[ovs-dev] [PATCH 2/2] flow: Workaround for GCC false-positive compilation error.

[Jarno Rajahalme]

Without an explicit bounds check GCC 4.9 issues an array out of bounds
error.  This patch adds explicit checks which will however be
optimized away as the relevant parameters are compile-time constants.

Signed-off-by: Jarno Rajahalme <jrajahalme at nicira.com>
---
 lib/flow.h | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/lib/flow.h b/lib/flow.h
index 323cbdb..25172e0 100644
--- a/lib/flow.h
+++ b/lib/flow.h
@@ -478,7 +478,9 @@ flowmap_are_set(const struct flowmap *fm, size_t idx, unsigned int n_bits)
     if (fm->bits[unit] & (n_bits_mask << idx)) {
         return true;
     }
-    if (idx + n_bits > MAP_T_BITS) {
+    /* The seemingly unnecessary bounds check on 'unit' is a workaround for a
+     * false-positive array out of bounds error by GCC 4.9. */
+    if (unit + 1 < FLOWMAP_UNITS && idx + n_bits > MAP_T_BITS) {
         /* Check the remaining bits from the next unit. */
         return fm->bits[unit + 1] & (n_bits_mask >> (MAP_T_BITS - idx));
     }
@@ -496,7 +498,9 @@ flowmap_set(struct flowmap *fm, size_t idx, unsigned int n_bits)
     idx %= MAP_T_BITS;
 
     fm->bits[unit] |= n_bits_mask << idx;
-    if (idx + n_bits > MAP_T_BITS) {
+    /* The seemingly unnecessary bounds check on 'unit' is a workaround for a
+     * false-positive array out of bounds error by GCC 4.9. */
+    if (unit + 1 < FLOWMAP_UNITS && idx + n_bits > MAP_T_BITS) {
         /* 'MAP_T_BITS - idx' bits were set on 'unit', set the remaining
          * bits from the next unit. */
         fm->bits[unit + 1] |= n_bits_mask >> (MAP_T_BITS - idx);
@@ -514,7 +518,9 @@ flowmap_clear(struct flowmap *fm, size_t idx, unsigned int n_bits)
     idx %= MAP_T_BITS;
 
     fm->bits[unit] &= ~(n_bits_mask << idx);
-    if (idx + n_bits > MAP_T_BITS) {
+    /* The seemingly unnecessary bounds check on 'unit' is a workaround for a
+     * false-positive array out of bounds error by GCC 4.9. */
+    if (unit + 1 < FLOWMAP_UNITS && idx + n_bits > MAP_T_BITS) {
         /* 'MAP_T_BITS - idx' bits were cleared on 'unit', clear the
          * remaining bits from the next unit. */
         fm->bits[unit + 1] &= ~(n_bits_mask >> (MAP_T_BITS - idx));
-- 
2.1.4