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kripken merged 3 commits into from
Aug 27, 2026
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Constraints: Do not optimize floats without fast-math #9049

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fix
kripken Aug 27, 2026
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kripken Aug 27, 2026
88f168a
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kripken Aug 27, 2026
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5 changes: 5 additions & 0 deletions src/ir/constraint.h
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Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,11 @@
// inference whether other things are true given a set of constraints, like
// { x == 10 } => { x >= 5 }.
//
// This code follows the basic rules of logic, like the law of the excluded
// middle and so forth. Things that do not follow basic logic, like floating-
// point NaNs (where both x < y and x >= y are possible, with NaNs), may not be
// handled correctly. The caller must ensure that no such input is possible.
//

#ifndef wasm_ir_constraint_h
#define wasm_ir_constraint_h
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18 changes: 17 additions & 1 deletion src/passes/ConstraintAnalysis.cpp
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Expand Up @@ -146,15 +146,31 @@ struct ConstraintAnalysis

void maybeMarkRelevant(Expression* curr) {
// If this parses into a constraint on a local, that local is relevant.
if (auto parsed = LocalConstraint::parseCondition(curr)) {
if (auto parsed = LocalConstraint::parseCondition(curr);
parsed && isRelevantType(getFunction()->getLocalType(parsed->local))) {
relevantLocals[parsed->local] = true;
if (auto* other = std::get_if<Index>(&parsed->constraint.term)) {
relevantLocals[*other] = true;
}
}
}

bool fastMath;

bool isRelevantType(Type type) {
// Floating-point math does not follow the basic rules of logic (for
// example, NaN < NaN and NaN >= NaN are both false, despite the law of the
// excluded middle). Constraints follow the rules of logic, so we cannot
// operate on floats unless we have fast-math enabled (which assures us we
// can ignore NaNs).
// TODO: when values are constant and non-NaN, we could optimize even
// without fast-math
return !type.isFloat() || fastMath;
}

void doWalkFunction(Function* func) {
fastMath = getPassOptions().fastMath;

relevantLocals.assign(func->getNumLocals(), false);

Super::doWalkFunction(func);
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138 changes: 138 additions & 0 deletions test/lit/passes/constraint-analysis-float.wast
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@@ -0,0 +1,138 @@
;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.

;; Floating-point numbers are not optimized without fast-math, as NaN operations
;; do not follow the rules of logic and we might misoptimize them.

;; RUN: wasm-opt %s --constraint-analysis -all -S -o - | filecheck %s
;; RUN: wasm-opt %s --constraint-analysis --fast-math -all -S -o - | filecheck %s --check-prefix=FASTM

(module
;; CHECK: (func $float-zero (type $0)
;; CHECK-NEXT: (local $f f64)
;; CHECK-NEXT: (if
;; CHECK-NEXT: (f64.eq
;; CHECK-NEXT: (local.get $f)
;; CHECK-NEXT: (local.get $f)
;; CHECK-NEXT: )
;; CHECK-NEXT: (then
;; CHECK-NEXT: (nop)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; FASTM: (func $float-zero (type $0)
;; FASTM-NEXT: (local $f f64)
;; FASTM-NEXT: (if
;; FASTM-NEXT: (i32.const 1)
;; FASTM-NEXT: (then
;; FASTM-NEXT: (nop)
;; FASTM-NEXT: )
;; FASTM-NEXT: )
;; FASTM-NEXT: )
(func $float-zero
(local $f f64)
;; When fast-math is enabled, we can optimize f == f to 1, as NaNs are not
;; important.
(if
(f64.eq
(local.get $f)
(local.get $f)
)
(then
(nop)
)
)
)

;; CHECK: (func $float-negative-zero (type $0)
;; CHECK-NEXT: (local $f f64)
;; CHECK-NEXT: (if
;; CHECK-NEXT: (f64.eq
;; CHECK-NEXT: (local.get $f)
;; CHECK-NEXT: (f64.const -0)
;; CHECK-NEXT: )
;; CHECK-NEXT: (then
;; CHECK-NEXT: (nop)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (if
;; CHECK-NEXT: (f64.eq
;; CHECK-NEXT: (local.get $f)
;; CHECK-NEXT: (f64.const 0)
;; CHECK-NEXT: )
;; CHECK-NEXT: (then
;; CHECK-NEXT: (nop)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; FASTM: (func $float-negative-zero (type $0)
;; FASTM-NEXT: (local $f f64)
;; FASTM-NEXT: (if
;; FASTM-NEXT: (i32.const 1)
;; FASTM-NEXT: (then
;; FASTM-NEXT: (nop)
;; FASTM-NEXT: )
;; FASTM-NEXT: )
;; FASTM-NEXT: (if
;; FASTM-NEXT: (i32.const 1)
;; FASTM-NEXT: (then
;; FASTM-NEXT: (nop)
;; FASTM-NEXT: )
;; FASTM-NEXT: )
;; FASTM-NEXT: )
(func $float-negative-zero
(local $f f64)
;; Negative zero is equal to zero, even though it has a different bit
;; pattern. Both conditions here should be optimized to 1 in fast-math
;; mode (but not otherwise).
(if
(f64.eq
(local.get $f)
(f64.const -0)
)
(then
(nop)
)
)
(if
(f64.eq
(local.get $f)
(f64.const 0)
)
(then
(nop)
)
)
)

;; CHECK: (func $float-ne-nan (type $1) (result i32)
;; CHECK-NEXT: (local $nan f64)
;; CHECK-NEXT: (local.set $nan
;; CHECK-NEXT: (f64.const nan:0x8000000000000)
;; CHECK-NEXT: )
;; CHECK-NEXT: (f64.ne
;; CHECK-NEXT: (local.get $nan)
;; CHECK-NEXT: (local.get $nan)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; FASTM: (func $float-ne-nan (type $1) (result i32)
;; FASTM-NEXT: (local $nan f64)
;; FASTM-NEXT: (local.set $nan
;; FASTM-NEXT: (f64.const nan:0x8000000000000)
;; FASTM-NEXT: )
;; FASTM-NEXT: (i32.const 0)
;; FASTM-NEXT: )
(func $float-ne-nan (result i32)
(local $nan f64)
(local.set $nan
(f64.const nan)
)
;; This should be 1: the number is a NaN, and NaN != NaN. However, we
;; misoptimize this to 0 in fast-math mode (which is fine, as that mode
;; ignores the possibility of NaNs). In normal mode we do not optimize,
;; avoiding a misoptimization.
(f64.ne
(local.get $nan)
(local.get $nan)
)
)
)
54 changes: 0 additions & 54 deletions test/lit/passes/constraint-analysis.wast
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Expand Up @@ -4551,60 +4551,6 @@
)
)

;; CHECK: (func $float-negative-zero (type $1)
;; CHECK-NEXT: (local $f f64)
;; CHECK-NEXT: (if
;; CHECK-NEXT: (i32.const 1)
;; CHECK-NEXT: (then
;; CHECK-NEXT: (nop)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (if
;; CHECK-NEXT: (i32.const 1)
;; CHECK-NEXT: (then
;; CHECK-NEXT: (nop)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; OPTIN: (func $float-negative-zero (type $1)
;; OPTIN-NEXT: (local $f f64)
;; OPTIN-NEXT: (if
;; OPTIN-NEXT: (i32.const 1)
;; OPTIN-NEXT: (then
;; OPTIN-NEXT: (nop)
;; OPTIN-NEXT: )
;; OPTIN-NEXT: )
;; OPTIN-NEXT: (if
;; OPTIN-NEXT: (i32.const 1)
;; OPTIN-NEXT: (then
;; OPTIN-NEXT: (nop)
;; OPTIN-NEXT: )
;; OPTIN-NEXT: )
;; OPTIN-NEXT: )
(func $float-negative-zero
(local $f f64)
;; Negative zero is equal to zero, even though it has a different bit
;; pattern. Both conditions here should be optimized to 1.
(if
(f64.eq
(local.get $f)
(f64.const -0)
)
(then
(nop)
)
)
(if
(f64.eq
(local.get $f)
(f64.const 0)
)
(then
(nop)
)
)
)

;; CHECK: (func $tee (type $1)
;; CHECK-NEXT: (local $x i32)
;; CHECK-NEXT: (local $y i32)
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