https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87502
Bug ID: 87502
Summary: Poor code generation for std::string("c-style string")
Product: gcc
Version: 8.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: terra at gnome dot org
Target Milestone: ---
Created attachment 44776
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44776&action=edit
Preprocessed source code
It appears that gcc is creating quite poor code when "c-style strings"
are used to construct std::string objects. Ideally, the result ought
to be just a few move instructions for small strings.
Host: Linux x86_64 4.4.140-62-default (OpenSuSE)
Test code:
---------------------------------------------------------------
#include <string>
extern void foo (const std::string &);
void
bar ()
{
foo ("abc");
foo (std::string("abc"));
}
---------------------------------------------------------------
# /usr/local/products/gcc/8.2.0/bin/g++ -std=gnu++1z -S -m32 -O3 ttt.C
# grep 'call.*construct' ttt.s
call
_ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE12_M_constructIPKcEEvT_S8_St20forward_iterator_tag.constprop.18
call
_ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE12_M_constructIPKcEEvT_S8_St20forward_iterator_tag.constprop.18
Here gcc generates complete calls to the generic string construction
even though the strings are constructed from small, known strings.
"-std=gnu++1z" is important; "-m32" and "-O3" (as opposed to "-m64" and
"-O2") are not.
# /usr/local/products/gcc/8.2.0/bin/g++ -S -m32 -O3 ttt.C
# grep 'call.*construct' ttt.s
# (nada)
No calls -- good. In this case gcc generates this fragment:
_Z3barv:
.LFB1084:
.cfi_startproc
.cfi_personality 0,__gxx_personality_v0
.cfi_lsda 0,.LLSDA1084
pushl %ebp
.cfi_def_cfa_offset 8
.cfi_offset 5, -8
movl $25185, %edx
movl %esp, %ebp
.cfi_def_cfa_register 5
pushl %edi
pushl %esi
.cfi_offset 7, -12
.cfi_offset 6, -16
leal -48(%ebp), %esi
pushl %ebx
.cfi_offset 3, -20
leal -40(%ebp), %ebx
subl $56, %esp
movl %ebx, -48(%ebp)
pushl %esi
movw %dx, -40(%ebp)
movb $99, -38(%ebp)
movl $3, -44(%ebp)
movb $0, -37(%ebp)
.LEHB6:
.cfi_escape 0x2e,0x10
call _Z3fooRKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE
[...]
This is better than a call, but not great:
1. The string is moved into position in three chunks (25185, 99, 0).
This probably comes from inlined memcpy of 3 bytes, but the source
is zero-terminated so rounding the memcpy size up to 4 would have
been better.
2. It's unclear why 25185 is passed through a register.
