5.11 Stack use in C and C++

C and C++ both use the stack intensively.

For example, the stack holds:

  • The return address of functions.

  • Registers that must be preserved, as determined by the ARM Architecture Procedure Call Standard (AAPCS), for instance, when register contents are saved on entry into subroutines.

  • Local variables, including local arrays, structures, unions, and in C++, classes.

Some stack usage is not obvious, such as:

  • Local integer or floating point variables are allocated stack memory if they are spilled (that is, not allocated to a register).

  • Structures are normally allocated to the stack. A space equivalent to sizeof(struct) padded to a multiple of four bytes is reserved on the stack. The compiler tries to allocate structures to registers instead.

  • If the size of an array size is known at compile time, the compiler allocates memory on the stack. Again, a space equivalent to sizeof(struct) padded to a multiple of four bytes is reserved on the stack.

    Note:

    Memory for variable length arrays is allocated at runtime, on the heap.
  • Several optimizations can introduce new temporary variables to hold intermediate results. The optimizations include: CSE elimination, live range splitting and structure splitting. The compiler tries to allocate these temporary variables to registers. If not, it spills them to the stack.

  • Generally, code compiled for processors that support only 16-bit encoded Thumb instructions makes more use of the stack than ARM code and code compiled for processors that support 32-bit encoded Thumb instructions. This is because 16-bit encoded Thumb instructions have only eight registers available for allocation, compared to fourteen for ARM code and 32-bit encoded Thumb instructions.

  • The AAPCS requires that some function arguments are passed through the stack instead of the registers, depending on their type, size, and order.

Methods of estimating stack usage

Stack use is difficult to estimate because it is code dependent, and can vary between runs depending on the code path that the program takes on execution. However, it is possible to manually estimate the extent of stack utilization using the following methods:

  • Link with --callgraph to produce a static callgraph. This shows information on all functions, including stack use.

  • Link with --info=stack or --info=summarystack to list the stack usage of all global symbols.

  • Use the debugger to set a watchpoint on the last available location in the stack and see if the watchpoint is ever hit.

    Note:

    Running your program under a debug monitor like a Real-Time System Model (RTSM), in DS-5 Debugger or RealView Debugger, has a severe performance penalty, because the watched address is checked for every instruction. Using DSTREAM or RealView ICE and RealView Trace has no such penalty.

  • Use the debugger, and:

    1. Allocate space in memory for the stack that is much larger than you expect to require.

    2. Fill the stack space with copies of a known value, for example, 0xDEADDEAD.

    3. Run your application, or a fixed portion of it. Aim to use as much of the stack space as possible in the test run. For example, try to execute the most deeply nested function calls and the worst case path found by the static analysis. Try to generate interrupts where appropriate, so that they are included in the stack trace.

    4. After your application has finished executing, examine the stack space of memory to see how many of the known values have been overwritten. The space has garbage in the used part and the known values in the remainder.

    5. Count the number of garbage values and multiply by four, to give their size, in bytes.

    The result of the calculation shows how the size of the stack has grown, in bytes.

  • Use RTSM, and define a region of memory where access is not allowed directly below your stack in memory, with a map file. If the stack overflows into the forbidden region, a data abort occurs, which can be trapped by the debugger.

Methods of reducing stack usage

In general, you can lower the stack requirements of your program by:

  • Writing small functions that only require a small number of variables.

  • Avoiding the use of large local structures or arrays.

  • Avoiding recursion, for example, by using an alternative algorithm.

  • Minimizing the number of variables that are in use at any given time at each point in a function.

  • Using C block scope and declaring variables only where they are required, so overlapping the memory used by distinct scopes.

The use of C block scope involves declaring variables only where they are required. This minimizes use of the stack by overlapping memory required by distinct scopes.

Note:

Code performance is optimized by locating the stack in fast (zero wait-state), on-chip, 32-bit RAM. The ARM (LDMFD and STMFD) and Thumb (PUSH and POP) stack access instructions both push and pop a number of 32-bit registers on or off the stack. If the stack is in 32-bit memory, each register access takes one cycle. However, if the stack is in 16-bit memory then each register access takes two cycles, reducing overall performance.

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