The alloc and nr fields of a prio-queue tell us how much memory is
allocated and used in the array. So the natural type for them is size_t,
which prevents overflow on 64-bit systems where "int" is still 32 bits.
This is unlikely to happen in practice, as we typically use it for
storing commits, and having 2^31 of those is rather a lot. But it's good
to keep our generic data structures as flexible as possible. And as we
start to enforce -Wsign-compare, it means that callers need to use
"int", too, and the problem proliferates. Let's fix it at the source.
The changes here can be put into a few groups:
1. Changing the alloc/nr fields in the struct to size_t. This requires
swapping out int for size_t in negotiator/skipping.c, as well as
in prio_queue_get(), because those all iterate over the array.
Building with -Wsign-compare complains about these.
2. Other code that assigns or passes around indexes into the array
(e.g., the swap() and compare() functions) won't trigger
-Wsign-compare because we are simply truncating the values. These
are caught by -Wconversion, but I've adjusted them here to
future-proof us.
3. In prio_queue_reverse() we compute "queue->nr - 1" without checking
if anything is in the queue, which underflows now that nr is
unsigned. We can fix that by returning early when the queue is
empty (there is nothing to reverse).
4. The insertion_ctr variable is currently unsigned, but can likewise
grow (it is actually worse, because adding and removing an element
many times will keep increasing the counter, even though "nr" does
not). I've bumped that to size_t here, as well.
But -Wconversion notes that computing the "cmp" result by
subtracting the counters and assigning to "int" is a potential
problem. And that's true even before this patch, since we use an
unsigned counter (imagine comparing "2^32-1" and "0", which should
be a high positive value, but instead is "-1" as a signed int).
Since we only care about the sign (and not the magnitude) of the
result, we could fix this by swapping out the subtraction for a
ternary comparison. Probably the performance impact would be
negligible, since we just called into a custom compare function and
branched on its result anyway. But it's easy enough to do a
branchless version by subtracting the comparison results.
Signed-off-by: Jeff King <peff@xxxxxxxx>
---
Noticed this because the -Wsign-compare topic hit next, which raised a
few warnings with my local branches (but I think prio-queue is in the
wrong here, hence this patch). It did turn out a little more involved
than I had expected, but I think the end result is more future-proof.
negotiator/skipping.c | 5 ++---
prio-queue.c | 15 +++++++++------
prio-queue.h | 2 +-
3 files changed, 12 insertions(+), 10 deletions(-)
diff --git a/negotiator/skipping.c b/negotiator/skipping.c
index abedb70a48..616df6bf3a 100644
--- a/negotiator/skipping.c
+++ b/negotiator/skipping.c
@@ -134,7 +134,6 @@ static int push_parent(struct data *data, struct entry *entry,
struct entry *parent_entry;
if (to_push->object.flags & SEEN) {
- int i;
if (to_push->object.flags & POPPED)
/*
* The entry for this commit has already been popped,
@@ -145,7 +144,7 @@ static int push_parent(struct data *data, struct entry *entry,
/*
* Find the existing entry and use it.
*/
- for (i = 0; i < data->rev_list.nr; i++) {
+ for (size_t i = 0; i < data->rev_list.nr; i++) {
parent_entry = data->rev_list.array[i].data;
if (parent_entry->commit == to_push)
goto parent_found;
@@ -248,7 +247,7 @@ static int ack(struct fetch_negotiator *n, struct commit *c)
static void release(struct fetch_negotiator *n)
{
struct data *data = n->data;
- for (int i = 0; i < data->rev_list.nr; i++)
+ for (size_t i = 0; i < data->rev_list.nr; i++)
free(data->rev_list.array[i].data);
clear_prio_queue(&data->rev_list);
FREE_AND_NULL(data);
diff --git a/prio-queue.c b/prio-queue.c
index 450775a374..d0581dcc5c 100644
--- a/prio-queue.c
+++ b/prio-queue.c
@@ -1,26 +1,29 @@
#include "git-compat-util.h"
#include "prio-queue.h"
-static inline int compare(struct prio_queue *queue, int i, int j)
+static inline int compare(struct prio_queue *queue, size_t i, size_t j)
{
int cmp = queue->compare(queue->array[i].data, queue->array[j].data,
queue->cb_data);
if (!cmp)
- cmp = queue->array[i].ctr - queue->array[j].ctr;
+ cmp = (queue->array[i].ctr > queue->array[j].ctr) -
+ (queue->array[i].ctr < queue->array[j].ctr);
return cmp;
}
-static inline void swap(struct prio_queue *queue, int i, int j)
+static inline void swap(struct prio_queue *queue, size_t i, size_t j)
{
SWAP(queue->array[i], queue->array[j]);
}
void prio_queue_reverse(struct prio_queue *queue)
{
- int i, j;
+ size_t i, j;
if (queue->compare)
BUG("prio_queue_reverse() on non-LIFO queue");
+ if (!queue->nr)
+ return;
for (i = 0; i < (j = (queue->nr - 1) - i); i++)
swap(queue, i, j);
}
@@ -35,7 +38,7 @@ void clear_prio_queue(struct prio_queue *queue)
void prio_queue_put(struct prio_queue *queue, void *thing)
{
- int ix, parent;
+ size_t ix, parent;
/* Append at the end */
ALLOC_GROW(queue->array, queue->nr + 1, queue->alloc);
@@ -58,7 +61,7 @@ void prio_queue_put(struct prio_queue *queue, void *thing)
void *prio_queue_get(struct prio_queue *queue)
{
void *result;
- int ix, child;
+ size_t ix, child;
if (!queue->nr)
return NULL;
diff --git a/prio-queue.h b/prio-queue.h
index 4f9a37e6be..36f370625f 100644
--- a/prio-queue.h
+++ b/prio-queue.h
@@ -30,7 +30,7 @@ struct prio_queue {
prio_queue_compare_fn compare;
unsigned insertion_ctr;
void *cb_data;
- int alloc, nr;
+ size_t alloc, nr;
struct prio_queue_entry *array;
};
--
2.48.0.rc0.406.g65874f98ea
