[wasm] [jiterp] Use wasm if opcode for null checks and conditional branches

[kg]

Right now conditional branches look like:

block (
  ... compute condition ...
  i32.eqz
  br_if // take branch
)

and null checks look like

block (
  ... load pointer ...
  br_if // exit block
  ... bailout ...
)

Since wasm doesn't have branch hinting, it's hard to be sure how to properly hint an unlikely branch. I suspect that using different opcode layouts might produce better performance here, so this PR changes the generated code for both kinds of branches and moves more of the codegen into the Cfg class.

Local benchmarking results were inconclusive, but that's to be expected. The impact of this would likely appear in browser-bench and the perf autofiling if it appears anywhere. On the bright side it also makes null checks and branches much easier to spot in traces, so that's nice.

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Issue Details

---

Right now conditional branches look like:

block (
  ... compute condition ...
  i32.eqz
  br_if // take branch
)

and null checks look like

block (
  ... load pointer ...
  br_if // exit block
  ... bailout ...
)

Since wasm doesn't have branch hinting, it's hard to be sure how to properly hint an unlikely branch. I suspect that using different opcode layouts might produce better performance here, so this PR changes the generated code for both kinds of branches and moves more of the codegen into the Cfg class.

Local benchmarking results were inconclusive, but that's to be expected. The impact of this would likely appear in browser-bench and the perf autofiling if it appears anywhere. On the bright side it also makes null checks and branches much easier to spot in traces, so that's nice.

Author:	kg
Assignees:	-
Labels:	area-Codegen-Interpreter-mono
Milestone:	-