OP-913: ensured that UAVO data structs in .bss/.data sections and on the stack have 4 byte alignment. Also changed linker options to sort common and sort sections by alignment, which reduces the amount of fill/padding in the .bss and .data sections.

+review OPReview-444

flight/PiOS/inc/pios_debug.h

@@ -68,6 +68,12 @@ void PIOS_DEBUG_Panic(const char *msg);
 #define PIOS_Assert(test) if (!(test)) while (1);
 #endif
 
+/* Static (compile-time) assertion for use in a function.
+   If test evaluates to 0 (ie fails) at compile time then compilation will
+   fail with the error: "size of unnamed array is negative" */
+#define PIOS_STATIC_ASSERT(test) ((void)sizeof(int[1 - 2*!(test)]))
+
+
 #endif /* PIOS_DEBUG_H */
 
 /**

flight/targets/UAVObjects/inc/uavobjecttemplate.h

@@ -51,11 +51,19 @@ int32_t $(NAME)Initialize();
 UAVObjHandle $(NAME)Handle();
 void $(NAME)SetDefaults(UAVObjHandle obj, uint16_t instId);
 
-// Object data
+// Packed Object data (unaligned).
+// Should only be used where 4 byte alignment can be guaranteed
+// (eg a single instance on the heap)
 typedef struct {
 $(DATAFIELDS)
-} __attribute__((packed)) $(NAME)Data;
+} __attribute__((packed)) $(NAME)DataPacked;
 
+// Packed Object data.
+// Alignment is forced to 4 bytes so as to avoid the potential for CPU usage faults 
+// on Cortex M4F during load/store of float UAVO fields
+typedef $(NAME)DataPacked __attribute__((aligned(4))) $(NAME)Data;
+
+    
 // Typesafe Object access functions
 /**
  * @function $(NAME)Get(dataOut)

flight/targets/UAVObjects/uavobjecttemplate.c

@@ -49,6 +49,11 @@ static UAVObjHandle handle __attribute__((section("_uavo_handles")));
  */
 int32_t $(NAME)Initialize(void)
 {
+	// Compile time assertion that the $(NAME)DataPacked and $(NAME)Data structs
+	// have the same size (though instances of $(NAME)Data
+	// should be placed in memory by the linker/compiler on a 4 byte alignment).
+	PIOS_STATIC_ASSERT(sizeof($(NAME)DataPacked) == sizeof($(NAME)Data));
+    
 	// Don't set the handle to null if already registered
 	if(UAVObjGetByID($(NAMEUC)_OBJID) != NULL)
 		return -2;

make/common-defs.mk

@@ -153,7 +153,7 @@ ASFLAGS += -Wa,-adhlns=$(addprefix $(OUTDIR)/, $(notdir $(addsuffix .lst, $(base
 #    -Map:      create map file
 #    --cref:    add cross reference to  map file
 LDFLAGS += -nostartfiles
-LDFLAGS += -Wl,--warn-common,--fatal-warnings,--gc-sections
+LDFLAGS += -Wl,--warn-common,--fatal-warnings,--sort-common,--sort-section=alignment,--gc-sections
 LDFLAGS += -Wl,-Map=$(OUTDIR)/$(TARGET).map,--cref
 LDFLAGS += $(patsubst %,-L%,$(EXTRA_LIBDIRS))
 LDFLAGS += $(patsubst %,-l%,$(EXTRA_LIBS))