base: Fix CircularQueue when diffing iterators

This patch is fixing CircularQueue iterators' subtraction, in particular
the behaviour when head and tail round multiple times.

Change-Id: Ie79ac8accd30a10cf039cf4def87675b01375d6b
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Gabor Dozsa <gabor.dozsa@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17188
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

1 files changed, 13 insertions, 4 deletions

diff --git a/src/base/circular_queue.hh b/src/base/circular_queue.hh
index 5023661b9..f368ed0d1 100644
--- a/src/base/circular_queue.hh
+++ b/src/base/circular_queue.hh
@@ -106,10 +106,19 @@ class CircularQueue : private std::vector<T>
     static uint32_t
     moduloSub(uint32_t op1, uint32_t op2, uint32_t size)
     {
-        int ret = (uint32_t)(op1 - op2) % size;
+        int32_t ret = sub(op1, op2, size);
         return ret >= 0 ? ret : ret + size;
     }
 
+    static int32_t
+    sub(uint32_t op1, uint32_t op2, uint32_t size)
+    {
+        if (op1 > op2)
+            return (op1 - op2) % size;
+        else
+            return -((op2 - op1) % size);
+    }
+
     void increase(uint32_t& v, size_t delta = 1)
     {
         v = moduloAdd(v, delta, _capacity);
@@ -355,10 +364,10 @@ class CircularQueue : private std::vector<T>
         difference_type operator-(const iterator& that)
         {
             /* If a is already at the end, we can safely return 0. */
-            auto ret = _cq->moduloSub(this->_idx, that._idx);
+            auto ret = _cq->sub(this->_idx, that._idx, _cq->capacity());
 
-            if (ret == 0 && this->_round != that._round) {
-                ret += this->_round * _cq->capacity();
+            if (this->_round != that._round) {
+                ret += ((this->_round - that._round) * _cq->capacity());
             }
             return ret;
         }