[LLVMdev] Passing undef to functions

Peter Housel
Thu, 07 Apr 2011 17:33:37 -0000

In my back-end I'd like to be able to pass dummy values to functions
that ignore them. The following code:

    declare i32 @foo(i32, i32, i32)

    define void @bar() nounwind {
    entry:
      %call = call i32 @foo(i32 8, i32 6, i32 undef)
      ret void
    }

ends up generating (using llc -O3):

bar:                                    # @bar
# BB#0:                                 # %entry
        subl    $12, %esp
        movl    %eax, 8(%esp)
        movl    $6, 4(%esp)
        movl    $8, (%esp)
        calll   foo
        addl    $12, %esp
        ret

Shouldn't the "movl %eax, 8(%esp)" be unnecessary?

P.S. On ARM the mov to r2 is successfully elided:

bar:                                    @ @bar
@ BB#0:                                 @ %entry
        push    {r11, lr}
        mov     r0, #8
        mov     r1, #6
        bl      foo
        pop     {r11, pc}