Curious compiler error, "j=j++"

Altera_Forum · ‎01-02-2007

Hey, I made an error coding yesterday while developing an application for the NIOS II processor. I accidentally typed "j=j++;" instead of just "j++;". What I found curious was the resulting assembly:

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gcc version 3.4.1 (Altera Nios II 6.1 b197)

j=j++;

0x00102bb0 <process_packet+940>: addi r3,fp,60

0x00102bb4 <process_packet+944>: ldw r2,0(r3)

0x00102bb8 <process_packet+948>: mov r4,r2

0x00102bbc <process_packet+952>: addi r2,r2,1

0x00102bc0 <process_packet+956>: stw r2,0(r3)

0x00102bc4 <process_packet+960>: stw r4,60(fp)[/b]

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I don't mind so much that j doesn't get incremented, j=j++ is a quirky case, but why two memory writes? I compiled the code using the default options associated with the "hello world stand alone" template.

--Jordan

Altera_Forum · ‎01-02-2007

I wrote the following function:

void TestCompiler2(int j)
{
   j = j++;
}

and upon compiling received this warning message:

../hello_world_small.c: In function `TestCompiler2':

../hello_world_small.c:189: warning: operation on `j' may be undefined

I have all warnings enabled, and for my real projects I have -Wall and -Werror on the compiler flags line (with the -Wall checkbox off, as it can override values on the compiler flags line).

Here is the produced code, built with the release configuration:

010002b4 <TestCompiler2>:
 10002b4:    f800283a  ret

So, to answer your question, you're building the debug version where the compiler is supposed to produce code that does literally what you are telling it to do.

What you are telling it to do, is the following:

1. Set j = to j.

2. At some point, either before or after, increment j.

The behavior is undefined according to the C standard because j is modified twice in the same expression. The compiler is free to perform the side-effect (incrementing j) at any time, although it must of course assign the previous value to whatever is to the left of the =. But because j is there, the behavior is undefined and the C compiler is allowed to do *anything* at this point, whether it makes sense or not.