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* [Bug rtl-optimization/11873] inefficient use of registers induces size and time overhead
[not found] <bug-11873-6707@http.gcc.gnu.org/bugzilla/>
@ 2007-11-09 23:51 ` rask at gcc dot gnu dot org
0 siblings, 0 replies; 5+ messages in thread
From: rask at gcc dot gnu dot org @ 2007-11-09 23:51 UTC (permalink / raw)
To: gcc-bugs
------- Comment #4 from rask at gcc dot gnu dot org 2007-11-09 23:51 -------
This has improved (-O2 -fomit-frame-pointer):
test:
movl 4(%esp), %eax # 32 *movsi_1/1 [length = 4]
movl 8(%esp), %edx # 44 *movsi_1/1 [length = 4]
orl %eax, %edx # 6 *iorsi_1/1 [length = 2]
addl $1, %eax # 35 *addsi_1/1 [length = 3]
cmpl $1, %edx # 38 *cmpsi_1_insn/1 [length = 3]
sbbl %edx, %edx # 39 x86_movsicc_0_m1 [length = 2]
notl %edx # 40 *one_cmplsi2_1 [length = 2]
andl %edx, %eax # 41 *andsi_1/1 [length = 2]
ret # 47 return_internal [length = 1]
.ident "GCC: (GNU) 4.3.0 20071102 (experimental)"
With -Os -fomit-frame-pointer we get:
test:
movl 4(%esp), %edx # 32 *movsi_1/1 [length = 4]
xorl %eax, %eax # 48 *movsi_xor [length = 2]
movl 8(%esp), %ecx # 43 *movsi_1/1 [length = 4]
orl %edx, %ecx # 7 *iorsi_3 [length = 2]
je .L3 # 8 *jcc_1 [length = 2]
leal 1(%edx), %eax # 44 *lea_1 [length = 3]
.L3:
ret # 47 return_internal [length = 1]
With -O2/-Os -fomit-frame-pointer -march=pentiumpro:
test:
movl 4(%esp), %edx # 32 *movsi_1/1 [length = 4]
xorl %eax, %eax # 46 *movsi_xor [length = 2]
leal 1(%edx), %ecx # 41 *lea_1 [length = 3]
orl 8(%esp), %edx # 36 *iorsi_3 [length = 4]
cmovne %ecx, %eax # 38 *movsicc_noc/1 [length = 3]
ret # 44 return_internal [length = 1]
I would probably code it like so:
movl 4(%esp), %eax ; 4
movl 8(%esp), %edx ; 4
orl %eax, %edx ; 2
addl $-1, %edx ; 3
adcl $0, %eax ; 3
ret ; 1
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11873
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug optimization/11873] New: inefficient use of registers induces size and time overhead
@ 2003-08-10 8:39 willy at w dot ods dot org
2004-06-03 4:27 ` [Bug rtl-optimization/11873] " pinskia at gcc dot gnu dot org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: willy at w dot ods dot org @ 2003-08-10 8:39 UTC (permalink / raw)
To: gcc-bugs
PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11873
Summary: inefficient use of registers induces size and time
overhead
Product: gcc
Version: 3.3.1
Status: UNCONFIRMED
Severity: enhancement
Priority: P2
Component: optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: willy at w dot ods dot org
CC: gcc-bugs at gcc dot gnu dot org
GCC build triplet: i586-linux-gnu
GCC host triplet: i586-linux-gnu
GCC target triplet: i586-linux-gnu
I wrote a simple test function which uses an unsigned long long arg and returns
its integer part + 1 if not 0, otherwise 0. GCC 3.3.1 passes some intermediate
values through several registers while it's unneeded. Here comes all my analysis.
I hope this can help improving the optimizer.
The following code :
int test(unsigned long long x) {
if (x) {
return (int)x + 1;
}
else {
return (int)x;
}
}
processed this way :
# 1 "bool.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "bool.c"
int test(unsigned long long x) {
if (x) {
return (int)x + 1;
}
else {
return (int)x;
}
}
produces the following code when compiled with gcc 2.95.3 :
(gcc -c -O2 -fomit-frame-pointer bool.c)
00000000 <test>:
0: 8b 54 24 04 mov 0x4(%esp,1),%edx
4: 8b 4c 24 08 mov 0x8(%esp,1),%ecx
8: 89 d0 mov %edx,%eax
a: 09 c8 or %ecx,%eax
c: 75 03 jne 11 <test+0x11>
e: 89 d0 mov %edx,%eax
10: c3 ret
11: 8d 42 01 lea 0x1(%edx),%eax
14: c3 ret
15: 8d 76 00 lea 0x0(%esi),%esi
18:
and this one with gcc-3.3.1 :
(gcc-3.3.1 -c -O2 -fomit-frame-pointer bool.c)
00000000 <test>:
0: 53 push %ebx
1: 8b 4c 24 0c mov 0xc(%esp,1),%ecx
5: 8b 54 24 08 mov 0x8(%esp,1),%edx
9: 89 cb mov %ecx,%ebx
b: 89 d0 mov %edx,%eax
d: 09 d3 or %edx,%ebx
f: 74 03 je 14 <test+0x14>
11: 8d 42 01 lea 0x1(%edx),%eax
14: 5b pop %ebx
15: c3 ret
16: 8d 76 00 lea 0x0(%esi),%esi
19: 8d bc 27 00 00 00 00 lea 0x0(%edi,1),%edi
20:
the EBX register is used and clobbered for nothing here. The
same code could be written this way, which is fully equivalent
and saves some cycles and bytes :
00000000 <test>:
0: 8b 4c 24 0c mov 0xc(%esp,1),%ecx
4: 8b 54 24 08 mov 0x8(%esp,1),%edx
8: 89 d0 mov %edx,%eax
a: 09 d1 or %edx,%ecx
c: 74 03 je 11 <test+0x11>
e: 8d 42 01 lea 0x1(%edx),%eax
11: c3 ret
12:
Now we can also save EDX and some more bytes :
00000000 <test>:
0: 8b 4c 24 0c mov 0xc(%esp,1),%ecx
4: 8b 54 24 08 mov 0x8(%esp,1),%eax
a: 09 c1 or %eax,%ecx
c: 74 01 je f <test+0xf>
e: 41 inc %eax
f: c3 ret
10:
Here are the compilers versions :
wtap:~/dev$ gcc -v
Reading specs from /usr/lib/gcc-lib/i586-pc-linux-gnu/2.95.3/specs
gcc version 2.95.3 20010315 (release)
wtap:~/dev$ gcc-3.3.1 -v
Reading specs from /usr/lib/gcc-lib/i586-pc-linux-gnu/3.3.1/specs
Configured with: ./configure --prefix=/usr --with-cpu=i386 --host=i586-pc-linux-gnu --enable-languages=c,c++ --disable-nls --disable-locale --enable-shared --enable-target-optspace --enable-version-specific-runtime-libs --program-suffix=-3.3.1 --enable-threads
Thread model: posix
gcc version 3.3.1
^ permalink raw reply [flat|nested] 5+ messages in thread
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[not found] <bug-11873-6707@http.gcc.gnu.org/bugzilla/>
2007-11-09 23:51 ` [Bug rtl-optimization/11873] inefficient use of registers induces size and time overhead rask at gcc dot gnu dot org
2003-08-10 8:39 [Bug optimization/11873] New: " willy at w dot ods dot org
2004-06-03 4:27 ` [Bug rtl-optimization/11873] " pinskia at gcc dot gnu dot org
2004-06-03 4:29 ` pinskia at gcc dot gnu dot org
2004-11-08 23:44 ` pinskia at gcc dot gnu dot org
2004-11-13 6:01 ` pinskia at gcc dot gnu dot org
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