[Bug c/24177] New: function returning structure produce very long/slow assembly

["etienne_lorrain at yahoo dot fr" <gcc-bugzilla at gcc dot gnu dot org>]

Compiling this file with -O2 optimisation:
----------------
struct str {
        int a, b, c, d;
};

void fct3 (struct str *);

extern inline struct str fct (void)
{
        struct str returned = { 1, 2, 3, 4 };
        return returned;
}

void fct2 (void)
{
        struct str tmp;

        tmp = fct ();
        fct3 (&tmp);
}
----------------
  with this compiler:
gcc version 4.0.2 20050913 (prerelease) (Debian 4.0.1-7)
  creates this assembler having three copies of the structure
 in the stack, and one as constant in .rodata:
$ gcc -O2 tmp.c -S -o tmp.s
$ cat tmp.s
        .file   "tmp.c"
        .section        .rodata
        .align 4
        .type   C.0.1141, @object
        .size   C.0.1141, 16
C.0.1141:
        .long   1
        .long   2
        .long   3
        .long   4
        .text
        .p2align 4,,15
.globl fct2
        .type   fct2, @function
fct2:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %edi
        pushl   %esi
        subl    $76, %esp
        leal    -56(%ebp), %edi
        movl    $C.0.1141, %esi
        cld
        movl    $4, %ecx
        rep
        movsl
        leal    -24(%ebp), %edi
        leal    -56(%ebp), %esi
        movb    $4, %cl
        rep
        movsl
        leal    -40(%ebp), %edi
        leal    -24(%ebp), %esi
        movb    $4, %cl
        rep
        movsl
        leal    -40(%ebp), %eax
        pushl   %eax
        call    fct3
        addl    $16, %esp
        leal    -8(%ebp), %esp
        popl    %esi
        popl    %edi
        popl    %ebp
        ret
        .size   fct2, .-fct2
        .ident  "GCC: (GNU) 4.0.2 20050913 (prerelease) (Debian 4.0.1-7)"
        .section        .note.GNU-stack,"",@progbits
----------------
  If compiled with -Os, the "memcpy" function is called three times.
$ gcc -Os tmp.c -S -o tmp.s
$ cat tmp.s
        .file   "tmp.c"
        .section        .rodata
        .align 4
        .type   C.0.1141, @object
        .size   C.0.1141, 16
C.0.1141:
        .long   1
        .long   2
        .long   3
        .long   4
        .text
.globl fct2
        .type   fct2, @function
fct2:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %esi
        pushl   %ebx
        subl    $48, %esp
        leal    -56(%ebp), %ebx
        pushl   $16
        pushl   $C.0.1141
        pushl   %ebx
        call    memcpy
        leal    -24(%ebp), %esi
        pushl   $16
        pushl   %ebx
        pushl   %esi
        call    memcpy
        leal    -40(%ebp), %ebx
        pushl   $16
        pushl   %esi
        pushl   %ebx
        call    memcpy
        addl    $36, %esp
        pushl   %ebx
        call    fct3
        popl    %eax
        leal    -8(%ebp), %esp
        popl    %ebx
        popl    %esi
        popl    %ebp
        ret
        .size   fct2, .-fct2
        .ident  "GCC: (GNU) 4.0.2 20050913 (prerelease) (Debian 4.0.1-7)"
        .section        .note.GNU-stack,"",@progbits
----------------
  That is not a regression, gcc-3.4* and gcc-2.95 do not produce very good
 assembler code neither for this source file.

  Etienne.


-- 
           Summary: function returning structure produce very long/slow
                    assembly
           Product: gcc
           Version: 4.0.2
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: etienne_lorrain at yahoo dot fr
 GCC build triplet: i486-linux-gnu
  GCC host triplet: i486-linux-gnu
GCC target triplet: i486-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24177