It seems that GCC 4.2.1 generates better code than GCC 4.4.0 in this case:
The following code (extracted from Android's
Dalvik_java_lang_System_currentTimeMillis in native/java_lang_System.c):
// compilation options: -march=armv5te -mthumb -Os
struct timeval
{
long tv_sec;
long tv_usec;
};
extern void get_time(struct timeval*);
void test(long long *res)
{
struct timeval tv;
get_time(&tv);
*res = tv.tv_sec * 1000LL + tv.tv_usec / 1000;
}
is compiled by gcc-4.4.0 in sub-optimal way, so it takes 110 bytes (vs 74 bytes
when compiled by gcc-4.2.1). Assembly files shows that it spills some registers
on stack because code that multiply on 1000LL uses more registers that it need
(that is use when compiled by gcc-4.2.1). Multiplication code is similar, but
gcc 4.4 emits several additional MOVs that can be easily eliminated.
This bug can be more easily demonstrated with multiplication of tv_sec by 10
and tv_usec/ 1000 removed.
gcc.4.2.1:
push {r4, r5, lr}
sub sp, sp, #12
mov r5, r0
mov r0, sp
bl get_time
ldr r2, [sp]
add sp, sp, #12
@ sp needed for prologue
asr r4, r2, #31
mov r3, r2
lsr r0, r2, #30
lsl r2, r4, #2
orr r2, r2, r0
lsl r1, r3, #2
add r1, r1, r3
adc r2, r2, r4
lsr r0, r1, #31
lsl r4, r2, #1
orr r4, r4, r0
lsl r3, r1, #1
str r3, [r5]
str r4, [r5, #4]
pop {r4, r5, pc}
gcc 4.4.0:
push {r4, r5, r6, r7, lr} // note that gcc 4.2.1 uses only
{r4, r5, lr}
sub sp, sp, #12
mov r4, r0
mov r0, sp
bl get_time
ldr r6, [sp]
add sp, sp, #12
@ sp needed for prologue
mov r0, r6
asr r6, r6, #31
lsr r7, r0, #30
lsl r3, r6, #2
orr r3, r3, r7
mov r1, r6 // not needed actually, r6 can be used directly
lsl r2, r0, #2
add r0, r0, r2
adc r1, r1, r3
lsr r2, r0, #31
lsl r3, r1, #1
orr r3, r3, r2
lsl r0, r0, #1
str r0, [r4]
str r3, [r4, #4]
pop {r4, r5, r6, r7, pc}
--
Summary: Bad register allocation in multiplication code by
constant
Product: gcc
Version: 4.4.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: sliao at google dot com
GCC build triplet: i686-linux
GCC host triplet: i686-linux
GCC target triplet: arm-eabi
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42499
