[llvm-bugs] [Bug 50996] New: LLVM unable to optimize same associative addition in if-else condition in loop

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50996

            Bug ID: 50996
           Summary: LLVM unable to optimize same associative addition in
                    if-else condition in loop
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: TejasSanjay.Joshi at amd.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 25004
  --> https://bugs.llvm.org/attachment.cgi?id=25004&action=edit
test case

For the same associative calculation in if and else conditions, LLVM seems
unable to recognize it:

void foo(int *a, int *b, int *c)
{
    for (int i=0; i<100; ++i) {
        if (c[i] > 0)
            c[i] = a[i]+b[i];
        else
            c[i] = b[i]+a[i];
    }
}

Even with -Ofast -fno-unroll-loops -ffast-math -S foo.c: Gives the following
main loop:

.LBB0_1:
    cmpl    $0, (%rdx,%rax,4)
    movq    %rsi, %r8
    cmovgq  %rdi, %r8
    movq    %rdi, %rcx
    cmovgq  %rsi, %rcx
    movl    (%rcx,%rax,4), %ecx
    addl    (%r8,%rax,4), %ecx
    movl    %ecx, (%rdx,%rax,4)
    addq    $1, %rax
    cmpq    $100, %rax
    jne .LBB0_1

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