Add a special handler for the case of doing a subtraction as in:

x -= y;

where abs(x) < abs(y).

This change alone increases ECDSA performance by 5 to 15%

1 files changed, 14 insertions, 1 deletions

diff --git a/src/math/bigint/mp_amd64/mp_asmi.h b/src/math/bigint/mp_amd64/mp_asmi.h
index d8f681d77..adf7774ef 100644
--- a/src/math/bigint/mp_amd64/mp_asmi.h
+++ b/src/math/bigint/mp_amd64/mp_asmi.h
@@ -1,6 +1,6 @@
 /*
 * Lowest Level MPI Algorithms
-* (C) 1999-2007 Jack Lloyd
+* (C) 1999-2010 Jack Lloyd
 *     2006 Luca Piccarreta
 *
 * Distributed under the terms of the Botan license
@@ -131,6 +131,19 @@ inline word word8_sub2(word x[8], const word y[8], word carry)
    }
 
 /*
+* Eight Word Block Subtraction, Two Argument
+*/
+inline word word8_sub2_rev(word x[8], const word y[8], word carry)
+   {
+   asm(
+      ADD_OR_SUBTRACT(DO_8_TIMES(ADDSUB3_OP, "sbbq"))
+      : [carry]"=r"(carry)
+      : [x]"r"(y), [y]"r"(x), [z]"r"(x), "0"(carry)
+      : "cc", "memory");
+   return carry;
+   }
+
+/*
 * Eight Word Block Subtraction, Three Argument
 */
 inline word word8_sub3(word z[8], const word x[8], const word y[8], word carry)