How to check if a register is zero in x86_64 assembly

I'm trying to check if a value is zero in x86_64 assembly code. I know that this usually consist of a `cmp` variant followed by a `jmp` variant, but I'm not sure of the exact instructions to use.

test %eax, %eax ; set ZF to 1 if eax == 0
je 0x804f430 ; jump to 0x00804f4 if ZF == 1

`ZF` is a single bit zero flag which will be set to 1 if `eax` be equal to zero. `je` will take the jump to `0x804f430` if the `ZF` be set to 1.

If you've just used an instruction that modifies ZF before, simply check that flag and jump using `JZ` or `JE`. For example

and rax, rbx ; ZF was modified
jz is_zero ; so to check if rax is zero, a single jump is enough

If ZF was not set, you need to do that explicitly. The obvious way is

cmp rax, 0
je equal_zero

However since `cmp` is longer if you look at the output binary, [`test`](

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) or sometimes [`and`, `or`](

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) is preferred

83F800 cmp eax, 0
09C0 or eax, eax
85C0 test eax, eax

The resulting code will be

test rax, rax
jz is_zero

You can get the assembly output from a compiler and check or view it in an online tool like [gcc godbolt][1]

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[1]:

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:'//+Type+your+code+here,+or+load+an+example.%0Aint+is_zero(int+x,+int+*y,+int+*z)+%7B%0A+++if+(x+%3D%3D+0)%0A+++++*y+%3D+*z%3B%0A+++else%0A+++++*y+%3D+-*z%3B%0A%7D%0A'),l:'5',n:'0',o:'C%2B%2B+source+%231',t:'0')),k:25.650950254211345,l:'4',n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:g71,filters:(b:'0',commentOnly:'0',directives:'0',intel:'0'),options:'-O2',source:1),l:'5',n:'0',o:'x86-64+gcc+7.1+(Editor+%231,+Compiler+%231)',t:'0')),k:35.5645638630054,l:'4',m:100,n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:icc17,filters:(b:'0',commentOnly:'0',directives:'0',intel:'0'),options:'-O2',source:1),l:'5',n:'0',o:'x86-64+icc+17+(Editor+%231,+Compiler+%232)',t:'0')),k:38.78448588278327,l:'4',m:100,n:'0',o:'',s:0,t:'0')),l:'2',n:'0',o:'',t:'0')),version:4