This is the mail archive of the
gcc-bugs@gcc.gnu.org
mailing list for the GCC project.
[Bug c/31238] New: Too many instructions in C loop
- From: "sigra at home dot se" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 17 Mar 2007 10:15:40 -0000
- Subject: [Bug c/31238] New: Too many instructions in C loop
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
Build the following code with "gcc -std=c99 -Wall -Wextra -Werror -Os -S":
void q(const unsigned int);
void f() {for (unsigned int x = 0; x != 10; ++x) q(77);}
The loop becomes:
.L2:
subl $12, %esp
incl %ebx
pushl $77
call q
addl $16, %esp
cmpl $10, %ebx
jne .L2
This is 7 instructions, which is too much. The following equivalent program:
void q(const unsigned int);
void f() {for (unsigned int x = 10; x; --x) q(77);}
becomes:
.L2:
subl $12, %esp
pushl $77
call q
addl $16, %esp
decl %ebx
jne .L2
which is only 6 instructions. Since the programs are equivalent (both just call
q(77) 10 times) and the second version becomes shorter than the first, the
first version is not optimized properly.
The corresponding Ada program with q.ads:
procedure Q(N : in Natural);
and f.adb:
with Q;
procedure F is begin for i in 1 .. 10 loop Q(77); end loop; end F;
built with "gnatgcc -Os -Wall -Wextra -Wextra -Werror -S f.adb" produces the
following loop:
.L5:
pushl $77
.LCFI3:
call _ada_q
popl %eax
decl %ebx
jns .L5
which is only 5 instructions. I know that situations are often encountered
where C code can not be optimized as much as Ada code, because it would break
some bizarre C feature. I do not know it this is such a situation, or if the C
code could actually become as tight as the Ada code when compiled and
optimized.
But at least the first version of the C code should be optimized to be as tight
as the second version.
(Tested with gcc 4.1.1 (Gentoo 4.1.1-r3) and gnatgcc 3.4.5 (from Gentoo
dev-lang/gnat-3.45).)
--
Summary: Too many instructions in C loop
Product: gcc
Version: 4.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: sigra at home dot se
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31238