Compile the following code with options -O2
extern "C" void foo(int a[4]);
extern "C" void bar(int a, int b, int c, int d)
{
int v[4] = {a,b,c,d};
foo(v);
foo(v);
}
GCC generates
bar:
stmfd sp!, {r4, lr}
.save {r4, lr}
.LCFI0:
.pad #16
sub sp, sp, #16
.LCFI1:
mov lr, #0
add ip, sp, #8
str lr, [ip], #4
str r0, [sp, #0]
mov r0, sp
str lr, [ip, #0]
stmib sp, {r1, r2, r3} @ phole stm
bl foo
mov r0, sp
mov r4, sp // A
bl foo
add sp, sp, #16
ldmfd sp!, {r4, lr}
bx lr
Instruction A move sp to r4, but r4 is never used again.
If compile the code with options -Os, the result is even worse:
bar:
stmfd sp!, {r0, r1, r2, r3, r4, r5, r6, r7, r8, lr}
.save {r0, r1, r2, r3, r4, r5, r6, r7, r8, lr}
.LCFI0:
mov r6, r0
mov r5, r1
mov r4, r2
mov r1, #0
mov r2, #16
mov r0, sp
mov r8, r3
bl memset
mov r0, sp
str r6, [sp, #0]
str r5, [sp, #4]
str r4, [sp, #8]
str r8, [sp, #12]
bl foo
mov r0, sp
mov r7, sp
bl foo
ldmfd sp!, {r0, r1, r2, r3, r4, r5, r6, r7, r8, lr}
bx lr
It calls memset to set the array to 0, then store parameters to that array. It
is quite inefficient.
--
Summary: unnecessary register move
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: carrot 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=42500
