[Guile-commits] 03/06: Less pessimal scm_integer_sub_zi

[Andy Wingo]

wingo pushed a commit to branch wip-inline-digits
in repository guile.

commit 774b863bfcfba724e4caf7dc4b580260acf55138
Author: Andy Wingo <wingo@pobox.com>
AuthorDate: Sun Jan 9 20:48:45 2022 +0100

    Less pessimal scm_integer_sub_zi
    
    * libguile/integers.c (scm_integer_sub_zi): Only delegate to
    scm_integer_add_ii if y is negative.
---
 libguile/integers.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/libguile/integers.c b/libguile/integers.c
index 60c0fd34a..b368f52db 100644
--- a/libguile/integers.c
+++ b/libguile/integers.c
@@ -2875,10 +2875,22 @@ scm_integer_sub_iz (scm_t_inum x, struct scm_bignum *y)
 SCM
 scm_integer_sub_zi (struct scm_bignum *x, scm_t_inum y)
 {
-  // Assumes that -INUM_MIN can fit in a scm_t_inum, even if that
-  // scm_t_inum is not fixable, and that scm_integer_add_ii can handle
-  // scm_t_inum inputs outside the fixable range.
-  return scm_integer_add_zi (x, -y);
+  if (y == 0)
+    return scm_from_bignum (x);
+  if (y < 0)
+    // Assumes that -INUM_MIN can fit in a scm_t_inum, even if that
+    // scm_t_inum is not fixable, and that scm_integer_add_ii can handle
+    // scm_t_inum inputs outside the fixable range.
+    return scm_integer_add_zi (x, -y);
+
+  mpz_t result, zx;
+  mpz_init (result);
+  alias_bignum_to_mpz (x, zx);
+  mpz_sub_ui (result, zx, y);
+  scm_remember_upto_here_1 (x);
+  // FIXME: We know that if X is negative, no need to check if
+  // result is fixable.
+  return take_mpz (result);
 }
 
 SCM