diff --git a/src/accumulator.jl b/src/accumulator.jl
index 447fbf19..5539d1b2 100644
--- a/src/accumulator.jl
+++ b/src/accumulator.jl
@@ -84,17 +84,31 @@ resolve_acctype(op, ::Type, ::Type, acctype) =
 # gives the same result as seeding with a neutral element.
 #
 # An unset `Optional` of a bits type (every one kernels see) holds zero bits; one of another type
-# leaves the value undefined, which is how it is recognised.
-struct Optional{T}
+# leaves the value undefined, which is how it is recognised. The flag `F` is `flag_type(T)`.
+struct Optional{T, F}
     value::T
-    isset::Bool
-    Optional{T}() where {T} = isinline(T) ? new{T}(zero_bits(T), false) : new{T}()
-    Optional{T}(value) where {T} = new{T}(value, true)
-    Optional{T}(value, isset::Bool) where {T} = new{T}(value, isset)
+    isset::F
+    Optional{T, F}() where {T, F} = new{T, F}()
+    Optional{T, F}(value, isset::Bool) where {T, F} = new{T, F}(value, isset)
+end
+Optional{T}() where {T} =
+    isinline(T) ? Optional{T, flag_type(T)}(zero_bits(T), false) : Optional{T, flag_type(T)}()
+Optional{T}(value) where {T} = Optional{T, flag_type(T)}(value, true)
+Optional{T}(value, isset::Bool) where {T} = Optional{T, flag_type(T)}(value, isset)
+
+# The flag of an `Optional{T}`. A `Bool` would leave the bytes up to `T`'s alignment as padding,
+# which stores of an `Optional` don't write but kernels load (partials are loaded whole) and the
+# host copies, so Compute Sanitizer's initcheck reports reading them. For a bits type, the flag is
+# the unsigned integer that fills those bytes, up to 8 of them: OpenCL's SPIR-V has no wider
+# integers, so an `Optional` of a 16-byte aligned type keeps some padding (as does a `T` with
+# padding of its own).
+@generated function flag_type(::Type{T}) where {T}
+    isbitstype(T) || return Bool
+    return (UInt8, UInt16, UInt32, UInt64)[trailing_zeros(min(Base.datatype_alignment(T), 8)) + 1]
 end
 
 isinline(::Type{T}) where {T} = isbitstype(T) || Base.isbitsunion(T)
-@inline isset(l::Optional{T}) where {T} = isinline(T) ? l.isset : isdefined(l, :value)
+@inline isset(l::Optional{T}) where {T} = isinline(T) ? !iszero(l.isset) : isdefined(l, :value)
 
 # A value of the bits type or bits union `T` with all-zero bits
 function zero_bits(::Type{T}) where {T}
diff --git a/test/algorithms.jl b/test/algorithms.jl
index b441c93d..2b64a11e 100644
--- a/test/algorithms.jl
+++ b/test/algorithms.jl
@@ -533,6 +533,15 @@ end
     @test_throws ArgumentError resolve_reduce(AK.Auto(); T=Union{Missing, Int32})
     @test_throws ArgumentError resolve_reduce(AK.BlockReduce(); T=String)
     @test resolve_reduce(AK.Auto(); T=String, backend=HOST) isa AK.CPUThreads.Partitioned
+
+    # Partial results of an operator without a neutral element are `Optional`s, which kernels load
+    # whole and the host copies: for these types, they have no padding that stores leave unwritten
+    for T in (Bool, Int8, Int32, Float32, Float64, ComplexF32, NTuple{3, Int32},
+              Tuple{Float64, Float64})
+        @test !Base.datatype_haspadding(typeof(AK.reduce_seed((a, b) -> a, T, nothing)))
+    end
+    # (but their flag is no wider than 64 bits, which OpenCL's SPIR-V can't go beyond)
+    @test AK.flag_type(NTuple{4, VecElement{Float32}}) === UInt64
 end
 
 
@@ -594,13 +603,13 @@ end
     @test_throws ArgumentError resolve_scan(AK.ScanPrefixes(); dims=1)
     @test_throws ArgumentError resolve_scan(AK.SliceScan())
 
-    # Tiles of partial results: lanes (a Bool and a value) are wider than elements, so fewer fit.
+    # Tiles of partial results: lanes (a value and a flag) are wider than elements, so fewer fit.
     # NTuple{3, Int32} is 4-byte aligned everywhere, so its lanes take 16 bytes on every platform
     # (Int64 lanes take 12 on 32-bit x86)
     with_tuning(AK.ScanTuning(block_size=256, local_mem_bytes=32 * 1024, max_items=8)) do
         @test resolve_scan(AK.Auto(); T=NTuple{3, Int32}) === AK.ScanPrefixes(256, 8)
         @test AK.resolve_scan(AK.Auto(), FAKE, NTuple{3, Int32}, nothing,
-                              AK.Optional{NTuple{3, Int32}}) === AK.ScanPrefixes(256, 7)
+                              typeof(AK.Optional{NTuple{3, Int32}}())) === AK.ScanPrefixes(256, 7)
     end
 
     # Kernels need a bits-type element
diff --git a/test/generic/accumulate.jl b/test/generic/accumulate.jl
index 117578ea..461bcd6e 100644
--- a/test/generic/accumulate.jl
+++ b/test/generic/accumulate.jl
@@ -213,7 +213,7 @@ end
     # in turn. Tiles of exactly one and two blocks' worth, and one element more, for both block
     # sizes.
     for neutral in (I2, nothing), inclusive in (true, false), block_size in (64, 256)
-        S = neutral === nothing ? AK.Optional{Mat2} : Mat2
+        S = neutral === nothing ? typeof(AK.Optional{Mat2}()) : Mat2
         tile = !TEST_KERNELS ? 2 * block_size :
                block_size * AK.resolve_scan(AK.SliceScan(; block_size), BACKEND, Mat2, 1,
                                             S).items_per_thread
diff --git a/test/generic/tunings.jl b/test/generic/tunings.jl
index f5014a01..45bb8684 100644
--- a/test/generic/tunings.jl
+++ b/test/generic/tunings.jl
@@ -34,7 +34,8 @@ end
         end
         for dims in (nothing, 1)
             @test AK.resolve_scan(AK.Auto(), BACKEND, T, dims) isa AK.Algorithm
-            @test AK.resolve_scan(AK.Auto(), BACKEND, T, dims, AK.Optional{T}) isa AK.Algorithm
+            @test AK.resolve_scan(AK.Auto(), BACKEND, T, dims, typeof(AK.Optional{T}())) isa
+                  AK.Algorithm
         end
         if kernels
             @test AK.resolve_scan(AK.ScanPrefixes(), BACKEND, T, nothing) isa AK.ScanPrefixes