CODEWITHSUNDEEP
assemblyavravr-gcc
xandy
xandy

Reputation: 27421

Why is GCC generate the code with bunch of useless JMP instruction?

I am currently working on a Bootloader in Atmel Studio for Atmega328P (Arduino UNO), and from the disassembly I found the following code (My bootloader starts from 0x3800): 

--- ../../../../crt1/gcrt1.S ---------------------------------------------------
00003800  JMP 0x00003834        Jump 
00003802  JMP 0x00003979        Jump 
00003804  JMP 0x00003979        Jump 
00003806  JMP 0x00003979        Jump 
00003808  JMP 0x00003979        Jump 
0000380A  JMP 0x00003979        Jump 
0000380C  JMP 0x00003979        Jump 
0000380E  JMP 0x00003979        Jump 
--- ../../../../crt1/gcrt1.S ---------------------------------------------------
00003810  JMP 0x00003979        Jump 
00003812  JMP 0x00003979        Jump 
00003814  JMP 0x00003979        Jump 
00003816  JMP 0x00003979        Jump 
00003818  JMP 0x00003979        Jump 
0000381A  JMP 0x00003979        Jump 
0000381C  JMP 0x00003979        Jump 
0000381E  JMP 0x00003979        Jump 
00003820  JMP 0x00003979        Jump 
00003822  JMP 0x00003979        Jump 
00003824  JMP 0x00003979        Jump 
00003826  JMP 0x00003979        Jump 
00003828  JMP 0x00003979        Jump 
0000382A  JMP 0x00003979        Jump 
0000382C  JMP 0x00003979        Jump 
0000382E  JMP 0x00003979        Jump 
00003830  JMP 0x00003979        Jump 
00003832  JMP 0x00003979        Jump 
--- C:\Users\andy\Documents\Atmel Studio\7.0\CannyFlashy\CannyFlashy\CannyFlashy\Debug/.././Sketch.cpp 
int main(){
00003834  IN R28,0x3D       In from I/O location

Why is GCC generating such code and is there anyway to avoid them?

Upvotes: 3

Views: 531

Answers (1)

Mephistt
Mephistt

Reputation: 186

WHAT IS IT

As Javier Silva Ortíz mentioned in the comment, it is an interrupt table placed at the most beginning of the program memory.

The first instruction is a RESET vector, and we can see the address it is pointing at. In the other words, just after reset, MCU executes first JMP 0x00003834 instruction and moves exactly to the beginning of the program.

By the way, have a look at the other JMP instructions: all of them pointing at the same address, so called __bad_interrupt(), where the only instruction RETI is placed. If one of these interrupts accidentally hits (and your program have no idea what to do with this interrupt), then the single RETI instruction finalizes this interrupt instantly and the program returns to it's normal execution.

AVOIDING IT

If you are absolutely sure there are no possible interrupt hits in your Bootloader, you could avoid interrupt table, passing CFLAGS = -nostartfiles flag to the compiler.

BE CAREFUL!

Some other things will be broken with -nostartfiles flag!

The problem is that avr-gcc makes some magic at the beginning of the compiled program. There are some common operations that compiler places just after interrupt table. These operations are devided by the sections, such as __ctors_end, __do_copy_data, __do_clear_bss and so on. What are they needed for?

First of all, r1 register is set to zero during __ctors_end section. Why is it so important? r1 is used by default every time the compiler compares other registers with zero. Yes, avr-gcc knows about this "magical" r1 and does not write to it, but there are some cases, when it changes (for example, the result of multiplication is placed at the r0:r1 register pair), and every time it changes, avr-gcc nullify it. Everything can go wrong if r1 is not set to zero during startup...

The other thing done at the beginning of the program is setting to zero variables that are supposed to be zero. It is done at the __do_clear_bss section. Just imagine you expect something to be zero and it is not. What are the consequences?

The other important operation is to copy data from the program memory to SRAM. It is done at __do_copy_data section. Switching off __do_copy_data brokes all the static arrays.

AND?..

You should be very careful trying to optimize interrupt table with -nostartfiles. Do this only if you are 200% sure what are going on.

Upvotes: 5

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