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Re: [PATCH] Use intermediate variable to compute exponent in __muland __sqr

On 02/14/2013 11:53 AM, Siddhesh Poyarekar wrote:
Hi,

Here's a tiny bit of optimization for __mul and __sqr that keeps the
values in a register during computation and then writes the result out
to EZ only once.  This gives about a 1% boost in performance.
Verified that it does not cause regressions on x86_64.  I've not
changed the ppc bits since I intend to post a separate patch to sync
up this part of code once this is in (and provided that the result is
positive for ppc as well).

OK to commit?

Siddhesh

	* sysdeps/ieee754/dbl-64/mpa.c (__mul): Use intermediate
	variable to calculate EZ.
	(__sqr): Likewise.

diff --git a/sysdeps/ieee754/dbl-64/mpa.c b/sysdeps/ieee754/dbl-64/mpa.c
index a5aace7..bbe9648 100644
--- a/sysdeps/ieee754/dbl-64/mpa.c
+++ b/sysdeps/ieee754/dbl-64/mpa.c
@@ -737,15 +737,16 @@ __mul (const mp_no *x, const mp_no *y, mp_no *z, int p)
      }
    Z[k] = zk;

-  EZ = EX + EY;
+  int e = EX + EY;

I think this kind of optimization needs a comment. Could you add one, please?

The code itself is fine, thanks,

Andreas

    /* 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];
  }

@@ -830,14 +831,16 @@ __sqr (const mp_no *x, mp_no *y, int p)
    /* Squares are always positive.  */
    Y[0] = 1.0;

-  EY = 2 * EX;
+  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:



--
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