[PATCH 2/5] mpi: avoid MPI copy at gcry_mpi_sub

[Jussi Kivilinna]

* mpi/mpi-add.c (_gcry_mpi_add): Rename function...
(_gcry_mpi_add_inv_sign): ... to this and add parameter for inverting
sign of second operand.
(_gcry_mpi_add): New.
(_gcry_mpi_sub): Remove mpi_copy and instead use new
'_gcry_mpi_add_inv_sign' function with inverted sign for second
operand.
--

Benchmark on AMD Ryzen 9 7900X:

 Before:
                    |  nanosecs/byte   mebibytes/sec   cycles/byte  auto Mhz
 add                |     0.052 ns/B     18301 MiB/s     0.287 c/B      5500
 sub                |     0.098 ns/B      9768 MiB/s     0.537 c/B      5500

 After:
                    |  nanosecs/byte   mebibytes/sec   cycles/byte  auto Mhz
 add                |     0.030 ns/B     31771 MiB/s     0.165 c/B      5500
 sub                |     0.031 ns/B     31187 MiB/s     0.168 c/B      5500

Signed-off-by: Jussi Kivilinna <jussi.kivilinna at iki.fi>
---
 mpi/mpi-add.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/mpi/mpi-add.c b/mpi/mpi-add.c
index 38dd352f..2fd19e55 100644
--- a/mpi/mpi-add.c
+++ b/mpi/mpi-add.c
@@ -84,8 +84,8 @@ _gcry_mpi_add_ui (gcry_mpi_t w, gcry_mpi_t u, unsigned long v )
 }
 
 
-void
-_gcry_mpi_add(gcry_mpi_t w, gcry_mpi_t u, gcry_mpi_t v)
+static void
+_gcry_mpi_add_inv_sign(gcry_mpi_t w, gcry_mpi_t u, gcry_mpi_t v, int inv_v_sign)
 {
     mpi_ptr_t wp, up, vp;
     mpi_size_t usize, vsize, wsize;
@@ -93,7 +93,7 @@ _gcry_mpi_add(gcry_mpi_t w, gcry_mpi_t u, gcry_mpi_t v)
 
     if( u->nlimbs < v->nlimbs ) { /* Swap U and V. */
 	usize = v->nlimbs;
-	usign = v->sign;
+	usign = v->sign ^ inv_v_sign;
 	vsize = u->nlimbs;
 	vsign = u->sign;
 	wsize = usize + 1;
@@ -106,7 +106,7 @@ _gcry_mpi_add(gcry_mpi_t w, gcry_mpi_t u, gcry_mpi_t v)
 	usize = u->nlimbs;
 	usign = u->sign;
 	vsize = v->nlimbs;
-	vsign = v->sign;
+	vsign = v->sign ^ inv_v_sign;
 	wsize = usize + 1;
 	RESIZE_IF_NEEDED(w, wsize);
 	/* These must be after realloc (u or v may be the same as w).  */
@@ -211,13 +211,16 @@ _gcry_mpi_sub_ui(gcry_mpi_t w, gcry_mpi_t u, unsigned long v )
     w->sign   = wsign;
 }
 
+void
+_gcry_mpi_add(gcry_mpi_t w, gcry_mpi_t u, gcry_mpi_t v)
+{
+  _gcry_mpi_add_inv_sign (w, u, v, 0);
+}
+
 void
 _gcry_mpi_sub(gcry_mpi_t w, gcry_mpi_t u, gcry_mpi_t v)
 {
-  gcry_mpi_t vv = mpi_copy (v);
-  vv->sign = ! vv->sign;
-  mpi_add (w, u, vv);
-  mpi_free (vv);
+  _gcry_mpi_add_inv_sign (w, u, v, 1);
 }
 
 
-- 
2.39.2