literal constant vs variable in math library

Question

So, I know that in C you need to link the code to the math library, libm, to be able to use its functions. Today, while I was trying to demonstrate

Answer 1

As everyone mentions, yes it has to do with constant folding.

With optimizations off, GCC only seems to do it when sqrt(2.0) is used. Here's the evidence:

Case 1: With the variable.

    .file   "main.c"
    .section    .rodata
.LC1:
    .string "b = %lf\n"
    .text
.globl main
    .type   main, @function
main:
    pushl   %ebp
    movl    %esp, %ebp
    andl    $-16, %esp
    subl    $32, %esp
    fldl    .LC0
    fstpl   24(%esp)
    fldl    24(%esp)
    fsqrt
    fucom   %st(0)
    fnstsw  %ax
    sahf
    jp  .L5
    je  .L2
    fstp    %st(0)
    jmp .L4
.L5:
    fstp    %st(0)
.L4:
    fldl    24(%esp)
    fstpl   (%esp)
    call    sqrt
.L2:
    fstpl   16(%esp)
    movl    $.LC1, %eax
    fldl    16(%esp)
    fstpl   4(%esp)
    movl    %eax, (%esp)
    call    printf
    movl    $0, %eax
    leave
    ret
    .size   main, .-main
    .section    .rodata
    .align 8
.LC0:
    .long   0
    .long   1073741824
    .ident  "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
    .section    .note.GNU-stack,"",@progbits

You can see that it emits a call to the sqrt function. So you'll get a linker error if you don't link the math library.

Case 2: With the Literal.

    .file   "main.c"
    .section    .rodata
.LC1:
    .string "b = %lf\n"
    .text
.globl main
    .type   main, @function
main:
    pushl   %ebp
    movl    %esp, %ebp
    andl    $-16, %esp
    subl    $32, %esp
    fldl    .LC0
    fstpl   24(%esp)
    movl    $.LC1, %eax
    fldl    24(%esp)
    fstpl   4(%esp)
    movl    %eax, (%esp)
    call    printf
    movl    $0, %eax
    leave
    ret
    .size   main, .-main
    .section    .rodata
    .align 8
.LC0:
    .long   1719614413
    .long   1073127582
    .ident  "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
    .section    .note.GNU-stack,"",@progbits

There's no call to sqrt. Hence no linker error.

---

With optimizations on, GCC will do constant propagation in both cases. So no linker error in either case.

$ gcc main.c -save-temps
main.o: In function `main':
main.c:(.text+0x30): undefined reference to `sqrt'
collect2: ld returned 1 exit status
$ gcc main.c -save-temps -O2
$