Skip to content

RemoveUnusedBrs: optimize unreachable control flow mixed with side-effecting branches #7639

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
wants to merge 11 commits into
base: main
Choose a base branch
from
Open

RemoveUnusedBrs: optimize unreachable control flow mixed with side-effecting branches #7639

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
11 commits
Select commit Hold shift + click to select a range
8b5a617
opt for #7637
xuruiyang2002 Jun 4, 2025
b9db9cf
tests
xuruiyang2002 Jun 4, 2025
223bfc0
add a test
xuruiyang2002 Jun 4, 2025
1f44ad6
Add comments
xuruiyang2002 Jun 4, 2025
1feb9ca
Use ChildLocalizer if there is no curr->value
xuruiyang2002 Jun 5, 2025
df10dce
Update test
xuruiyang2002 Jun 5, 2025
bae9749
NFC
xuruiyang2002 Jun 5, 2025
6eafa8c
Bugfix
xuruiyang2002 Jun 5, 2025
3dacea0
refinalize the type
xuruiyang2002 Jun 6, 2025
e41f344
Add test of two-branches-unreachable
xuruiyang2002 Jun 6, 2025
abbe4ce
Use auto_update script
xuruiyang2002 Jun 6, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
38 changes: 38 additions & 0 deletions src/passes/RemoveUnusedBrs.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -24,6 +24,8 @@
#include "ir/effects.h"
#include "ir/gc-type-utils.h"
#include "ir/literal-utils.h"
#include "ir/localize.h"
#include "ir/properties.h"
#include "ir/utils.h"
#include "parsing.h"
#include "pass.h"
Expand Down Expand Up @@ -1881,6 +1883,42 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
start = end;
}
}

void visitBreak(Break* curr) {
if (!curr->condition) {
return;
}
auto* value = Properties::getFallthrough(
curr->condition, passOptions, *getModule());
// optimize if condition's fallthrough is a constant
if (auto* c = value->dynCast<Const>()) {
if (curr->value) {
curr->condition = getDroppedChildrenAndAppend(
curr->condition,
*getModule(),
passOptions,
Builder(*getModule()).makeConst(c->value),
DropMode::NoticeParentEffects);
} else {
ChildLocalizer localizer(
curr, getFunction(), *getModule(), passOptions);
auto* block = localizer.getChildrenReplacement();
if (c->value.geti32()) {
// the branch is always taken, make it unconditional
curr->condition = nullptr;
curr->type = Type::unreachable;
Copy link
Contributor Author

@xuruiyang2002 xuruiyang2002 Jun 5, 2025 •
edited
Loading

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Because of this line curr->type = Type::unreachable; (which fixes type mismatch after transforming br_if to br)

As for the case found by the fuzzer, it transforms code from

    block  ;; label = @1
      global.get 0
      if  ;; label = @2
        i32.const 1
        br_if 1 (;@1;)
      else
        unreachable
      end
      local.get 0
      call 0
    end

to

    block  ;; label = @1
      global.get 0
      if  ;; label = @2
        br 1 (;@1;)
      else
        unreachable
      end
      local.get 0
      call 0
    end

which in SimplifyLocals triggers the assertion:

binaryen/src/passes/SimplifyLocals.cpp

Lines 659 to 666 in 700fa15

if (iff->ifTrue->type == Type::unreachable) {
// since the if type is none
assert(iff->ifFalse->type != Type::unreachable);
if (!ifFalse.empty()) {
goodIndex = ifFalse.begin()->first;
found = true;
}
} else if (iff->ifFalse->type == Type::unreachable) {

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Should this assertion always hold true?

Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Yes. If an if has two branches of unreachable then the if should be unreachable: logically, if neither if arm returns, neither does the if itself.

So we need to figure out how things got to that situation, and prevent it. Likely a refinalize() call is missing somewhere (which updates the types of expression after changes - that would update an if to unreachable if it needs to be, etc.)

Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Probably the issue is that turning a br_if into a br makes it unreachable, when it wasn't before. Setting refinalize = true; in that case should fix things.

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Got it, and I've updated.

And I've also fuzzing for about one hour; all is well.

block->list.push_back(curr);
block->finalize();
// the type is changed , so refinalize
refinalize = true;
replaceCurrent(block);
} else {
// the branch is never taken, allow control flow to fall through
replaceCurrent(block);
}
}
}
}
};
FinalOptimizer finalOptimizer(getPassOptions());
finalOptimizer.setModule(getModule());
Expand Down
34 changes: 34 additions & 0 deletions test/lit/passes/O1.wast
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,8 @@
;; RUN: foreach %s %t wasm-opt -O1 -S -o - | filecheck %s

(module

(import "env" "fimport$0" (func $fimport$0 (param i32)))
;; CHECK: (type $0 (func (result i32)))

;; CHECK: (memory $0 1 1)
Expand Down Expand Up @@ -38,5 +40,37 @@
(i32.const -2147483648)
)
)
(func $two-branches-unreachable
;; remove-unused-brs makes the break unconditional,
;; thus in this case the two branches are unreachable,
;; refinalization is required.
(local $0 i32)
(block $label
(block $block
(if
(local.get $0)
(then
(if
(local.tee $0
(i32.const 1)
)
(then
(br $label)
)
(else
(br $block)
)
)
)
(else
(unreachable)
)
)
)
(call $fimport$0
(local.get $0)
)
)
)
)

78 changes: 77 additions & 1 deletion test/lit/passes/remove-unused-brs.wast
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -340,6 +340,77 @@
)
)

;; CHECK: (func $restructure-br_if-constant-branch-1 (type $0) (param $x i32) (result i32)
;; CHECK-NEXT: (block $x (result i32)
;; CHECK-NEXT: (br_if $x
;; CHECK-NEXT: (unreachable)
;; CHECK-NEXT: (block (result i32)
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (local.tee $x
;; CHECK-NEXT: (i32.const 42)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (i32.const 42)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (unreachable)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $restructure-br_if-constant-branch-1 (param $x i32) (result i32)
(block $x (result i32)
(br_if $x
(unreachable)
(local.tee $x
(i32.const 42)
)
)
(unreachable)
)
)
;; CHECK: (func $restructure-br_if-constant-branch-2 (type $5) (param $x i32)
;; CHECK-NEXT: (local $1 i32)
;; CHECK-NEXT: (local $2 i32)
;; CHECK-NEXT: (block $x
;; CHECK-NEXT: (block
;; CHECK-NEXT: (local.set $1
;; CHECK-NEXT: (local.tee $x
;; CHECK-NEXT: (i32.const 1)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (br $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (block $x0
;; CHECK-NEXT: (block
;; CHECK-NEXT: (local.set $2
;; CHECK-NEXT: (local.tee $x
;; CHECK-NEXT: (i32.const 0)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $restructure-br_if-constant-branch-2 (param $x i32)
;; there is no value, thus it can be simpler as below
(block $x
(br_if $x
;; the branch is always taken, make it unconditional
(local.tee $x
(i32.const 1)
)
)
)
(block $x
(br_if $x
;; the branch is never taken, allow control flow to fall through
(local.tee $x
(i32.const 0)
)
)
)
)


;; CHECK: (func $restructure-br_if-value-redundant-in-block-tail-1 (type $2) (result i32)
;; CHECK-NEXT: (block $parent (result i32)
;; CHECK-NEXT: (call $nothing)
Expand Down Expand Up @@ -472,7 +543,12 @@
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (br_if $block
;; CHECK-NEXT: (local.get $temp)
;; CHECK-NEXT: (local.tee $temp
;; CHECK-NEXT: (block (result i32)
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (local.tee $temp
;; CHECK-NEXT: (i32.const 1)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (i32.const 1)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
Expand Down
Loading
Loading