[llvm-bugs] [Bug 52355] New: Missed DCE/loop deletion

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52355

            Bug ID: 52355
           Summary: Missed DCE/loop deletion
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

inline unsigned int
stringLen(const short* const src)
{
    if (src == 0 || *src == 0) {
        return 0;
    } else {
        const short* pszTmp = src + 1;

        while (*pszTmp)
            ++pszTmp;

        return (unsigned int)(pszTmp - src);
    }
}

extern void bar();

void foo(const short* const str) {
    unsigned int len = stringLen(str);
    if (!len) {
        bar();
    }
}

LLVM:
foo(short const*):                              # @foo(short const*)
        test    rdi, rdi
        je      .LBB0_6
        cmp     word ptr [rdi], 0
        je      .LBB0_6
        xor     eax, eax
.LBB0_3:                                # =>This Inner Loop Header: Depth=1
        cmp     word ptr [rdi + rax + 2], 0
        lea     rax, [rax + 2]
        jne     .LBB0_3
        shr     rax
        test    eax, eax
        je      .LBB0_6
        ret
.LBB0_6:
        jmp     bar()                         # TAILCALL

ICC is smarter here:
foo(short const*):
        test      rdi, rdi                                      #4.16
        je        ..B1.4        # Prob 5%                       #4.16
        movsx     eax, WORD PTR [rdi]                           #4.22
        test      eax, eax                                      #4.29
        jne       ..B1.5        # Prob 72%                      #4.29
..B1.4:                         # Preds ..B1.2 ..B1.1
        jmp       bar()                                       #21.9
..B1.5:                         # Preds ..B1.2
        ret


https://godbolt.org/z/erWa6efKa                                                
    #23.1

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