Hello List,
Duff’s device is a very dangerous trick for the strange & non-standard c syntax usage.
There is a compiler problem in duff’s device (SDL_blit.h). The comment above it says:
“There’s a bug in the Visual C++ 7 optimizer when compiling this code”
I don’t think it is a optimizer bug, but a duff’s bug – non-standard c syntax usage.
/* 4-times unrolled loop */
#define DUFFS_LOOP4(pixel_copy_increment, width)
{ int n = (width+3)/4;
switch (width & 3) {
case 0: do { pixel_copy_increment;
case 3: pixel_copy_increment;
case 2: pixel_copy_increment;
case 1: pixel_copy_increment;
} while ( --n > 0 );
}
}
it can be replaced with clean-c code without any compiler error:
#define DUFFS_LOOP4_FIXED(pixel_copy_increment, width)
{
unsigned long __width = (unsigned long)(width);
unsigned long __increment = __width >> 2;
for (; __increment > 0; __increment–) {
pixel_copy_increment;
pixel_copy_increment;
pixel_copy_increment;
pixel_copy_increment;
}
switch (__width & 3)
{
case 3: pixel_copy_increment;
case 2: pixel_copy_increment;
case 1: pixel_copy_increment;
case 0: break;
}
}
The only disadvantage is that it will generate more code. Because of the strange & non-
standard c syntax usage, we find bugs in VC7 today, maybe in other compilers tomorrow.
Duff maybe feel elated by mixing switch/do/case/while together, but it is irrelevant
in loop unrolling.
DUFFS_LOOP_DOUBLE2 can also be replaced with clean-c:
#define DUFFS_LOOP_DOUBLE2_FIXED(working, workingx2, width)
{
unsigned long __width = (unsigned long)(width);
unsigned long __increment = __width >> 2;
for (; __increment > 0; __increment–) {
workingx2;
workingx2;
}
switch (__width & 3) {
case 2: workingx2; break;
case 3: workingx2;
case 1: working; break;
}
}
The old version - “DUFFS_LOOP_DOUBLE2” is likely to copy 1-3 pixels first before
unrolling when “width % 4 != 0”. It will not get the best performance when src/dst
address are both aligned and the width mod 4 isn’t zero.
I think it is good for us to write every thing in standard c syntax .------------------
skywind
2007-09-23