Avoid unnecessary doublings in precomputation

peterdettman · sipa · commit f663209b772a · 2021-01-30T11:53:04.000-08:00
diff --git a/src/ecmult_gen_impl.h b/src/ecmult_gen_impl.h
@@ -64,8 +64,10 @@ static void secp256k1_ecmult_gen_context_build(secp256k1_ecmult_gen_context *ctx
                 secp256k1_gej_add_var(&sum, &sum, &u, NULL);
                 secp256k1_gej_double_var(&u, &u, NULL);
                 ds[tooth] = u;
-                for (spacing = 1; spacing < COMB_SPACING; ++spacing) {
-                    secp256k1_gej_double_var(&u, &u, NULL);
+                if (block + tooth != COMB_BLOCKS + COMB_TEETH - 2) {
+                    for (spacing = 1; spacing < COMB_SPACING; ++spacing) {
+                        secp256k1_gej_double_var(&u, &u, NULL);
+                    }
                 }
             }
             secp256k1_gej_neg(&vs[vs_pos++], &sum);

Original file line number	Diff line number	Diff line change
`@@ -64,8 +64,10 @@ static void secp256k1_ecmult_gen_context_build(secp256k1_ecmult_gen_context *ctx`
`64`	`64`	`secp256k1_gej_add_var(&sum, &sum, &u, NULL);`
`65`	`65`	`secp256k1_gej_double_var(&u, &u, NULL);`
`66`	`66`	`ds[tooth] = u;`
`67`		`- for (spacing = 1; spacing < COMB_SPACING; ++spacing) {`
`68`		`- secp256k1_gej_double_var(&u, &u, NULL);`
	`67`	`+ if (block + tooth != COMB_BLOCKS + COMB_TEETH - 2) {`
	`68`	`+ for (spacing = 1; spacing < COMB_SPACING; ++spacing) {`
	`69`	`+ secp256k1_gej_double_var(&u, &u, NULL);`
	`70`	`+ }`
`69`	`71`	`}`
`70`	`72`	`}`
`71`	`73`	`secp256k1_gej_neg(&vs[vs_pos++], &sum);`