Skip to content

perf: fold LSB-test i32.and X 1 into i32.ctz in boolean contexts #8562

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
ggreif wants to merge 3 commits into
base: main
Choose a base branch
from
+119 −0
Open

perf: fold LSB-test i32.and X 1 into i32.ctz in boolean contexts #8562

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
9b55439
perf(OptimizeInstructions): fold `i32.and X 1; if T E` into `i32.ctz …
ggreif Apr 1, 2026
bff1a50
perf(OptimizeInstructions): fold `eqz(and X 1)` into `ctz X` in boole…
ggreif Apr 1, 2026
575cd27
chore: remove nix files (not for upstream)
ggreif Apr 1, 2026
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
40 changes: 40 additions & 0 deletions src/passes/OptimizeInstructions.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1193,6 +1193,28 @@ struct OptimizeInstructions
BranchHints::flip(curr, getFunction());
}
}
// (i32.and X 1) as if-else condition => (i32.ctz X) with swapped arms,
// since ctz(X) == 0 iff LSB(X) == 1 (saves one instruction).
if (auto* binary = curr->condition->dynCast<Binary>()) {
if (binary->op == AndInt32) {
Expression* other = nullptr;
if (auto* c = binary->right->dynCast<Const>()) {
if (c->value.geti32() == 1) {
other = binary->left;
}
} else if (auto* c = binary->left->dynCast<Const>()) {
if (c->value.geti32() == 1) {
other = binary->right;
}
}
if (other) {
Builder builder(*getModule());
curr->condition = builder.makeUnary(CtzInt32, other);
std::swap(curr->ifTrue, curr->ifFalse);
BranchHints::flip(curr, getFunction());
}
}
}
// Note that we do not consider metadata here. Like LLVM, we ignore
// metadata when trying to fold code together, preferring certain
// optimization over possible benefits of profiling data.
Expand Down Expand Up @@ -3114,6 +3136,24 @@ struct OptimizeInstructions
binary->op = op;
return binary;
}
// eqz(and X 1) ==> ctz X in boolean context:
// both are truthy iff LSB(X) == 0, saving one instruction.
if (binary->op == AndInt32) {
Expression* other = nullptr;
if (auto* c = binary->right->dynCast<Const>()) {
if (c->value.geti32() == 1) {
other = binary->left;
}
} else if (auto* c = binary->left->dynCast<Const>()) {
if (c->value.geti32() == 1) {
other = binary->right;
}
}
if (other) {
Builder builder(*getModule());
return builder.makeUnary(CtzInt32, other);
}
}
}
}
if (unary->op == EqZInt32 || unary->op == EqZInt64) {
Expand Down
79 changes: 79 additions & 0 deletions test/lit/passes/optimize-instructions-lsb-if.wast
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,79 @@
;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
;; RUN: wasm-opt %s --optimize-instructions -S -o - | filecheck %s

;; Test that (if (i32.and X (i32.const 1)) T E) is optimized to
;; (if (i32.ctz X) E T), and (br_if N V (i32.eqz (i32.and X 1))) to
;; (br_if N V (i32.ctz X)), saving one instruction in each case.

(module
;; CHECK: (func $lsb-if (param $x i32) (result i32)
;; CHECK-NEXT: (if (result i32)
;; CHECK-NEXT: (i32.ctz
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: (then
;; CHECK-NEXT: (i32.const 0)
;; CHECK-NEXT: )
;; CHECK-NEXT: (else
;; CHECK-NEXT: (i32.const 1)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $lsb-if (param $x i32) (result i32)
;; if LSB is set, return 1; else return 0
;; optimizes to: if ctz(x) != 0, return 0; else return 1
(if (result i32)
(i32.and (local.get $x) (i32.const 1))
(then (i32.const 1))
(else (i32.const 0))
)
)

;; CHECK: (func $lsb-if-const-left (param $x i32) (result i32)
;; CHECK-NEXT: (if (result i32)
;; CHECK-NEXT: (i32.ctz
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: (then
;; CHECK-NEXT: (i32.const 0)
;; CHECK-NEXT: )
;; CHECK-NEXT: (else
;; CHECK-NEXT: (i32.const 1)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $lsb-if-const-left (param $x i32) (result i32)
;; same but constant on the left
(if (result i32)
(i32.and (i32.const 1) (local.get $x))
(then (i32.const 1))
(else (i32.const 0))
)
)

;; CHECK: (func $lsb-brif (param $x i32) (result i32)
;; CHECK-NEXT: (block $done (result i32)
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (br_if $done
;; CHECK-NEXT: (i32.const 99)
;; CHECK-NEXT: (i32.ctz
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (i32.const 42)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $lsb-brif (param $x i32) (result i32)
;; br_if (eqz (and X 1)) => br_if (ctz X): the typical is_skewed/is_scalar pattern
(block $done (result i32)
(drop
(br_if $done
(i32.const 99)
(i32.eqz (i32.and (local.get $x) (i32.const 1)))
)
)
(i32.const 42)
)
)
)