vulkan: don't zero-initialize STACK_ARRAY()'s stack array

STACK_ARRAY() is used in a lot of places. When games are running we
see STACK_ARRAY() arrays being used all the time: each queue
submission uses 6, WaitSemaphores and syncobj waiting also uses them:
they're constantly present in Vulkan runtime.

There's no need for STACK_ARRAY()'s stack array to be initialized,
callers cannot not depend on it. If the number of elements is greater
than STACK_ARRAY_SIZE, then STACK_ARRAY() will just malloc() the array
and return it not initialized: anybody depending of
zero-initialization is going to break when the array is big.

The reason why we're zero-intializing STACK_ARRAY()'s stack array is
to silence -Wmaybe-uninitialized warnings: see commit d7957df318
("vulkan: fix uninitialized variables"). I don't think that commit is
the ideal way to deal with the problem, so this patch proposes a
better solution.

The problem here is that zero-initializing it adds code we don't need
for every single caller. STACK_ARRAY() already has 63 callers and only
3 of them are affected by the -Wmaybe-uninitialized warining. So here
we undo what commit d7957df318 did and instead we fix the 3 cases
that actually generate the -Wmaybe-uninitialized warnings.

Gcc is only emitting those warinings because it knows that the number
of elements in the array may be zero, so the loops we have that set
elements to the array may end up do nothing, and then we pass the
array uninitialized to other functions.

For the cases related to vk_sync this is just returning VK_SUCCESS
earlier, instead of relying on the check that eventually happens at
__vk_sync_wait_many(). For the vkCmdWaitEvents() function, the Vulkan
spec says that "eventCount must be greater than 0", so the early
return doesn't hurt anybody either. In both cases we make the zero
case faster by not defining an 8-sized array, zero-initializing it,
then returning success without using it.

Reference: d7957df318 ("vulkan: fix uninitialized variables")
Acked-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Yiwei Zhang <zzyiwei@chromium.org>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28288>

src/vulkan/runtime/vk_queue.c

@@ -1037,6 +1037,10 @@ vk_queue_wait_before_present(struct vk_queue *queue,
       return VK_SUCCESS;
 
    const uint32_t wait_count = pPresentInfo->waitSemaphoreCount;
+
+   if (wait_count == 0)
+      return VK_SUCCESS;
+
    STACK_ARRAY(struct vk_sync_wait, waits, wait_count);
 
    for (uint32_t i = 0; i < wait_count; i++) {

src/vulkan/runtime/vk_sync_binary.c

@@ -91,6 +91,9 @@ vk_sync_binary_wait_many(struct vk_device *device,
                          enum vk_sync_wait_flags wait_flags,
                          uint64_t abs_timeout_ns)
 {
+   if (wait_count == 0)
+      return VK_SUCCESS;
+
    STACK_ARRAY(struct vk_sync_wait, timeline_waits, wait_count);
 
    for (uint32_t i = 0; i < wait_count; i++) {

src/vulkan/runtime/vk_synchronization.c

@@ -249,6 +249,9 @@ vk_common_CmdWaitEvents(
    VK_FROM_HANDLE(vk_command_buffer, cmd_buffer, commandBuffer);
    struct vk_device *device = cmd_buffer->base.device;
 
+   if (eventCount == 0)
+      return;
+
    STACK_ARRAY(VkDependencyInfo, deps, eventCount);
 
    /* Note that dstStageMask and srcStageMask in the CmdWaitEvent2() call

src/vulkan/util/vk_util.h

@@ -352,14 +352,14 @@ vk_spec_info_to_nir_spirv(const VkSpecializationInfo *spec_info,
 
 #define STACK_ARRAY_SIZE 8
 
-#ifdef __cplusplus
-#define STACK_ARRAY_ZERO_INIT {}
-#else
-#define STACK_ARRAY_ZERO_INIT {0}
-#endif
-
+/* Sometimes gcc may claim -Wmaybe-uninitialized for the stack array in some
+ * places it can't verify that when size is 0 nobody down the call chain reads
+ * the array. Please don't try to fix it by zero-initializing the array here
+ * since it's used in a lot of different places. An "if (size == 0) return;"
+ * may work for you.
+ */
 #define STACK_ARRAY(type, name, size) \
-   type _stack_##name[STACK_ARRAY_SIZE] = STACK_ARRAY_ZERO_INIT; \
+   type _stack_##name[STACK_ARRAY_SIZE]; \
    type *const name = \
      ((size) <= STACK_ARRAY_SIZE ? _stack_##name : (type *)malloc((size) * sizeof(type)))