Google Bard explains this code (very badly)

Warning: This explanation is incredibly wrong, see this post for details.

The initial prompt, which aside from the actual code is the default for Bard’s “explain code” option:

Explain this code. Go step by step, explaining each line in detail:

.text
.global call_func_ptr
call_func_ptr:
   PUSH   {R4-R5,LR}
   MOV    R12, R0
   MOV    R4, SP

   ADD    R5, R1, R2,LSL #2
   RSBS   R0, R2, #4
   BLO    stack_setup
   ADD    PC, PC, R0,LSL #2
   NOP
   LDR    R3, [R5,#-4]!
   LDR    R2, [R5,#-4]!
   LDR    R1, [R5,#-4]!
   LDR    R0, [R5,#-4]!
   B      do_call

stack_setup:
   SUB    R2, R2, #4
   SUB    R2, R5, R2,LSL #2

stack_setup_loop:
   LDR    R0, [R5,#-4]!
   STR    R0, [SP,#-4]!
   CMP    R5, R2
   BNE    stack_setup_loop

   LDMIA  R1, {R0-R3}

do_call:
   BLX    R12
   MOV    SP, R4
   POP    {R4-R5,LR}
   BX     LR

Bard’s response

Sure. Here is a step-by-step explanation of the assembly language code you provided:

Text section:

.text

This line tells the assembler that the following code is part of the text section, which contains executable instructions.

Global symbol:

.global call_func_ptr

This line tells the assembler to make the symbol call_func_ptr globally visible. This means that other code modules can call the function at this address.

Function definition:

call_func_ptr:

This line defines the start of the function call_func_ptr.

Stack saving:

   PUSH   {R4-R5,LR}

This instruction pushes the registers R4 through R5 and LR onto the stack. This is done to preserve the values of these registers while the function is running.

Register setup:

   MOV    R12, R0
   MOV    R4, SP

These instructions move the values of the R0 and SP registers to the R12 and R4 registers, respectively.

Stack setup:

   ADD    R5, R1, R2,LSL #2
   RSBS   R0, R2, #4
   BLO    stack_setup
   ADD    PC, PC, R0,LSL #2
   NOP

These instructions perform the following steps:

  1. Add the values of the R1 and R2 registers, shifted left by 2 bits, to the R5 register. This calculates the address of the function’s stack frame.
  2. Subtract 4 from the R2 register. This is because the function’s stack frame will need to store 4 return addresses.
  3. Branch to the stack_setup label if the R2 register is less than zero. This means that the stack frame is too large and the function needs to return an error.
  4. Add the value of the R0 register, shifted left by 2 bits, to the program counter (PC). This effectively branches to the return address at the top of the stack frame.
  5. Insert a NOP instruction. This is necessary because the branch instruction in the previous step takes up two bytes, but the PC must be aligned on a four-byte boundary.

Stack setup loop:

   LDR    R0, [R5,#-4]!
   STR    R0, [SP,#-4]!
   CMP    R5, R2
   BNE    stack_setup_loop

This loop copies the caller’s stack frame to the function’s stack frame. The LDR instruction loads a value from the stack frame at the address R5 and decrements R5. The STR instruction stores the value from R0 to the stack frame at the address SP and decrements SP. The CMP instruction compares the values of R5 and R2. If R5 is greater than R2, the loop branches back to the beginning.

Register restoration:

   LDMIA  R1, {R0-R3}

This instruction loads the registers R0 through R3 from the stack frame at the address R1 and increments R1.

Function call:

   BLX    R12

This instruction branches to the function at the address R12.

Stack cleanup:

   MOV    SP, R4
   POP    {R4-R5,LR}

These instructions restore the stack pointer (SP) to its original value and pop the registers R4 through R5 and LR from the stack.

Function return:

   BX     LR

This instruction branches to the return address stored in the LR register.

Pseudo-code:

Here is a pseudo-code representation of the entire function:

call_func_ptr(func_ptr, arg1, arg2, arg3):

    # Save the calle

This is where Bard’s original response ended, minus the markdown closing code tag. This appears to be due to some ~32K limit on output (although this wasn’t obviously indicated or documented that I could find), so I prompted it again with

The psuedo-code above appears incomplete. Please provide complete pseudo-code for the assembly language function you explained previously

(“psuedo” typo in the original)

Bard’s response:

Sure, here is a complete pseudo-code representation of the entire assembly language function:

call_func_ptr(func_ptr, arg1, arg2, arg3):

    # Save the caller's registers on the stack
    save_registers()

    # Set up the function's stack frame
    setup_stack_frame(func_ptr, arg1, arg2, arg3)

    # Restore the caller's registers
    restore_registers()

    # Call the function
    call_func(func_ptr)

    # Clean up the function's stack frame
    cleanup_stack_frame()

    # Return to the caller
    return()

Here is a more detailed explanation of each step:

Save the caller’s registers on the stack:

The save_registers() function saves the registers R4 through R5 and LR onto the stack. This is done to preserve the values of these registers while the function is running.

Set up the function’s stack frame:

The setup_stack_frame() function performs the following steps:

  1. Calculates the address of the function’s stack frame by adding the values of the R1 and R2 registers, shifted left by 2 bits.
  2. Subtracts 4 from the R2 register to account for the space needed for the return address and the four function arguments.
  3. Copies the caller’s stack frame to the function’s stack frame.
  4. Stores the function pointer in the first slot of the stack frame.
  5. Stores the values of the arg1, arg2, and arg3 registers in the next three slots of the stack frame.

Restore the caller’s registers:

The restore_registers() function restores the registers R0 through R3 from the stack frame at the address R1 and increments R1.

Call the function:

The call_func() function branches to the function at the address R12.

Clean up the function’s stack frame:

The cleanup_stack_frame() function restores the stack pointer (SP) to its original value and pops the registers R4 through R5 and LR from the stack.

Return to the caller:

The return() function branches to the return address stored in the LR register.

I hope this more detailed explanation is helpful. Please let me know if you have any other questions.