Sema: automatically optimize order of struct fields

This is a simple starting version of the optimization described in #168
where the fields are just sorted by order of descending alignment.

1 files changed, 36 insertions, 0 deletions

diff --git a/src/Sema.zig b/src/Sema.zig
index 3db5eb5ba1..fcf25ab9bf 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -29919,6 +29919,42 @@ fn resolveStructLayout(sema: *Sema, ty: Type) CompileError!void {
             );
             return sema.failWithOwnedErrorMsg(msg);
         }
+
+        if (struct_obj.layout == .Auto and sema.mod.backendSupportsFeature(.field_reordering)) {
+            const optimized_order = blk: {
+                const decl = sema.mod.declPtr(struct_obj.owner_decl);
+                var decl_arena = decl.value_arena.?.promote(sema.mod.gpa);
+                defer decl.value_arena.?.* = decl_arena.state;
+                const decl_arena_allocator = decl_arena.allocator();
+
+                break :blk try decl_arena_allocator.alloc(u32, struct_obj.fields.count());
+            };
+
+            for (struct_obj.fields.values()) |field, i| {
+                optimized_order[i] = if (field.ty.hasRuntimeBits())
+                    @intCast(u32, i)
+                else
+                    Module.Struct.omitted_field;
+            }
+
+            const AlignSortContext = struct {
+                struct_obj: *Module.Struct,
+                sema: *Sema,
+
+                fn lessThan(ctx: @This(), a: u32, b: u32) bool {
+                    if (a == Module.Struct.omitted_field) return false;
+                    if (b == Module.Struct.omitted_field) return true;
+                    const target = ctx.sema.mod.getTarget();
+                    return ctx.struct_obj.fields.values()[a].ty.abiAlignment(target) >
+                        ctx.struct_obj.fields.values()[b].ty.abiAlignment(target);
+                }
+            };
+            std.sort.sort(u32, optimized_order, AlignSortContext{
+                .struct_obj = struct_obj,
+                .sema = sema,
+            }, AlignSortContext.lessThan);
+            struct_obj.optimized_order = optimized_order.ptr;
+        }
     }
     // otherwise it's a tuple; no need to resolve anything
 }