Optimize control-flow-y conditional traps into single CLIF trapnz instructions #10941
Description
Wasm does not have conditional trap instructions, just control flow and unconditional traps. CLIF does have conditional trap instructions, but we do not translate the control-flow-y Wasm equivalent into these single conditionally-trapping instructions. This is unfortunate because Cranelift's mid-end cannot currently turn control-flow-y conditional traps into single trap[n]z instructions either, but the single instructions allow the optimizer to do more and also ultimately get better codegen in the backend.
I'm sure that conditional traps show up frequently in real Wasm code. They additionally show up frequently in the fused adapters we generate when linking components together.
Might be worth identifying blocks that contain a single unconditional trap instruction and rewrite conditional branches to these blocks during egraph construction or something.
#10940 introduced the following disas tests that highlight the problem:
(module
;; This function body should ideally get compiled down into a single `trapz`
;; CLIF instruction.
(func (export "trapnz") (param i32)
local.get 0
if
unreachable
end
)
;; And this one into a single `trapnz` instruction.
(func (export "trapz") (param i32)
local.get 0
i32.eqz
if
unreachable
end
)
)
;; function u0:0(i64 vmctx, i64, i32) tail {
;; gv0 = vmctx
;; gv1 = load.i64 notrap aligned readonly gv0+8
;; gv2 = load.i64 notrap aligned gv1+16
;; stack_limit = gv2
;;
;; block0(v0: i64, v1: i64, v2: i32):
;; @002f brif v2, block2, block3
;;
;; block2:
;; @0031 trap user11
;;
;; block3:
;; @0033 jump block1
;;
;; block1:
;; @0033 return
;; }
;;
;; function u0:1(i64 vmctx, i64, i32) tail {
;; gv0 = vmctx
;; gv1 = load.i64 notrap aligned readonly gv0+8
;; gv2 = load.i64 notrap aligned gv1+16
;; stack_limit = gv2
;;
;; block0(v0: i64, v1: i64, v2: i32):
;; v5 = iconst.i32 0
;; @0038 v3 = icmp eq v2, v5 ; v5 = 0
;; @0038 v4 = uextend.i32 v3
;; @0039 brif v4, block2, block3
;;
;; block2:
;; @003b trap user11
;;
;; block3:
;; @003d jump block1
;;
;; block1:
;; @003d return
;; }