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From: "hiraditya at msn dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/109442] Dead local copy of std::vector not removed
 from function
Date: Thu, 15 Jun 2023 18:38:32 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
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X-Bugzilla-Keywords: missed-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109442

--- Comment #17 from AK <hiraditya at msn dot com> ---
With recent changes in libc++ (https://reviews.llvm.org/D147741) clang
optimizes away the new-delete pair. https://godbolt.org/z/a6PG54Pvb

$ clang++ -O3 -stdlib=3Dlibc++ -fno-exceptions

vat1(std::__1::vector<int, std::__1::allocator<int> >): #
@vat1(std::__1::vector<int, std::__1::allocator<int> >)
  sub rsp, 24
  xorps xmm0, xmm0
  movaps xmmword ptr [rsp], xmm0
  mov qword ptr [rsp + 16], 0
  mov rax, qword ptr [rdi + 8]
  sub rax, qword ptr [rdi]
  je .LBB0_2
  js .LBB0_3
.LBB0_2:
  mov eax, 10
  add rsp, 24
  ret
.LBB0_3:
  mov rdi, rsp
  call std::__1::vector<int, std::__1::allocator<int>
>::__throw_length_error[abi:v170000]() const
.L.str:
  .asciz "vector"

.L.str.1:
  .asciz "length_error was thrown in -fno-exceptions mode with message \"%s=
\""


Previously clang couldn't even convert the copy to a memmove and would gene=
rate
a raw loop e.g., https://godbolt.org/z/G8ax1o5bc

.LBB0_6: # =3D>This Inner Loop Header: Depth=3D1
  movups xmm0, xmmword ptr [r15 + 4*rdi]
  movups xmm1, xmmword ptr [r15 + 4*rdi + 16]
  movups xmmword ptr [rax + 4*rdi], xmm0
  movups xmmword ptr [rax + 4*rdi + 16], xmm1
  add rdi, 8
  cmp rsi, rdi
  jne .LBB0_6
  cmp rbx, rsi
  jne .LBB0_8
  jmp .LBB0_9
.LBB0_3:=