[lldb-dev] [Bug 36155] New: FastDemangle performance problem

via lldb-dev lldb-dev at lists.llvm.org
Tue Jan 30 10:29:26 PST 2018


            Bug ID: 36155
           Summary: FastDemangle performance problem
           Product: lldb
           Version: 6.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: lldb-dev at lists.llvm.org
          Reporter: feishenniubi at gmail.com
                CC: llvm-bugs at lists.llvm.org

Here's a simple python program that generates a mangled string with size n:

def gen(n):                                                                     
    prefix = '_Z0'                                                              
    suffix = 'Mm0'                                                              
    content = 'M0' * (n-6)                                                      
    return prefix + content + suffix

lldb's FastDemangler exhibit quadratic behavior when demangling such a string.
On my laptop (2015 Thinkpad t440p Arch Linux), setting n to ~20000 will cause a
noticable slowdown, and ~50000 would just segfault the program.

The same inputs are rejected instantly by itaniumDemangle() as well as c++filt.

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