Static Branch prediction for the ARM with __builtin_expect is not functional!!? - performance

Question

Im doing the optimization in the C code running in the Cortex-R4.
first of all I haven't seen any change in the assembly code output when I indicated the "__builtin_expect" in condition check.
It seem like the compiler generate the unnecessary Jump.
My C code:
bit ++; (Likely)
if(__builtin_expect(bit >= 32),0)
{
bit -=32; // unlikely code
xxxxxx; // unlikely code
xxxxxx; // unlikely code
xxxxxx; // unlikely code
}
bit = bit*2 // something (Likely)
return bit;
---- Generated ASM code --------
(bit => r0)
ADD r2,r2,#1
CMP r0,#0x20
BCC NoDecrement
SUB r0,r0,#0x20
XXXXXXXXX
XXXXXXXXX
XXXXXXXXX
NoDecrement LSL r0,r0,#1
BX lr
---- My expected ASM Code --------
ADD r2,r2,#1
CMP r0,#0x20
BHE Decrement
JumbBack LSL r0,r0,#1
BX lr
Decrement SUB r0,r0,#0x20
XXXXXXXXX
XXXXXXXXX
XXXXXXXXX
B JumbBack
suppose if this piece of C code runs in a loop, then each time it has to jump (because the if condition is passed only once).
Is there any other compiler setting which actually, generates the code as expected..??

Answer 1

You wrote:
if(__builtin_expect(bit >= 32),0)
{
...
}
The code inside the curly braces will never be executed, because it's surrounded by if(foo,0) which is equivalent to if(0) for any value of foo, no matter what builtin you're trying to use. If you turn on optimization with -O2, you'll see that the compiler removes the dead code completely, rather than just jumping around it. I think you probably meant to write
if (__builtin_expect(bit >= 32, 0)) {
bit -= 32;
}
If I do this, I get exactly the forward branch I'd expect (with clang -O1 or higher).
extern void something();
int foo(int bit)
{
++bit;
if (__builtin_expect(bit >= 32, 0)) {
bit -= 32; // "Decrement"
something();
}
bit = bit*2;
something();
return bit;
}
Here's the code from clang -arch armv7 -O2 -S:
_foo:
# BB#0:
push {r4, r7, lr}
adds r4, r0, #1
add r7, sp, #4
cmp r4, #32
bge LBB0_2 // a forward branch for the unlikely case
LBB0_1:
lsls r4, r4, #1
blx _something
mov r0, r4
pop {r4, r7, pc}
LBB0_2: // "Decrement"
sub.w r4, r0, #31
blx _something
b LBB0_1