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

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

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54f70f1
perf(OptimizeInstructions): fold `i32.and X 1; if T E` into `i32.ctz …
ggreif Apr 1, 2026
3bacb80
perf(OptimizeInstructions): fold `eqz(and X 1)` into `ctz X` in boole…
ggreif Apr 1, 2026
1f29def
perf(OptimizeInstructions): gate LSB→ctz fold on shrinkLevel >= 1
ggreif May 8, 2026
784ed83
chore: remove nix files (not for upstream)
ggreif Apr 1, 2026
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50 changes: 50 additions & 0 deletions src/passes/OptimizeInstructions.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1193,6 +1193,35 @@ 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).
//
// Gated on shrinkLevel >= 1 (i.e. -Os or -Oz) only: TZCNT can cost
// 1-2 cycles more than AND on common JIT VMs (per Agner Fog's
// tables), and JIT-less interpreters (wasm3, smart-contract
// runtimes) lack a fast path for ctz at all. The byte-saving is the
// win we want under shrink modes; under speed modes the AND form
// stays. See WebAssembly/binaryen#8562.
if (auto* binary = curr->condition->dynCast<Binary>()) {
if (binary->op == AndInt32 && getPassOptions().shrinkLevel >= 1) {
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@kripken kripken May 8, 2026

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How about making this >= 2, i.e., only in -Oz? -Os is meant to be a good balance between size and speed, and without more data I'm not sure how balanced this is. -Oz is "size at all costs".

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@ggreif ggreif May 8, 2026

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I don't think so, as (i32.and X (i32.const 1)) would often feed conditionals and the proposed transform would unlock ripple effects. This is not only space but also time saving.

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 +3143,27 @@ 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.
// Gated on shrinkLevel >= 1 (-Os, -Oz) — see the matching
// comment in visitIf and WebAssembly/binaryen#8562.
if (binary->op == AndInt32 &&
getPassOptions().shrinkLevel >= 1) {
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
186 changes: 186 additions & 0 deletions test/lit/passes/optimize-instructions-lsb-if.wast
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@tlively tlively May 8, 2026

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Let's add tests for select as well.

Original file line number Diff line number Diff line change
@@ -0,0 +1,186 @@
;; 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 --check-prefix=DEFAULT
;; RUN: wasm-opt %s --shrink-level=1 --optimize-instructions -S -o - | filecheck %s --check-prefix=SHRINK

;; Test the LSB→ctz fold: under shrink modes (-Os, -Oz, equivalent to
;; --shrink-level >= 1) `(if (i32.and X 1) T E)` becomes
;; `(if (i32.ctz X) E T)`, and `(br_if N V (i32.eqz (i32.and X 1)))`
;; becomes `(br_if N V (i32.ctz X))` — one instruction less, but
;; potentially 1-2 cycles slower on JIT VMs and unconditionally slower
;; on JIT-less interpreters. The fold is therefore suppressed under
;; default and speed-optimised modes, and only fires when the user has
;; opted into shrinking. See WebAssembly/binaryen#8562.

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

;; DEFAULT: (func $lsb-if-const-left (param $x i32) (result i32)
;; DEFAULT-NEXT: (if (result i32)
;; DEFAULT-NEXT: (i32.and
;; DEFAULT-NEXT: (local.get $x)
;; DEFAULT-NEXT: (i32.const 1)
;; DEFAULT-NEXT: )
;; DEFAULT-NEXT: (then
;; DEFAULT-NEXT: (i32.const 1)
;; DEFAULT-NEXT: )
;; DEFAULT-NEXT: (else
;; DEFAULT-NEXT: (i32.const 0)
;; DEFAULT-NEXT: )
;; DEFAULT-NEXT: )
;; DEFAULT-NEXT: )
;; SHRINK: (func $lsb-if-const-left (param $x i32) (result i32)
;; SHRINK-NEXT: (if (result i32)
;; SHRINK-NEXT: (i32.ctz
;; SHRINK-NEXT: (local.get $x)
;; SHRINK-NEXT: )
;; SHRINK-NEXT: (then
;; SHRINK-NEXT: (i32.const 0)
;; SHRINK-NEXT: )
;; SHRINK-NEXT: (else
;; SHRINK-NEXT: (i32.const 1)
;; SHRINK-NEXT: )
;; SHRINK-NEXT: )
;; SHRINK-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))
)
)

;; DEFAULT: (func $lsb-brif (param $x i32) (result i32)
;; DEFAULT-NEXT: (block $done (result i32)
;; DEFAULT-NEXT: (drop
;; DEFAULT-NEXT: (br_if $done
;; DEFAULT-NEXT: (i32.const 99)
;; DEFAULT-NEXT: (i32.eqz
;; DEFAULT-NEXT: (i32.and
;; DEFAULT-NEXT: (local.get $x)
;; DEFAULT-NEXT: (i32.const 1)
;; DEFAULT-NEXT: )
;; DEFAULT-NEXT: )
;; DEFAULT-NEXT: )
;; DEFAULT-NEXT: )
;; DEFAULT-NEXT: (i32.const 42)
;; DEFAULT-NEXT: )
;; DEFAULT-NEXT: )
;; SHRINK: (func $lsb-brif (param $x i32) (result i32)
;; SHRINK-NEXT: (block $done (result i32)
;; SHRINK-NEXT: (drop
;; SHRINK-NEXT: (br_if $done
;; SHRINK-NEXT: (i32.const 99)
;; SHRINK-NEXT: (i32.ctz
;; SHRINK-NEXT: (local.get $x)
;; SHRINK-NEXT: )
;; SHRINK-NEXT: )
;; SHRINK-NEXT: )
;; SHRINK-NEXT: (i32.const 42)
;; SHRINK-NEXT: )
;; SHRINK-NEXT: )
(func $lsb-brif (param $x i32) (result i32)
;; br_if (eqz (and X 1)) — 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)
)
)

;; DEFAULT: (func $lsb-select (param $x i32) (param $a i32) (param $b i32) (result i32)
;; DEFAULT-NEXT: (select
;; DEFAULT-NEXT: (local.get $b)
;; DEFAULT-NEXT: (local.get $a)
;; DEFAULT-NEXT: (i32.and
;; DEFAULT-NEXT: (local.get $x)
;; DEFAULT-NEXT: (i32.const 1)
;; DEFAULT-NEXT: )
;; DEFAULT-NEXT: )
;; DEFAULT-NEXT: )
;; SHRINK: (func $lsb-select (param $x i32) (param $a i32) (param $b i32) (result i32)
;; SHRINK-NEXT: (select
;; SHRINK-NEXT: (local.get $a)
;; SHRINK-NEXT: (local.get $b)
;; SHRINK-NEXT: (i32.ctz
;; SHRINK-NEXT: (local.get $x)
;; SHRINK-NEXT: )
;; SHRINK-NEXT: )
;; SHRINK-NEXT: )
(func $lsb-select (param $x i32) (param $a i32) (param $b i32) (result i32)
;; select with the eqz-and-1 boolean condition.
;; Non-constant arms keep the select itself in the IR — otherwise
;; an unrelated `select c1 c0 P` simplification would eat it.
(select
(local.get $a)
(local.get $b)
(i32.eqz (i32.and (local.get $x) (i32.const 1)))
)
)

;; DEFAULT: (func $lsb-select-const-left (param $x i32) (param $a i32) (param $b i32) (result i32)
;; DEFAULT-NEXT: (select
;; DEFAULT-NEXT: (local.get $b)
;; DEFAULT-NEXT: (local.get $a)
;; DEFAULT-NEXT: (i32.and
;; DEFAULT-NEXT: (local.get $x)
;; DEFAULT-NEXT: (i32.const 1)
;; DEFAULT-NEXT: )
;; DEFAULT-NEXT: )
;; DEFAULT-NEXT: )
;; SHRINK: (func $lsb-select-const-left (param $x i32) (param $a i32) (param $b i32) (result i32)
;; SHRINK-NEXT: (select
;; SHRINK-NEXT: (local.get $a)
;; SHRINK-NEXT: (local.get $b)
;; SHRINK-NEXT: (i32.ctz
;; SHRINK-NEXT: (local.get $x)
;; SHRINK-NEXT: )
;; SHRINK-NEXT: )
;; SHRINK-NEXT: )
(func $lsb-select-const-left (param $x i32) (param $a i32) (param $b i32) (result i32)
;; same but with the constant on the left of the AND.
(select
(local.get $a)
(local.get $b)
(i32.eqz (i32.and (i32.const 1) (local.get $x)))
)
)
)