为什么即使使用-O0标志，​​clang也会优化我的数组？

Question

I am trying to debug the following C Program using GDB:

// Program to generate a user specified number of 
// fibonacci numbers using variable length arrays
// Chapter 7  Program 8   2013-07-14

#include <stdio.h>

int main(void)
{
    int i, numFibs;
    printf("How many fibonacci numbers do you want (between 1 and 75)?\n");
    scanf("%i", &numFibs);

    if (numFibs < 1 || numFibs > 75)
    {
        printf("Between 1 and 75 remember?\n");
        return 1;
    }

    unsigned long long int fibonacci[numFibs];

    fibonacci[0] = 0;  // by definition
    fibonacci[1] = 1;  // by definition

    for(i = 2; i < numFibs; i++)
        fibonacci[i] = fibonacci[i-2] + fibonacci[i-1];

    for(i = 0; i < numFibs; i++)
        printf("%llu ", fibonacci[i]);

    printf("\n");

    return 0;
}

The issue I am having is when trying to compile the code using: clang -ggdb3 -O0 -Wall -Werror 7_8_FibonacciVarLengthArrays.c

When I try to run gdb on the a.out file created and I am stepping through the program execution. Anytime after the fibonacci[] array is decalared and I type: info locals the result says fibonacci <value optimized out> (until after the first iteration of my for loop) which then results in fibonacci holding the address 0xbffff128 for the rest of the program (but dereferencing that address does not appear to contain any meaningful data).

I am just confused why clang appears to be optimizing out this array when the -O0 flag is used?

I can use gcc to compile this code and the value displays as expected when using GDB.... Any thoughts?

Thank you.

Answer 1

You don't mention which version of clang you are using. I tried it with both 3.2 and a recent SVN install (3.4).

The code generated by the two versions looks pretty similar to me, but the debugging information is different. The clang 3.2 (which comes from a default ubuntu 13.04 install) produces an error when I try to examine fibonacci in gdb:

fibonacci = <error reading variable fibonacci (DWARF-2 expression error: DW_OP_reg operations must be used either alone or in conjunction with DW_OP_piece or DW_OP_bit_piece.)>

In the code compiled with clang 3.4, it all works fine. In neither case is the array "optimized out"; it's clearly allocated on the stack.

So I suspect the oddity that you're seeing has more to do with the emission of debugging information than with the actual code.

Answer 2

gdb does not yet support debugging stack allocated variable-length arrays. See https://sourceware.org/gdb/wiki/VariableLengthArray

Use a compile time constant or malloc to allocate fibonacci so that it will be visible to gdb.

See also GDB reports "no symbol in current context" upon array initialization

Answer 3

clang is not "optimizing out" the array at all! The array is declared as a variable-length array on the stack, so it has to be explicitly allocated (using techniques similar to those used by alloca() ) when its declaration is reached. The starting address of the array is unknown until that process is complete.