bar
@foo3 = alias i32, i32* @foo2
@baz = global i32* @foo3

Is redirection through multiple aliases supposed to work? If @baz is modified to reference @foo2 instead of @foo3, the IR lowers to an object file successfully.

Without asserts enabled, the following error is seen:
LLVM ERROR: unable to evaluate offset to undefined symbol '_foo2'

With asserts enabled, the following assert is seen:
Assertion failed: ((SymbolContents == SymContentsUnset || SymbolContents == SymContentsOffset) && "Cannot get offset for a common/variable symbol"), function getOffset, file /llvm-3.8-rc1/src/include/llvm/MC/MCSymbol.h, line 317.
0  llc                      0x00000001111c2a5e llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 46
1  llc                      0x00000001111c2e99 PrintStackTraceSignalHandler(void*) + 25
2  llc                      0x00000001111bf729 llvm::sys::RunSignalHandlers() + 425
3  llc                      0x00000001111c31e2 SignalHandler(int) + 354
4  libsystem_platform.dylib 0x00007fff885c2eaa _sigtramp + 26
5  llc                      0x0000000111e06635 ErrorCategory + 87253
6  llc                      0x00000001111c2ebb raise + 27
7  llc                      0x00000001111c2f62 abort + 18
8  llc                      0x00000001111c2f4e __assert_rtn + 126
9  llc                      0x0000000110bb988e llvm::MCSymbol::getOffset() const + 126
10 llc                      0x0000000110bbc25d getLabelOffset(llvm::MCAsmLayout const&, llvm::MCSymbol const&, bool, unsigned long long&) + 221
11 llc                      0x0000000110bba362 getSymbolOffsetImpl(llvm::MCAsmLayout const&, llvm::MCSymbol const&, bool, unsigned long long&) + 322
12 llc                      0x0000000110bba456 llvm::MCAsmLayout::getSymbolOffset(llvm::MCSymbol const&) const + 38
13 llc                      0x0000000110b82f21 llvm::MCAssembler::evaluateFixup(llvm::MCAsmLayout const&, llvm::MCFixup const&, llvm::MCFragment const*, llvm::MCValue&, unsigned long long&) const + 625
14 llc                      0x0000000110b85036 llvm::MCAssembler::handleFixup(llvm::MCAsmLayout const&, llvm::MCFragment&, llvm::MCFixup const&) + 182
15 llc                      0x0000000110b85a7f llvm::MCAssembler::layout(llvm::MCAsmLayout&) + 2223
16 llc                      0x0000000110b85f33 llvm::MCAssembler::Finish() + 83
17 llc                      0x0000000110bd07ff llvm::MCObjectStreamer::FinishImpl() + 191
18 llc                      0x0000000110bc2017 (anonymous namespace)::MCMachOStreamer::FinishImpl() + 807
19 llc                      0x0000000110bdba35 llvm::MCStreamer::Finish() + 149
20 llc                      0x0000000110296a21 llvm::AsmPrinter::doFinalization(llvm::Module&) + 5425
21 llc                      0x0000000110a4e5d4 llvm::FPPassManager::doFinalization(llvm::Module&) + 84
22 llc                      0x0000000110a4f7f3 (anonymous namespace)::MPPassManager::runOnModule(llvm::Module&) + 3603
23 llc                      0x0000000110a4e766 llvm::legacy::PassManagerImpl::run(llvm::Module&) + 342
24 llc                      0x0000000110a4fd81 llvm::legacy::PassManager::run(llvm::Module&) + 33
25 llc                      0x000000010eece223 compileModule(char**, llvm::LLVMContext&) + 19363
26 llc                      0x000000010eec9621 main + 225
27 libdyld.dylib            0x00007fff85f6a5ad start + 1
28 libdyld.dylib            0x0000000000000005 start + 2047433305
Stack dump:
0. Program arguments: /llvm-3.8-rc1/build/bin/llc double-alias.ll -filetype=obj -o double-alias.o
Illegal instruction: 4


Thanks for the help,
 Jason



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