[cc65] question about assignment statement

Just a quick question about how the compiler works.  When I do a simple 
assignment statement like:

a = 9;

the resulting assembly code is something like:

ldx     #$00
lda     #$09
ldy     #$05
sta     (sp),y

Why is the x register always cleared before the assignment?  Will the 
optimizer switch remove it?  (I'm asking because I still can't use the 
optimize switch for my code)

Thanks,
Paul
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