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: unassig...@gcc.gnu.org
ReportedBy: crusader.m...@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
