What's the difference between "block" and "loop" in WebAssembly spec?

Question

As titled, the spec says that "loop" is

a block with a label at the beginning which may be used to form loops.

and for "block":

the beginning of a block construct, a sequence of instructions with a label at the end.

But with the help of "br" (used for switching branch to a labeled block), I can form the same control structure even with "block", right?. So, what's the difference between these two instructions?

Andreas Rossberg · Accepted Answer · 2023-05-01T08:27:25.407

7

A br to a block label jumps to the end of the contained instruction sequence -- it behaves like a break statement in C.

A br to a loop label jumps to the start of the contained instruction sequence -- it behaves like a continue statement in C.

The former enables a forward jump, the latter a backwards jump. Neither can express the other.

edited May 01 '23 at 08:27

answered Nov 20 '19 at 17:59

Andreas Rossberg

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Thanks Andreas, the "br" and "br_if" instructions have one immediate operand stands for "relative_depth", what's the usage of this immediate? – Jason Yu Nov 20 '19 at 23:24
It denotes the jump target, counting blocks inside out. – Andreas Rossberg Nov 21 '19 at 05:47

score 0 · Answer 2 · answered Nov 20 '19 at 15:03

No you can't block has the label at end as it sayed block has the label at the end

LOOP START
label:
SOME CODE
IF condtion BR label:
EVEN MORE CODE
LOOP END

Will execute SOME CODE once. Than repeats SOME CODE until condition is not true. And than execute EVEN MORE CODE

BLOCK START
SOME CODE
IF condition BR label:
EVEN MORE CODE
label:
BLOCK END

Will execute SOME CODE only once. And than EVEN MORE CODE might be execute when contdtion is not true.

What's the difference between "block" and "loop" in WebAssembly spec?

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