From e83f140463f2144b84e2fe4fa249ab8c8426b5e9 Mon Sep 17 00:00:00 2001
From: Damien George <damien.p.george@gmail.com>
Date: Wed, 14 Dec 2016 10:39:41 +1100
Subject: [PATCH] py/mpz: Remove unreachable code in mpn_or_neg functions.
---
py/mpz.c | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)
diff --git a/py/mpz.c b/py/mpz.c
index 1dd177b8e..1861840e8 100644
--- a/py/mpz.c
+++ b/py/mpz.c
@@ -308,9 +308,13 @@ STATIC mp_uint_t mpn_or_neg(mpz_dig_t *idig, const mpz_dig_t *jdig, mp_uint_t jl
carryi >>= DIG_SIZE;
}
- if (0 != carryi) {
- *idig++ = carryi;
- }
+ // At least one of j,k must be negative so the above for-loop runs at least
+ // once. For carryi to be non-zero here it must be equal to 1 at the end of
+ // each iteration of the loop. So the accumulation of carryi must overflow
+ // each time, ie carryi += 0xff..ff. So carryj|carryk must be 0 in the
+ // DIG_MASK bits on each iteration. But considering all cases of signs of
+ // j,k one sees that this is not possible.
+ assert(carryi == 0);
return mpn_remove_trailing_zeros(oidig, idig);
}
@@ -334,9 +338,8 @@ STATIC mp_uint_t mpn_or_neg(mpz_dig_t *idig, const mpz_dig_t *jdig, mp_uint_t jl
carryi >>= DIG_SIZE;
}
- if (0 != carryi) {
- *idig++ = carryi;
- }
+ // See comment in above mpn_or_neg for why carryi must be 0.
+ assert(carryi == 0);
return mpn_remove_trailing_zeros(oidig, idig);
}
--
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