diff --git a/src/datatype.c b/src/datatype.c
index 593a2ededd169..d6604fb790ab1 100644
--- a/src/datatype.c
+++ b/src/datatype.c
@@ -282,7 +282,7 @@ static unsigned union_isinlinable(jl_value_t *ty, int pointerfree, size_t *nbyte
         size_t sz = jl_datatype_size(ty);
         size_t al = jl_datatype_align(ty);
         // primitive types in struct slots need their sizes aligned. issue #37974
-        if (asfield && jl_is_primitivetype(ty))
+        if (asfield)
             sz = LLT_ALIGN(sz, al);
         if (*nbytes < sz)
             *nbytes = sz;
@@ -465,6 +465,8 @@ void jl_compute_field_offsets(jl_datatype_t *st)
                     uint32_t fld_npointers = ((jl_datatype_t*)fld)->layout->npointers;
                     if (((jl_datatype_t*)fld)->layout->haspadding)
                         haspadding = 1;
+                    if (jl_datatype_size(fld) < fsz)
+                        haspadding = 1;
                     if (i >= nfields - st->name->n_uninitialized && fld_npointers &&
                         fld_npointers * sizeof(void*) != fsz) {
                         // field may be undef (may be uninitialized and contains pointer),
@@ -491,8 +493,13 @@ void jl_compute_field_offsets(jl_datatype_t *st)
             }
             if (isatomic && fsz > MAX_ATOMIC_SIZE)
                 needlock = 1;
-            if (isatomic && fsz <= MAX_ATOMIC_SIZE)
-                al = fsz = next_power_of_two(fsz);
+            if (isatomic && fsz <= MAX_ATOMIC_SIZE) {
+                size_t nfsz = next_power_of_two(fsz);
+                if (nfsz > fsz) {
+                    haspadding = 1;
+                }
+                al = fsz = nfsz;
+            }
             if (al != 0) {
                 size_t alsz = LLT_ALIGN(sz, al);
                 if (alsz != sz)
@@ -525,9 +532,7 @@ void jl_compute_field_offsets(jl_datatype_t *st)
             if (al > alignm)
                 alignm = al;
         }
-        st->size = LLT_ALIGN(sz, alignm);
-        if (st->size > sz)
-            haspadding = 1;
+        st->size = sz;
         if (should_malloc && npointers)
             pointers = (uint32_t*)malloc_s(npointers * sizeof(uint32_t));
         else
@@ -1470,6 +1475,7 @@ static inline void memassign_safe(int hasptr, jl_value_t *parent, char *dst, con
         memmove_refs((void**)dst, (void**)src, nptr);
         jl_gc_multi_wb(parent, src);
         src = (jl_value_t*)((char*)src + nptr * sizeof(void*));
+        dst += nptr * sizeof(void*);
         nb -= nptr * sizeof(void*);
     }
     else {
diff --git a/test/atomics.jl b/test/atomics.jl
index 15ffd84a2c0a2..1e9a5b087abba 100644
--- a/test/atomics.jl
+++ b/test/atomics.jl
@@ -46,7 +46,7 @@ swap(x, y) = y
 let T1 = Refxy{NTuple{3,UInt8}},
     T2 = ARefxy{NTuple{3,UInt8}}
     @test sizeof(T1) == 6
-    @test sizeof(T2) == 8
+    @test sizeof(T2) == 7
     @test fieldoffset(T1, 1) == 0
     @test fieldoffset(T2, 1) == 0
     @test fieldoffset(T1, 2) == 3
diff --git a/test/compiler/codegen.jl b/test/compiler/codegen.jl
index 5db536af6222f..eaa511b6aee10 100644
--- a/test/compiler/codegen.jl
+++ b/test/compiler/codegen.jl
@@ -540,10 +540,10 @@ let a = Core.Intrinsics.trunc_int(UInt24, 3),
     f(t) = t[2]
     @test f((a, true)) === true
     @test f((a, false)) === false
-    @test sizeof(Tuple{UInt24,Bool}) == 8
+    @test sizeof(Tuple{UInt24,Bool}) == 5
     @test sizeof(UInt24) == 3
     @test sizeof(Union{UInt8,UInt24}) == 3
-    @test sizeof(Base.RefValue{Union{UInt8,UInt24}}) == 8
+    @test sizeof(Base.RefValue{Union{UInt8,UInt24}}) == 5
 end
 
 # issue #39232
diff --git a/test/core.jl b/test/core.jl
index a0bd78df8e1ce..3dac23c34b2c1 100644
--- a/test/core.jl
+++ b/test/core.jl
@@ -6085,7 +6085,7 @@ let
     @test b.x === Int8(91)
     @test b.z === Int8(23)
     @test b.y === A23367((Int8(1), Int8(2), Int8(3), Int8(4), Int8(5), Int8(6), Int8(7)))
-    @test sizeof(b) == 12
+    @test sizeof(b) == 11
     @test A23367(Int8(1)).x === Int8(1)
     @test A23367(Int8(0)).x === Int8(0)
     @test A23367(Int16(1)).x === Int16(1)
@@ -6118,17 +6118,17 @@ struct UnionFieldInlineStruct
     y::Union{Float64, Missing}
 end
 
-@test sizeof(Vector{UnionFieldInlineStruct}(undef, 2)) == sizeof(UnionFieldInlineStruct) * 2
+@test sizeof(Vector{UnionFieldInlineStruct}(undef, 2)) >= sizeof(UnionFieldInlineStruct) * 2
 
 let x = UnionFieldInlineStruct(1, 3.14)
     AInlineUnion = [x for i = 1:10]
-    @test sizeof(AInlineUnion) == sizeof(UnionFieldInlineStruct) * 10
+    @test sizeof(AInlineUnion) >= sizeof(UnionFieldInlineStruct) * 10
     BInlineUnion = Vector{UnionFieldInlineStruct}(undef, 10)
     copyto!(BInlineUnion, AInlineUnion)
     @test AInlineUnion == BInlineUnion
     @test BInlineUnion[end] == x
     CInlineUnion = vcat(AInlineUnion, BInlineUnion)
-    @test sizeof(CInlineUnion) == sizeof(UnionFieldInlineStruct) * 20
+    @test sizeof(CInlineUnion) >= sizeof(UnionFieldInlineStruct) * 20
     @test CInlineUnion[end] == x
 end