[Bug c++/50771] New: redundant argument passing code (x64) + inefficient use of stack space

[Bug c++/50771] New: redundant argument passing code (x64) + inefficient use of stack space

[crusader.mike at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50771

             Bug #: 50771
           Summary: redundant argument passing code (x64) + inefficient
                    use of stack space
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: crusader.mike@gmail.com


I was checking how by-value argument passing looks in comparison with
by-reference and found strange things (looks like a compiler bug to me). 

This is tested on multiple versions of GCC (up to and including 4.6.1) and
every version produced similar results. Here is test code:

#include <cstdio>
using namespace std;

struct Foo
{
    size_t m1, m2;
    Foo(size_t a1, size_t a2) : m1(a1), m2(a2) {}
    // ~Foo() {}
};

void foo1(size_t, Foo, size_t) __attribute__((noinline));
void foo1(size_t, Foo, size_t) { asm("nop"); }

void foo2(size_t, Foo const&, size_t) __attribute__((noinline));
void foo2(size_t, Foo const&, size_t) { asm("nop"); }

int main()
{
    foo1(1, Foo(2, 3), 4);
    foo2(5, Foo(6, 7), 8);
    return 16;
}

Now if you compile this code ('gcc -O3 -S' on x64 platform) with ~Foo()
commented out, you get:

main:
.LFB17:
        subq    $32, %rsp
.LCFI0:
        movl    $4, %ecx
        movl    $2, %esi
        movl    $3, %edx
        movl    $1, %edi
        movq    $2, (%rsp)        # why?
        movq    $3, 8(%rsp)       # why?
        call    _Z4foo1m3Foom
        leaq    16(%rsp), %rsi
        movl    $8, %edx
        movl    $5, %edi
        movq    $6, 16(%rsp)
        movq    $7, 24(%rsp)
        call    _Z4foo2mRK3Foom
        movl    $16, %eax
        addq    $32, %rsp
.LCFI1:
        ret


there are two unnecessary commands marked "why?" -- they look like a leftover
code that optimizer forgot to take away. Now if we uncomment ~Foo() asm code
will look like:

main:
.LFB20:
        subq    $32, %rsp
.LCFI0:
        movl    $4, %edx
        movl    $1, %edi
        movq    %rsp, %rsi
        movq    $2, (%rsp)        # <-- here
        movq    $3, 8(%rsp)       # <-- here
        call    _Z4foo1m3Foom
        leaq    16(%rsp), %rsi
        movl    $8, %edx
        movl    $5, %edi
        movq    $6, 16(%rsp)
        movq    $7, 24(%rsp)
        call    _Z4foo2mRK3Foom
        movl    $16, %eax
        addq    $32, %rsp
.LCFI1:
        ret

Here same two ops are present and are actually required. It also came as a
shock to me to discover that trivial implicit destructor means different thing
for compiler than trivial explicit one. I would really like to know why if I
specify empty dtor -- my value ends up being passed (effectively) by reference
(I would expect this to be slower). I recall discovering in GCC 4.0.3 similar
problem when not declaring destructor meant 'no RVO for you', even if dtor is
empty.

Related notes:
- it is clear that stack is not used efficiently -- arguments for foo2 call
could be packed into the same location on the stack as arguments for foo1 call
- for some reason unused function arguments were not optimized away (-O3 should
do this by default since GCC 4.5, afaik). Looks like a problem
- surprisingly, if Foo structure contains only one member variable (just
comment out m2) asm code looks better (no redundant ops, but still passing by
reference if ~Foo is defined, unused args are not removed):

1 member, +dtor:

main:
.LFB20:
        subq    $32, %rsp
.LCFI0:
        movl    $4, %edx
        movl    $1, %edi
        movq    %rsp, %rsi    # why by-reference?
        movq    $2, (%rsp)
        call    _Z4foo1m3Foom
        leaq    16(%rsp), %rsi
        movl    $8, %edx
        movl    $5, %edi
        movq    $6, 16(%rsp)
        call    _Z4foo2mRK3Foom
        movl    $16, %eax
        addq    $32, %rsp
.LCFI1:
        ret

1 member, -dtor:

main:
.LFB17:
        subq    $16, %rsp
.LCFI0:
        movl    $4, %edx
        movl    $2, %esi
        movl    $1, %edi        # exactly how it should be
        call    _Z4foo1m3Foom
        movq    %rsp, %rsi
        movl    $8, %edx
        movl    $5, %edi
        movq    $6, (%rsp)
        call    _Z4foo2mRK3Foom
        movl    $16, %eax
        addq    $16, %rsp
.LCFI1:
        ret

[Bug c++/50771] redundant argument passing code (x64) + inefficient use of stack space

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50771

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2011-10-18
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-10-18 09:28:59 UTC ---
Because the ABI says so.

But you didn't provide information on what target architecture or OS you
are running on.

[Bug c++/50771] redundant argument passing code (x64) + inefficient use of stack space

[crusader.mike at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50771

--- Comment #2 from CM <crusader.mike at gmail dot com> 2011-10-18 18:20:18 UTC ---
It is RedHat Linux 64bit, platform x86_64:

emdhouapd07 (energy2) hoops: uname -a
Linux emdhouapd07 2.6.18-238.9.1.el5 #1 SMP Fri Mar 18 12:42:39 EDT 2011 x86_64
x86_64 x86_64 GNU/Linux


> Because the ABI says so.

Which question this statement answers?