[libcamera-devel] [PATCH 2/2] libcamera: controls: Don't over-optimize ControlValue layout

[Laurent Pinchart]

The ControlValue class size should be minimized to save space, as it can
be instantiated in large numbers. The current implementation does so by
specifying several members as bitfields, but does so too aggressively,
resulting in fields being packed in an inefficient to access way on some
platforms. For instance, on 32-bit x86, the numElements_ field is offset
by 7 bits in a 32-bit word. This additionally causes a static assert
that checks the size of the class to fail.

Relax the constraints on the isArray_ and numElements_ fields to avoid
inefficient access, and to ensure that the class size is identical
across all platforms. This will anyway need to be revisited when
stabilizing the ABI, so add a \todo comment as a reminder.

Fixes: 1fa4b43402a0 ("libcamera: controls: Support array controls in ControlValue")
Reported-by: Jan Engelhardt <jengelh at inai.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 include/libcamera/controls.h | 4 ++--
 src/libcamera/controls.cpp   | 1 +
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/libcamera/controls.h b/include/libcamera/controls.h
index 2fca975f7512..40a29189ba01 100644
--- a/include/libcamera/controls.h
+++ b/include/libcamera/controls.h
@@ -158,8 +158,8 @@ public:
 
 private:
 	ControlType type_ : 8;
-	bool isArray_ : 1;
-	std::size_t numElements_ : 16;
+	bool isArray_;
+	std::size_t numElements_ : 32;
 	union {
 		uint64_t value_;
 		void *storage_;
diff --git a/src/libcamera/controls.cpp b/src/libcamera/controls.cpp
index 4615e71d7e3c..3c910d38f05d 100644
--- a/src/libcamera/controls.cpp
+++ b/src/libcamera/controls.cpp
@@ -83,6 +83,7 @@ static constexpr size_t ControlValueSize[] = {
  * \brief Abstract type representing the value of a control
  */
 
+/** \todo Revisit the ControlValue layout when stabilizing the ABI */
 static_assert(sizeof(ControlValue) == 16, "Invalid size of ControlValue class");
 
 /**
-- 
Regards,

Laurent Pinchart