This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: [PATCH] Use intermediate variable to compute exponent in __muland __sqr
On Fri, Feb 15, 2013 at 04:42:32PM +0100, Andreas Jaeger wrote:
> On 02/14/2013 11:53 AM, Siddhesh Poyarekar wrote:
> I think this kind of optimization needs a comment. Could you add one,
> please?
>
Done, this is what I committed.
Siddhesh
diff --git a/sysdeps/ieee754/dbl-64/mpa.c b/sysdeps/ieee754/dbl-64/mpa.c
index 25f52f6..9da4257 100644
--- a/sysdeps/ieee754/dbl-64/mpa.c
+++ b/sysdeps/ieee754/dbl-64/mpa.c
@@ -693,15 +693,20 @@ __mul (const mp_no *x, const mp_no *y, mp_no *z, int p)
}
Z[k] = zk;
- EZ = EX + EY;
+ /* Get the exponent sum into an intermediate variable. This is a subtle
+ optimization, where given enough registers, all operations on the exponent
+ happen in registers and the result is written out only once into EZ. */
+ int e = EX + EY;
+
/* Is there a carry beyond the most significant digit? */
if (__glibc_unlikely (Z[1] == ZERO))
{
for (i = 1; i <= p; i++)
Z[i] = Z[i + 1];
- EZ--;
+ e--;
}
+ EZ = e;
Z[0] = X[0] * Y[0];
}
@@ -786,14 +791,20 @@ __sqr (const mp_no *x, mp_no *y, int p)
/* Squares are always positive. */
Y[0] = 1.0;
- EY = 2 * EX;
+ /* Get the exponent sum into an intermediate variable. This is a subtle
+ optimization, where given enough registers, all operations on the exponent
+ happen in registers and the result is written out only once into EZ. */
+ int e = EX * 2;
+
/* Is there a carry beyond the most significant digit? */
if (__glibc_unlikely (Y[1] == ZERO))
{
for (i = 1; i <= p; i++)
Y[i] = Y[i + 1];
- EY--;
+ e--;
}
+
+ EY = e;
}
/* Invert *X and store in *Y. Relative error bound: