Simple strength reduction on DUFFS_LOOP

SDL Development
1

A very easy optimization is just to do a strength reduction on the
operations preceeding the loop.

Instead of the following two lines…

int n = (width+7)/8;
switch (width % 8) {

…you can just use…

int n = (width+7)>>3;
switch (width & 0x03) {

I just don’t like the way the loop is nested. (Looks horrible.)
Something like…

switch (blah) {
	do {
		case 0:
		case 7:
		...
	} while (blah)
}

Seems to make a bit more sense. (However that might lead to one more jump
instruction, I’m not sure.) (It’s an asthetic thing, I know.)

Paul Braman
@Paul_Braman

2

A very easy optimization is just to do a strength reduction on the
operations preceeding the loop.

Instead of the following two lines…

int n = (width+7)/8;
switch (width % 8) {

…you can just use…

int n = (width+7)>>3;
switch (width & 0x03) {

Wouldn’t you expect the compiler to do this kind of strength reduction as
part of
its optimization anyway?

I just don’t like the way the loop is nested. (Looks horrible.)
Something like…

switch (blah) {
do {
case 0:
case 7:
…
} while (blah)
}

Seems to make a bit more sense. (However that might lead to one more jump
instruction, I’m not sure.) (It’s an asthetic thing, I know.)

Paul Braman
aeon at tampabay.rr.com

----- Original Message -----
From: aeon@tampabay.rr.com (Paul Braman)
To:
Sent: Monday, December 27, 1999 10:22 AM
Subject: [SDL] simple strength reduction on DUFFS_LOOP

3

Wouldn’t you expect the compiler to do this kind of strength reduction
as part of its optimization anyway?

Yeah, you probably would expect this to go on. That’s the only place I
can see where the loop can be generally optmized. You neither gain nor
lose anything for smart compilers by making the change, however.

Paul Braman
@Paul_BramanOn Mon, 27 Dec 1999, Prasanth Kumar wrote: