Difference between revisions of "WebAssembly/Internals"
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==Loop-block== | ==Loop-block== | ||
− | Loop is a block of instructions that has 2 flow labels. Begging (0) and end (1). All other block instructions only point to their "ends". | + | Loop is a block of instructions that has 2 flow labels. Begging (0) and end (1). All other block instructions only point to their "ends". |
+ | Using branching '''br''' or '''br_if''', it's possible to either leave or reiterate the loop. | ||
+ | |||
+ | Just like any other block, the loop has a result type. This is especially important when doing '''br_if''', which requires the result value to be on the stack. (Verified by the validation process) | ||
==See Also== | ==See Also== | ||
* [[WebAssembly]] | * [[WebAssembly]] | ||
[[Category:WebAssembly]] | [[Category:WebAssembly]] |
Revision as of 16:21, 10 September 2019
Code Branching/Flow Control
todo: rewrite to the better explanation
WebAssembly doesn't allow direct jumps to (address/label)/or jump by offset. It only allows to jump "out-of" a code block, where the code block would be identified by a label. I.e. beginning of the loop label (for continue) or end of the loop label (for break). Loop itself is a single code block, known as "loop" in WebAssembly ).
FPC basic implementation is based of the ability of conditional jump instructions to a specified label. For that reason, the default implementation of TCGIfNode doesn't apply. Also, not every conditional jump instructions can have a label assign to it. Only instructions that jump out of the block are should use labels. Other, simply rely on the fact, that the next instruction IS the point they needs to be.
IF-block
Wasm IF do return a value (one of the basic WebAsm types). Pascal IF-statement does not return a value.
Thus the feature of value returning is not used (todo: but should be).
A dummy 0 value is pushed on the stack. IF is hard-coded to return i32 at all times.
According to the official documentation, IF always comes with "ELSE" block. However, some earlier versions do allow IF without an else statement. (which is also allowed in Binary specification of WASM). Such IF comes without any resulting type, and the execution of IF should leave stack unmodified in the end.
At this time "ELSE" branch is always generated for any IF statement. However, if there's no actual ELSE code (in pascal), a single "nop" instruction and a dummy result value produced.
function cmp(a,b: integer): Integer;
begin
if a > b then
cmp := 1
else
cmp := 0;
end;
turns into:
(func $cmp
(param i32)
(param i32)
(result i32)
(local i32) ;; Temp 2,1 allocated
(local i32) ;; Temp 3,1 allocated
;; [3] begin
;; [4] if a > b then
get_local 0
set_local 3
get_local 3
get_local 1
i32.gt_s
if (result i32) ;; hard-coded i32 type of IF result
;; [5] cmp := 1
i32.const 1
set_local 2
i32.const 0 ;; mandatory wasm-IF result
else
;; [7] cmp := 0;
i32.const 0
set_local 2
i32.const 0 ;; mandatory wasm-IF result
end
drop ;; dropping IF-result from the stack, as unused
;; [8] end;
get_local 2
return
)
The code generation of cifnode is overridden.
Loop-block
Loop is a block of instructions that has 2 flow labels. Begging (0) and end (1). All other block instructions only point to their "ends". Using branching br or br_if, it's possible to either leave or reiterate the loop.
Just like any other block, the loop has a result type. This is especially important when doing br_if, which requires the result value to be on the stack. (Verified by the validation process)