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MergeBlocks: Optimize all dropped blocks #6984

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merged 12 commits into from
Oct 4, 2024
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MergeBlocks: Optimize all dropped blocks #6984

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Update src/passes/MergeBlocks.cpp
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9 changes: 9 additions & 0 deletions src/passes/MergeBlocks.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -434,6 +434,15 @@ struct MergeBlocks
optimizeBlock(curr, getModule(), getPassOptions(), branchInfo);
}

void visitDrop(Drop* curr) {
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@aheejin aheejin Oct 4, 2024 •
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What's the difference between the cases handled by this and this code in optimizeBlock?

binaryen/src/passes/MergeBlocks.cpp

Line 262 in 7f30b6c

if (auto* drop = list[i]->dynCast<Drop>()) {

Can't all cases be handled by this new visitDrop?

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The code in optimizeBlock runs on the children of a block, and we do need to run that more than once, unlike visitDrop: Each time we make a signficant change to a block, it's possible that merging with children of the block is possible, so we call optimizeBlock on such occasions.

On the other hand, for things whose parent isn't a block, the new visitDrop is enough, since the parent can't be merged with it, so we can just do that once.

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Actually, I was completely wrong here! You were right, we can remove this code. It looks like changes in the parent never lead to new improvements for the child, so we can just do the visitDrop part once.

I'll do some fuzzing to verify this.

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Ah, actually that was wrong, it turns out. There is at least one case where we do end up improving a child based on the parents, which I added a testcase for now (after reverting the last commit). What seems to happen is that after we optimize the parent there, we then look at the child and see that it can be removed with its name. (This was not noticed in the test suite since we run --remove-unused-names.)

Logically I still think it makes sense that we only need the call from visitDrop, but that would require changes to the pass, and I'm not sure it's worth a large refactoring.

if (auto* block = curr->value->dynCast<Block>()) {
if (optimizeDroppedBlock(
curr, block, *getModule(), getPassOptions(), branchInfo)) {
replaceCurrent(block);
}
}
}

// given
// (curr
// (block=child
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11 changes: 5 additions & 6 deletions test/lit/passes/merge-blocks.wast
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Expand Up @@ -405,17 +405,16 @@
)

;; CHECK: (func $toplevel (type $4)
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (block $label (result i32)
;; CHECK-NEXT: (br $label
;; CHECK-NEXT: (i32.const 42)
;; CHECK-NEXT: )
;; CHECK-NEXT: (block $label
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (i32.const 42)
;; CHECK-NEXT: )
;; CHECK-NEXT: (br $label)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $toplevel
;; Test we can remove a block value even when the drop is at the toplevel of
;; a function. This is not done yet TODO
;; a function.
(drop
(block $label (result i32)
(br $label
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28 changes: 28 additions & 0 deletions test/lit/passes/merge-blocks_names.wast
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@@ -0,0 +1,28 @@
;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
;; RUN: wasm-opt %s --merge-blocks -all -S -o - | filecheck %s
;;
;; Similar to merge-blocks.wast, but without --remove-unused-names. This tests
;; the pass entirely by itself.

(module
;; CHECK: (func $nested-dropped-blocks (type $0)
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (i32.const 42)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $nested-dropped-blocks
;; Fully removing unneeded blocks here requires multiple operations to happen.
;; Specifically, we remove all the outer blocks first, and only then get to
;; the inner named block, which we can then infer is not needed either.
(block
(drop
(block (result i32)
(block $named (result i32)
(i32.const 42)
)
)
)
)
)
)

7 changes: 1 addition & 6 deletions test/lit/passes/monomorphize-drop.wast
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Expand Up @@ -672,12 +672,7 @@

;; CAREFUL: (func $return-normal_4 (type $1)
;; CAREFUL-NEXT: (drop
;; CAREFUL-NEXT: (block
;; CAREFUL-NEXT: (drop
;; CAREFUL-NEXT: (call $import)
;; CAREFUL-NEXT: )
;; CAREFUL-NEXT: (return)
;; CAREFUL-NEXT: )
;; CAREFUL-NEXT: (call $import)
;; CAREFUL-NEXT: )
;; CAREFUL-NEXT: )

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10 changes: 3 additions & 7 deletions test/passes/Oz_fuzz-exec_all-features.txt
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Expand Up @@ -183,14 +183,10 @@
(nop)
)
(func $br-on_non_null-2 (type $void_func)
(drop
(block
(call $log
(i32.const 1)
)
(unreachable)
)
(call $log
(i32.const 1)
)
(unreachable)
)
(func $cast-on-func (type $void_func)
(call $log
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29 changes: 10 additions & 19 deletions test/passes/merge-blocks.txt
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Expand Up @@ -10,11 +10,12 @@
)
)
(func $drop-block-br
(drop
(block $x (result i32)
(br $x
(i32.const 1)
)
(block $x
(drop
(i32.const 1)
)
(br $x)
(drop
(i32.const 0)
)
)
Expand Down Expand Up @@ -77,15 +78,9 @@
)
)
(func $drop-block-squared-iloop
(drop
(block $label$0 (result i32)
(drop
(block $label$1
(loop $label$2
(br $label$2)
)
)
)
(block $label$0
(loop $label$2
(br $label$2)
)
)
)
Expand All @@ -109,11 +104,7 @@
(func $loop-block-drop-block-return
(loop $label$4
(block $label$5
(drop
(block $label$6 (result i32)
(return)
)
)
(return)
)
)
)
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64 changes: 20 additions & 44 deletions test/passes/remove-unused-names_merge-blocks_all-features.txt
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Expand Up @@ -286,15 +286,11 @@
(i32.const 30)
)
(drop
(block
(drop
(i32.const 10)
)
(i32.add
(unreachable)
(i32.const 20)
)
)
(i32.const 10)
)
(i32.add
(unreachable)
(i32.const 20)
)
(drop
(i32.const 20)
Expand Down Expand Up @@ -822,40 +818,30 @@
)
(func $drop-unreachable (type $2) (result i32)
(local $0 i32)
(drop
(block (result i32)
(unreachable)
)
)
(unreachable)
(unreachable)
)
(func $concrete_finale_in_unreachable (type $5) (result f64)
(drop
(block (result f64)
(unreachable)
(block
(unreachable)
(drop
(f64.const 6.322092475576799e-96)
)
)
(f64.const -1)
)
(func $dont-move-unreachable (type $3)
(loop $label$0
(br $label$0)
(drop
(block (result i32)
(br $label$0)
(i32.const 1)
)
(i32.const 1)
)
)
)
(func $dont-move-unreachable-last (type $3)
(loop $label$0
(drop
(block (result i32)
(call $dont-move-unreachable-last)
(br $label$0)
)
)
(call $dont-move-unreachable-last)
(br $label$0)
)
)
(func $move-around-unreachable-in-middle (type $3)
Expand All @@ -876,16 +862,10 @@
)
(func $drop-unreachable-block-with-concrete-final (type $3)
(drop
(block (result i32)
(drop
(block
(drop
(return)
)
)
)
(i32.const -452)
)
(return)
)
(drop
(i32.const -452)
)
)
(func $merging-with-unreachable-in-middle (type $2) (result i32)
Expand All @@ -901,13 +881,9 @@
)
(func $remove-br-after-unreachable (type $3)
(block $label$9
(drop
(block
(block
(return)
(br $label$9)
)
)
(block
(return)
(br $label$9)
)
)
)
Expand Down
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