Force gcc to use x86's built-in 64/32->32,32 division instruction

rather than swanning off to a helper function. This seems to make
woefully little actual performance difference, which annoys me, but
it's a just-about-detectable improvement so I'll leave it in.

git-svn-id: svn://svn.tartarus.org/sgt/putty@3136 cda61777-01e9-0310-a592-d414129be87e

diff --git a/sshbn.c b/sshbn.c
index a51c3a5..3742227 100644 (file)
--- a/sshbn.c
+++ b/sshbn.c
@@ -15,6 +15,10 @@ typedef unsigned long long BignumDblInt;
 #define BIGNUM_TOP_BIT   0x80000000UL
 #define BIGNUM_INT_BITS  32
 #define MUL_WORD(w1, w2) ((BignumDblInt)w1 * w2)
+#define DIVMOD_WORD(q, r, hi, lo, w) \
+    __asm__("div %2" : \
+           "=d" (r), "=a" (q) : \
+           "r" (w), "d" (hi), "a" (lo))
 #else
 typedef unsigned short BignumInt;
 typedef unsigned long BignumDblInt;
@@ -22,6 +26,11 @@ typedef unsigned long BignumDblInt;
 #define BIGNUM_TOP_BIT   0x8000U
 #define BIGNUM_INT_BITS  16
 #define MUL_WORD(w1, w2) ((BignumDblInt)w1 * w2)
+#define DIVMOD_WORD(q, r, hi, lo, w) do { \
+    BignumDblInt n = (((BignumDblInt)hi) << BIGNUM_INT_BITS) | lo; \
+    q = n / w; \
+    r = n % w; \
+} while (0)
 #endif
 
 #define BIGNUM_INT_BYTES (BIGNUM_INT_BITS / 8)
@@ -175,13 +184,11 @@ static void internal_mod(BignumInt *a, int alen,
            ai1 = a[i + 1];
 
        /* Find q = h:a[i] / m0 */
-       t = ((BignumDblInt) h << BIGNUM_INT_BITS) + a[i];
-       q = t / m0;
-       r = t % m0;
+       DIVMOD_WORD(q, r, h, a[i], m0);
 
        /* Refine our estimate of q by looking at
           h:a[i]:a[i+1] / m0:m1 */
-       t = (BignumDblInt) m1 * (BignumDblInt) q;
+       t = MUL_WORD(m1, q);
        if (t > ((BignumDblInt) r << BIGNUM_INT_BITS) + ai1) {
            q--;
            t -= m1;
@@ -193,7 +200,7 @@ static void internal_mod(BignumInt *a, int alen,
        /* Subtract q * m from a[i...] */
        c = 0;
        for (k = mlen - 1; k >= 0; k--) {
-           t = (BignumDblInt) q * (BignumDblInt) m[k];
+           t = MUL_WORD(q, m[k]);
            t += c;
            c = t >> BIGNUM_INT_BITS;
            if ((BignumInt) t > a[i + k])