[PATCH][MC] Emit an error if cfi_startproc is used before a symbol is defined

[Quentin Colombet]

Hi Eric and Jim,

@Eric, I do not know if you are the right person to review this, but compact frame information looks related enough to dwarf emission that I assume you know :).

The attached patch fixes an assertion failure[1] in the assembler when a .cfi_startproc is used before a function (or more generally a symbol) is defined.

Thanks for your reviews/feedbacks.

** Context **

Currently the assembler crashes when emitting the compact unwind information if the input assembly file starts with a .cfi_startproc directive before any symbol has been defined.

I am not a CFI expert, but that does not look right to me to use a .cfi_startproc directive without having started the related function!


** Proposed Solution **

Issue a backend fatal error if a frame is started whereas no symbol is defined. Although this diagnostic method is not pretty, this is how all cfi related errors are handled (e.g., looks for No open frame).

The patch also modifies a few test cases to define a function before .cfi_startproc is used.
I.e., test cases like this:
.cfi_startproc

Looks like this now:
_proc:
.cfi_startproc

If that does not make sense, please let me know!

<rdar://problem/15939159>

[1]
Assertion failed: (Symbol), function MCSymbolRefExpr, file include/llvm/MC/MCExpr.h, line 273.
0  llvm-mc                  0x0000000109d85e3e llvm::sys::PrintStackTrace(__sFILE*) + 46
1  llvm-mc                  0x0000000109d8614b PrintStackTraceSignalHandler(void*) + 27
2  llvm-mc                  0x0000000109d864c8 SignalHandler(int) + 408
3  libsystem_platform.dylib 0x00007fff8f8895aa _sigtramp + 26
4  libsystem_platform.dylib 0x00007f9452c05338 _sigtramp + 3275210152
5  llvm-mc                  0x0000000109d8617b raise + 27
6  llvm-mc                  0x0000000109d86232 abort + 18
7  llvm-mc                  0x0000000109d86211 __assert_rtn + 129
8  llvm-mc                  0x0000000109d019bf llvm::MCSymbolRefExpr::MCSymbolRefExpr(llvm::MCSymbol const*, llvm::MCSymbolRefExpr::VariantKind, llvm::MCAsmInfo const*) + 143
9  llvm-mc                  0x0000000109d0081b llvm::MCSymbolRefExpr::MCSymbolRefExpr(llvm::MCSymbol const*, llvm::MCSymbolRefExpr::VariantKind, llvm::MCAsmInfo const*) + 43
10 llvm-mc                  0x0000000109cfda56 llvm::MCSymbolRefExpr::Create(llvm::MCSymbol const*, llvm::MCSymbolRefExpr::VariantKind, llvm::MCContext&) + 134
11 llvm-mc                  0x0000000109b79df4 llvm::MCSymbolRefExpr::Create(llvm::MCSymbol const*, llvm::MCContext&) + 36
12 llvm-mc                  0x0000000109d16f85 llvm::MCStreamer::EmitSymbolValue(llvm::MCSymbol const*, unsigned int) + 69
13 llvm-mc                  0x0000000109ceb7ca (anonymous namespace)::FrameEmitterImpl::EmitCompactUnwind(llvm::MCStreamer&, llvm::MCDwarfFrameInfo const&) + 314
14 llvm-mc                  0x0000000109ceb2d6 llvm::MCDwarfFrameEmitter::Emit(llvm::MCStreamer&, llvm::MCAsmBackend*, bool, bool) + 470
15 llvm-mc                  0x0000000109d1b1cf llvm::MCStreamer::EmitFrames(llvm::MCAsmBackend*, bool) + 95
16 llvm-mc                  0x0000000109d04152 (anonymous namespace)::MCMachOStreamer::FinishImpl() + 66
17 llvm-mc                  0x0000000109d1b2df llvm::MCStreamer::Finish() + 159
18 llvm-mc                  0x0000000109c30582 (anonymous namespace)::AsmParser::Run(bool, bool) + 2546
19 llvm-mc                  0x000000010995457c AssembleInput(char const*, llvm::Target const*, llvm::SourceMgr&, llvm::MCContext&, llvm::MCStreamer&, llvm::MCAsmInfo&, llvm::MCSubtargetInfo&, llvm::MCInstrInfo&) + 940
20 llvm-mc                  0x0000000109952461 main + 9457
21 libdyld.dylib            0x00007fff959e65fd start + 1

Cheers,
-Quentin
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