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From: "rv at rasmusvillemoes dot dk" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/64753] New: Redundant cmp instruction on x86_64
Date: Fri, 23 Jan 2015 15:06:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64753

            Bug ID: 64753
           Summary: Redundant cmp instruction on x86_64
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rv at rasmusvillemoes dot dk
              Host: x86_64
            Target: x86_64

The linux kernel's library strncmp is this:

int strncmp(const char *cs, const char *ct, size_t count)
{
        unsigned char c1, c2;

        while (count) {
                c1 = *cs++;
                c2 = *ct++;
                if (c1 != c2)
                        return c1 < c2 ? -1 : 1;
                if (!c1)
                        break;
                count--;
        }
        return 0;
}

Compiling with gcc -O2 -S I get this:

strncmp:
.LFB0:
        .cfi_startproc
        testq   %rdx, %rdx
        je      .L10
        movzbl  (%rdi), %ecx
        movzbl  (%rsi), %r8d
#        cmpb    %r8b, %cl
#        jne     .L3
        testb   %cl, %cl
        je      .L10
        subq    $1, %rdx
        xorl    %eax, %eax
        jmp     .L4
        .p2align 4,,10
        .p2align 3
.L6:
        movzbl  1(%rdi,%rax), %ecx
        movzbl  1(%rsi,%rax), %r8d
#        cmpb    %r8b, %cl
#        jne     .L3
        addq    $1, %rax
        testb   %cl, %cl
        je      .L10
.L4:
        cmpq    %rdx, %rax
        jne     .L6
.L10:
        xorl    %eax, %eax
        ret
        .p2align 4,,10
        .p2align 3
.L3:
        cmpb    %r8b, %cl
        sbbl    %eax, %eax
        orl     $1, %eax
        ret
        .cfi_endproc

At the two places marked # we do a cmp and a conditional jump to .L3, where for
good measure the same cmp is done again... there's no other path to .L3, so it
would seem that simply omitting that extra cmp should be ok. 

This is with gcc-5.0 (GCC) 5.0.0 20150112 (experimental), but I see the same
with gcc (Debian 4.7.2-5) 4.7.2.