Do not use mp directly in mpz_class

This is the difference between f and g in the following,
where gcc generates "return 1;" for f and not for g.
The aliasing is caused by the functions get_num and get_den
in mpq_class.

struct mpz_struct { int i; };
typedef mpz_struct mpz_t[1];
struct mpq_struct { mpz_struct num, den; };
typedef mpq_struct mpq_t[1];
struct mpz_class { mpz_t mp; };
struct mpq_class { mpq_t mp; };

int f(mpq_class*q, mpz_class*z){
  q->mp->num.i = 1;
  z->mp->i = 2;
  return q->mp->num.i;
}
int g(mpq_class*q, mpz_class*z){
  q->mp->num.i = 1;
  int*p=&z->mp->i;
  *p = 2;
  return q->mp->num.i;
}

1 files changed, 4 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index a38896524..3dc9640c7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2022-04-18  Marc Glisse  <marc.glisse@inria.fr>
+
+	* gmpxx.h (mpz_class): Do not use mp directly.
+
 2022-03-18 Marco Bodrato <bodrato@mail.dm.unipi.it>
 
 	* tests/mpz/t-lucm.c: Remove an unused var.