[llvm-bugs] [Bug 45287] New: [missed optimization] failure to drop unused libstdc++ std::string

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45287

            Bug ID: 45287
           Summary: [missed optimization] failure to drop unused libstdc++
                    std::string
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: eyalroz at technion.ac.il
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

(GodBolt example: https://godbolt.org/z/rbkRSJ)

Consider the following program:

  #include <string>

  void foo() {
      std::string s { "This is not a small string" };
  }


If we compile this with clang++ and libc++, we get:

  foo():
          ret

but if we use libstdc++, we get:

  foo():
          sub     rsp, 40
          lea     rax, [rsp + 24]
          mov     qword ptr [rsp + 8], rax
          mov     edi, 27
          call    operator new(unsigned long)
          mov     qword ptr [rsp + 8], rax
          mov     qword ptr [rsp + 24], 26
          movups  xmm0, xmmword ptr [rip + .L.str+10]
          movups  xmmword ptr [rax + 10], xmm0
          movups  xmm0, xmmword ptr [rip + .L.str]
          movups  xmmword ptr [rax], xmm0
          mov     qword ptr [rsp + 16], 26
          mov     byte ptr [rax + 26], 0
          mov     rdi, rax
          call    operator delete(void*)
          add     rsp, 40
          ret
          mov     rdi, rax
          call    _Unwind_Resume
  .L.str:
          .asciz  "This is not a small string"

Yikes! What's going on here? Can't clang++/LLVM notice an unused string without
some libc++ special sauce? Is it the new/delete pair that's exotic somehow?

It seems that the hint is at the bottom "call _Unwind_Resume". James Garret
notes (in comments on this related SO answer:
https://stackoverflow.com/a/60820696/1593077) that libstdc++'s std::string code
apparently has some try/catch somewhere which LLVM can't optimize away, because
of bug 35052. So, marking this as dependent on that one - and pinging you LLVM
devs to give it some much needed attention...

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20200323/2b876b97/attachment.html>