https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53987
--- Comment #6 from Oleg Endo <olegendo at gcc dot gnu.org> ---
In the following example function
bool stack_is_background_free( unsigned short* _data, const size_t _size){
size_t num_pixels_above_128 = 0;
for(size_t index = 0;index< _size;++index){
if(_data[index] > 128)
num_pixels_above_128++;
}
return num_pixels_above_128 == _size;
}
unsigned 16 bit values are compared against an 8 bit constant, which results in
the following code:
mov.w @r4+,r1
extu.w r1,r1
cmp/hi r7,r1
mov #0,r1
addc r1,r3
dt r2
bf .L4
In this case, the zero extension can be omitted, since the comparison result
will always be 'true' if any bits >= 8 in the 16 bit value are set:
0x8000 (sign extend) -> 0xFFFF8000 (unsigned) > 0x80 = 1
0x8000 (zero extend) -> 0x00008000 (unsigned) > 0x80 = 1
The same would also apply for 8 bit values:
int test (unsigned char* x)
{
return x[0] >= 127;
}
compiles to:
mov.b @r4,r1
mov #126,r2
extu.b r1,r1
cmp/hi r2,r1
rts
movt r0
0x80 (sign extend) -> 0xFFFFFF80 (unsigned) > 0x7E = 1
0x80 (zero extend) -> 0x00000080 (unsigned) > 0x7E = 1
