diff --git a/mpint.c b/mpint.c
index 3d7a3987..8ac7a50e 100644
--- a/mpint.c
+++ b/mpint.c
@@ -33,6 +33,35 @@ static inline BignumInt mp_word(mp_int *x, size_t i)
     return i < x->nw ? x->w[i] : 0;
 }
 
+/*
+ * Shift an ordinary C integer by BIGNUM_INT_BITS, in a way that
+ * avoids writing a shift operator whose RHS is greater or equal to
+ * the size of the type, because that's undefined behaviour in C.
+ *
+ * In fact we must avoid even writing it in a definitely-untaken
+ * branch of an if, because compilers will sometimes warn about
+ * that. So you can't just write 'shift too big ? 0 : n >> shift',
+ * because even if 'shift too big' is a constant-expression
+ * evaluating to false, you can still get complaints about the
+ * else clause of the ?:.
+ *
+ * So we have to re-check _inside_ that clause, so that the shift
+ * count is reset to something nonsensical but safe in the case
+ * where the clause wasn't going to be taken anyway.
+ */
+static uintmax_t shift_right_by_one_word(uintmax_t n)
+{
+    bool shift_too_big = BIGNUM_INT_BYTES >= sizeof(n);
+    return shift_too_big ? 0 :
+        n >> (shift_too_big ? 0 : BIGNUM_INT_BITS);
+}
+static uintmax_t shift_left_by_one_word(uintmax_t n)
+{
+    bool shift_too_big = BIGNUM_INT_BYTES >= sizeof(n);
+    return shift_too_big ? 0 :
+        n << (shift_too_big ? 0 : BIGNUM_INT_BITS);
+}
+
 static mp_int *mp_make_sized(size_t nw)
 {
     mp_int *x = snew_plus(mp_int, nw * sizeof(BignumInt));
@@ -260,12 +289,8 @@ unsigned mp_get_bit(mp_int *x, size_t bit)
 uintmax_t mp_get_integer(mp_int *x)
 {
     uintmax_t toret = 0;
-    for (size_t i = x->nw; i-- > 0 ;) {
-        /* Shift in two stages to avoid undefined behaviour if the
-         * shift count equals the integer width */
-        toret = (toret << (BIGNUM_INT_BITS/2)) << (BIGNUM_INT_BITS/2);
-        toret |= x->w[i];
-    }
+    for (size_t i = x->nw; i-- > 0 ;)
+        toret = shift_left_by_one_word(toret) | x->w[i];
     return toret;
 }
 
@@ -757,8 +782,8 @@ static BignumCarry mp_add_masked_integer_into(
 {
     for (size_t i = 0; i < rw; i++) {
         BignumInt aword = mp_word(a, i);
-        size_t shift = i * BIGNUM_INT_BITS;
-        BignumInt bword = shift < CHAR_BIT*sizeof(b) ? b >> shift : 0;
+        BignumInt bword = b;
+        b = shift_right_by_one_word(b);
         BignumInt out;
         bword = (bword ^ b_xor) & b_and;
         BignumADC(out, carry, aword, bword, carry);
@@ -799,8 +824,7 @@ static void mp_add_integer_into_shifted_by_words(
          * shift n down. If it's 0, we add zero bits into r, and
          * leave n alone. */
         BignumInt bword = n & -(BignumInt)indicator;
-        uintmax_t new_n = (BIGNUM_INT_BYTES < sizeof(n) ?
-                           n >> BIGNUM_INT_BITS : 0);
+        uintmax_t new_n = shift_right_by_one_word(n);
         n ^= (n ^ new_n) & -(uintmax_t)indicator;
 
         BignumInt aword = mp_word(a, i);
@@ -846,8 +870,8 @@ unsigned mp_hs_integer(mp_int *x, uintmax_t n)
 {
     BignumInt carry = 1;
     for (size_t i = 0; i < x->nw; i++) {
-        size_t shift = i * BIGNUM_INT_BITS;
-        BignumInt nword = shift < CHAR_BIT*sizeof(n) ? n >> shift : 0;
+        BignumInt nword = n;
+        n = shift_right_by_one_word(n);
         BignumInt dummy_out;
         BignumADC(dummy_out, carry, x->w[i], ~nword, carry);
         (void)dummy_out;
@@ -872,8 +896,8 @@ unsigned mp_eq_integer(mp_int *x, uintmax_t n)
 {
     BignumInt diff = 0;
     for (size_t i = 0; i < x->nw; i++) {
-        size_t shift = i * BIGNUM_INT_BITS;
-        BignumInt nword = shift < CHAR_BIT*sizeof(n) ? n >> shift : 0;
+        BignumInt nword = n;
+        n = shift_right_by_one_word(n);
         diff |= x->w[i] ^ nword;
     }
     return 1 ^ normalise_to_1(diff);   /* return 1 if diff _is_ zero */