[Bug tree-optimization/104547] New: std::vector::resize(v.size() - n) produces poor code

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104547

            Bug ID: 104547
           Summary: std::vector::resize(v.size() - n) produces poor code
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

The codegen for this is pretty bad:

#include <vector>

void shrink(std::vector<int>& v, unsigned n) {
    v.resize(v.size() - n);
}

_Z6shrinkRSt6vectorIiSaIiEEj:
.LFB865:
        .cfi_startproc
        movq    8(%rdi), %rdx
        movq    (%rdi), %rcx
        movl    %esi, %esi
        movq    %rdx, %rax
        subq    %rcx, %rax
        sarq    $2, %rax
        movq    %rax, %r8
        subq    %rsi, %r8
        jb      .L3
        cmpq    %rax, %r8
        jnb     .L1
        leaq    (%rcx,%r8,4), %rax
        cmpq    %rax, %rdx
        je      .L1
        movq    %rax, 8(%rdi)
.L1:
        ret
.L3:
        pushq   %rax
        .cfi_def_cfa_offset 16
        movl    $.LC0, %edi
        call    _ZSt20__throw_length_errorPKc
        .cfi_endproc



Telling the compiler that v.size() - n doesn't wrap doesn't help at all:

#include <vector>

void shrink(std::vector<int>& v, unsigned n) {
    if (v.size() < n)
      __builtin_unreachable();
    v.resize(v.size() - n);
}

Why do we still have the call to __throw_length_error() there? It's
unreachable, because we're only shrinking, not growing.


This is better:

void shrink_pop(std::vector<int>& v, unsigned n) {
    while (n--)
      v.pop_back();
}

_Z10shrink_popRSt6vectorIiSaIiEEj:
.LFB866:
        .cfi_startproc
        testl   %esi, %esi
        je      .L10
        movq    8(%rdi), %rax
        movl    %esi, %esi
        negq    %rsi
        leaq    (%rax,%rsi,4), %rdx
        .p2align 4,,10
        .p2align 3
.L12:
        subq    $4, %rax
        movq    %rax, 8(%rdi)
        cmpq    %rax, %rdx
        jne     .L12
.L10:
        ret
        .cfi_endproc


And this:

void shrink_min(std::vector<int>& v, unsigned n) {
    v.resize(std::min<std::size_t>(v.size(), n));
}

_Z10shrink_minRSt6vectorIiSaIiEEj:
.LFB867:
        .cfi_startproc
        movq    8(%rdi), %rdx
        movq    (%rdi), %rcx
        movl    %esi, %esi
        movq    %rdx, %rax
        subq    %rcx, %rax
        sarq    $2, %rax
        cmpq    %rax, %rsi
        jb      .L54
.L52:
        ret
        .p2align 4,,10
        .p2align 3
.L54:
        leaq    (%rcx,%rsi,4), %rax
        cmpq    %rax, %rdx
        je      .L52
        movq    %rax, 8(%rdi)
        ret
        .cfi_endproc