Understanding the strcmp function of gnu libc

问题

Here is the strcmp function that i found in the glibc:

int
STRCMP (const char *p1, const char *p2)
{
  const unsigned char *s1 = (const unsigned char *) p1;
  const unsigned char *s2 = (const unsigned char *) p2;
  unsigned char c1, c2;

  do
    {
      c1 = (unsigned char) *s1++;
      c2 = (unsigned char) *s2++;
      if (c1 == '\0')
        return c1 - c2;
    }
  while (c1 == c2);

  return c1 - c2;
}

This is a pretty simple function where the body of while initiates c1 and c2 with the value of *s1 and *s2 and continues till either c1 is nul or the values of c1 and c2 are equal, then returns the difference between c1 and c2.

What i didn't understand is the use of s1 and s2 variables. I mean other than the fact that they are unsigned char they are also const like the 2 arguments p1 and p2, so why not just use the p1 and p2 inside the body of while and cast them ? Does in this case using those 2 extra variables make the function somehow more optimized? because here is the same function for FreeBSD I found on github:

int
strcmp(const char *s1, const char *s2)
{
    while (*s1 == *s2++)
        if (*s1++ == '\0')
            return (0);
    return (*(const unsigned char *)s1 - *(const unsigned char *)(s2 - 1));
}

In their version they didn't even bother using any extra variables.

Thanks in advance for your answers.

PS: I did search on the internet about this specific fact before asking here, but i didn't got anything.

I would also like to know if there are any particular reason why glibc used those extra variables instead of casting the parameters p1 and p2 directly inside while.

回答1:

What i didn't understand is the use of s1 and s2 variables. I mean other than the fact that they are unsigned char they are also const like the 2 arguments p1 and p2, so why not just use the p1 and p2 inside the body of while and cast them ?

For readability; to make it easier for us humans to maintain the code.

If you look at glibc sources, the code tends to readability rather than concise expressions. It seems to be a good policy, because it has kept it relevant and vibrant (actively maintained) for over 30 years now.

Does in this case using those 2 extra variables make the function somehow more optimized?

No, not at all.

I would also like to know if there are any particular reason why glibc used those extra variables instead of casting the parameters p1 and p2 directly inside while.

For readability only.

The authors know that the C compiler used should be able to optimize this code just fine. (And it is easy to prove that is the case, just by looking at the code compiler generateds. For GCC, you can use the -S option, or you can use the binutils' objdump -d to examine an object file or a binary executable.)

Note that the casts to unsigned char are required for the exact same reasons as they are for isspace(), isalpha() et cetera: the character codes compared must be treated as unsigned char for correct results.

回答2:

You're correct of course. One of the casts should be sufficient. Especially if the pointer is cast, casting the retrieved value is a no-op.

Here is the x86-64 compiled with gcc -O3 for the one with unnecessary cast:

STRCMP:
.L4:
        addq    $1, %rdi
        movzbl  -1(%rdi), %eax
        addq    $1, %rsi
        movzbl  -1(%rsi), %edx
        testb   %al, %al
        je      .L7
        cmpb    %dl, %al
        je      .L4
        subl    %edx, %eax
        ret
.L7:
        movzbl  %dl, %eax
        negl    %eax
        ret

and here's the one without the unnecessary cast:

STRCMP:
.L4:
        addq    $1, %rdi
        movzbl  -1(%rdi), %eax
        addq    $1, %rsi
        movzbl  -1(%rsi), %edx
        testb   %al, %al
        je      .L7
        cmpb    %dl, %al
        je      .L4
        subl    %edx, %eax
        ret
.L7:
        movzbl  %dl, %eax
        negl    %eax
        ret

They're identical

However there is one gotcha, that is now mostly of historical interest. If char is signed and the signed representation is not two's complement,

*(const unsigned char *)p1

and

(unsigned char)*p1

are not equivalent. The former reinterprets the bit pattern, while the latter converts the value using modulo arithmetic. This is of only historical interest since not even GCC supports any architecture that doesn't have 2's complement signed representation. And it is the compiler with most ports.

来源：https://stackoverflow.com/questions/53039818/understanding-the-strcmp-function-of-gnu-libc

标签

Linux

glibc

strcmp