commit eb7bcff0fd21e445dad1a3074ff96363faef5b07
parent f40b3574ba195bba7b7dfdad44fab8317922b6a2
Author: William Casarin <jb55@jb55.com>
Date: Tue, 6 Jul 2021 09:06:36 -0700
add note on labels
Diffstat:
1 file changed, 34 insertions(+), 0 deletions(-)
diff --git a/wasm/labels.txt b/wasm/labels.txt
@@ -0,0 +1,34 @@
+WASM Labels
+
+Each structured control instruction introduces an implicit label. Labels are
+targets for branch instructions that reference them with label indices. Unlike
+with other index spaces, indexing of labels is relative by nesting depth, that
+is, label 0 refers to the innermost structured control instruction enclosing
+the referring branch instruction, while increasing indices refer to those
+farther out. Consequently, labels can only be referenced from within the
+associated structured control instruction. This also implies that branches can
+only be directed outwards, “breaking” from the block of the control construct
+they target. The exact effect depends on that control construct. In case of
+block or if it is a forward jump, resuming execution after the matching end. In
+case of loop it is a backward jump to the beginning of the loop.
+
+Note
+This enforces structured control flow. Intuitively, a branch targeting a block
+or if behaves like a break statement in most C-like languages, while a branch
+targeting a loop behaves like a continue statement.
+
+Branch instructions come in several flavors: br performs an unconditional
+branch, br_if performs a conditional branch, and br_table performs an indirect
+branch through an operand indexing into the label vector that is an immediate
+to the instruction, or to a default target if the operand is out of bounds. The
+return instruction is a shortcut for an unconditional branch to the outermost
+block, which implicitly is the body of the current function. Taking a branch
+unwinds the operand stack up to the height where the targeted structured
+control instruction was entered. However, branches may additionally consume
+operands themselves, which they push back on the operand stack after unwinding.
+Forward branches require operands according to the output of the targeted
+block’s type, i.e., represent the values produced by the terminated block.
+Backward branches require operands according to the input of the targeted
+block’s type, i.e., represent the values consumed by the restarted block.
+
+https://webassembly.github.io/spec/core/syntax/instructions.html#syntax-instr-control
