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SimplifyLocals causes ineffective code #7672
Description
Given the following code:
(module
(type (;0;) (func))
(import "External" "external_function" (func $external_function (type 0)))
(func $_start (type 0)
i32.const 9576
i32.load
i32.load
i32.load
drop
i32.const 9576
i32.load
i32.load
i32.load
i32.const 9576
i32.load
i32.load
i32.load
i32.gt_s
if (result i32) ;; label = @1
call $external_function
i32.const 1
else
i32.const 0
end
drop
unreachable)
(memory $0 258 258)
(export "_start" (func $_start)))For wasm-opt (c91c052), -O3 -sp=simplify-locals can eliminate the unreachable code, while -O3 cannot:
(func $_start
(local $0 i32)
(if
(i32.gt_s
(local.tee $0
(i32.load
(i32.load
(i32.load
(i32.const 9576)
)
)
)
)
(local.get $0)
)
(then
(call $external_function)
)
)
(unreachable)
)After investigating, it is the Wasm-specific optimization simplify-locals causes the counter-intuitive code. Below is the change made by simplify-locals:
As you can see, the currently wasm-opt cannot optimize the if condition on the right---it can only do that on the left, which I think it is missed optimization (It is not the simplify-locals fault...).
Activity
[optimize-instructions] Fold relational comparisons of tee/get pairs
ConstraintAnalysis: Optimize self-comparisons (x ? x) (WebAssembly#9252)