[lldb-dev] llvm assertion while evaluating expressions for MIPS on Linux

[Greg Clayton via lldb-dev]

What is this not happening on any other architecture? Is the "$" special for MIPS and not for other architectures? We really don't want to remove the '$' as we want the symbol to be unique. The '$' symbol is fine for all x86/x86_64/arm and arm64 variants...

Greg


> On Oct 19, 2015, at 11:30 PM, Bhushan Attarde via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> 
> Hi,
>  
> I am facing issue (llvm assertion) in evaluating expressions for MIPS on Linux.
>  
> (lldb) p fooptr(a,b)
> lldb: /home/battarde/git/llvm/lib/MC/ELFObjectWriter.cpp:791: void {anonymous}::ELFObjectWriter::computeSymbolTable(llvm::MCAssembler&, const llvm::MCAsmLayout&, const SectionIndexMapTy&, const RevGroupMapTy&, {anonymous}::ELFObjectWriter::SectionOffsetsTy&): Assertion `Local || !Symbol.isTemporary()' failed.
>  
> I debugged it and found that, LLDB inserts calls to dynamic checker function for pointer validation at appropriate locations in expression’s IR.
>  
> The issue is that this checker function’s name (hard-coded in LLDB in lldb\source\Expression\IRDynamicChecks.cpp) starts with “$” i.e “$__lldb_valid_pointer_check”.
> While creating a MCSymbol (MCContext::createSymbol() in llvm/lib/MC/MCContext.cpp) for this function llvm detects the name starts with “$” and marks that symbol as ‘temporary’ symbol (PrivateGlobalPrefix is '$' for MIPS)
> Further while computing a symbol table in ELFObjectWriter::computeSymbolTable() the assertion triggers because this symbol is 'temporary'.
>  
> I tried couple of things that solves this issue for MIPS.
>  
> 1. Remove '$' from the function name.
> 2. Remove "C Language linkage" from the dynamic pointer validation function i.e the below piece of code in lldb\source\Expression\IRDynamicChecks.cpp
> ---------------------------------------------------------------------
> static const char g_valid_pointer_check_text[] =
> "extern \"C\" void\n"
> "$__lldb_valid_pointer_check (unsigned char *$__lldb_arg_ptr)\n"
> "{\n"
> "    unsigned char $__lldb_local_val = *$__lldb_arg_ptr;\n"
> "}";
> ----------------------------------------------------------------------
>  
> becomes 
>  
> --------------------------------------------------------------------
> static const char g_valid_pointer_check_text[] =
> "void\n"
> "$__lldb_valid_pointer_check (unsigned char *$__lldb_arg_ptr)\n"
> "{\n"
> "    unsigned char $__lldb_local_val = *$__lldb_arg_ptr;\n"
> "}";
> --------------------------------------------------------------------
>  
> Removing C Language linkage will enable mangling and will mangle "$__lldb_valid_pointer_check" to something like "_Z27$__lldb_valid_pointer_checkPh".
> So the mangled name won't start with '$' and the symbol will not be marked as Temporary and hence assertion won't be triggered.
>  
> Please let me know if there is any better solution to this issue.
>  
> Regards,
> Bhushan
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