[llvm-bugs] [Bug 42847] New: Ternary form is optimized better than corresponding if / if else

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42847

            Bug ID: 42847
           Summary: Ternary form is optimized better than corresponding if
                    / if else
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: jameshamm1995 at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

Compiling a function written as a ternary expression gave better assembly than
an equivalent function written with a single if statement and another with an
if else.

Example code (https://godbolt.org/z/vuDVaP) compiled with clang (trunk,
currently 10.0.0) -Ofast -march=skylake

// Case 1, let a != b => (true || a != b + 1) => (true)
// Case 2: let !(a != b) => (a == b), (false || a != a + 1) => (true)
// So the condition is a tautology.
int f(int a, int b) {
    return (a != b || a != b + 1) ? 200 : 500;
}

int g(int a, int b) {
    if (a != b || a != b + 1) {
        return 200;
    }
    return 500;
}

int h(int a, int b) {
    if (a != b || a != b + 1) {
        return 200;
    } else {
        return 500;
    }
}

Produces the following assembly

f(int, int):                                 # @f(int, int)
        mov     eax, 200
        ret

g(int, int):                                 # @g(int, int)
        mov     eax, edi
        xor     eax, esi
        inc     esi
        xor     esi, edi
        or      esi, eax
        mov     ecx, 500
        mov     eax, 200
        cmove   eax, ecx
        ret

(h is omitted as it is identical to g)

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