[cfe-dev] What is an address-significant symbol?
Gaier, Bjoern via cfe-dev
cfe-dev at lists.llvm.org
Fri Nov 22 03:20:47 PST 2019
Hello LLVM- and Clang-List,
I'm not sure if this a subject for LLVM or Clang - but there is something I don't understand. I wrote the following code in C++:
'searchPlanschi' is a function I provide. Clang generates the following assembly code for this:
lea rcx, [rip + __stdio_common_vswprintf]
call "?searchPlanschi@@YAXPEAX at Z"
I was surprised to see the register rip there, as far as I know this is the Instruction register, right? Why do I need the rip register to get the address of the function? I searched the assembly file for '__stdio_common_vswprintf' to get some hints about this. The only thing I found was:
So I googled ".addrsig" and found the following text:
"This section is used to mark symbols as address-significant, i.e. the address of the symbol is used in a comparison or leaks outside the translation unit. It has the same meaning as the absence of the LLVM attributes unnamed_addr and local_unnamed_addr.
Any sections referred to by symbols that are not marked as address-significant in any object file may be safely merged by a linker without breaking the address uniqueness guarantee provided by the C and C++ language standards.
The contents of the section are a sequence of ULEB128-encoded integers referring to the symbol table indexes of the address-significant symbols."
But sadly... this is way over my head. What does that actually mean? Does that explain the code construct with the rip register? Is that a form of optimization?
Thank you in advance for any help!
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