Re: [PATCH] common implementation of iterative div/mod

On Thu, 08 May 2008 16:16:41 +0100
Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:

We have a few instances of the open-coded iterative div/mod loop, used
when we don't expcet the dividend to be much bigger than the divisor.
Unfortunately modern gcc's have the tendency to strength "reduce" this
into a full mod operation, which isn't necessarily any faster, and
even if it were, doesn't exist if gcc implements it in libgcc.

The workaround is to put a dummy asm statement in the loop to prevent
gcc from performing the transformation.

This patch creates a single implementation of this loop, and uses it
to replace the open-coded versions I know about.

Fair enough. I'll plan on feeding this into 2.6.26 soon.

#endif /* BITS_PER_LONG == 32 */
+
+/*
+ * Iterative div/mod for use when dividend is not expected to be much
+ * bigger than divisor.
+ */
+unsigned iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
+{
+ unsigned ret = 0;
+
+ while(dividend >= divisor) {
+ /* The following asm() prevents the compiler from
+ optimising this loop into a modulo operation. */
+ asm("" : "+rm"(dividend));
+
+ dividend -= divisor;
+ ret++;
+ }
+
+ *remainder = dividend;
+
+ return ret;
+}
+EXPORT_SYMBOL(iter_div_u64_rem);


I think it would be better to do s/unsigned/u32/ here. It's cosmetic, but
all this sort of code is pretty formal about the sizes of the types which
it uses, and it sure needs to be.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/