rework generic function calls

Abridged summary:

 * Move `Module.Fn` into `InternPool`.
 * Delete a lot of confusing and problematic `Sema` logic related to
   generic function calls.

This commit removes `Module.Fn` and replaces it with two new
`InternPool.Tag` values:

 * `func_decl` - corresponding to a function declared in the source
   code. This one contains line/column numbers, zir_body_inst, etc.

 * `func_instance` - one for each monomorphization of a generic
   function. Contains a reference to the `func_decl` from whence the
   instantiation came, along with the `comptime` parameter values (or
   types in the case of `anytype`)

Since `InternPool` provides deduplication on these values, these fields
are now deleted from `Module`:

 * `monomorphed_func_keys`
 * `monomorphed_funcs`
 * `align_stack_fns`

Instead of these, Sema logic for generic function instantiation now
unconditionally evaluates the function prototype expression for every
generic callsite. This is technically required in order for type
coercions to work. The previous code had some dubious, probably wrong
hacks to make things work, such as `hashUncoerced`. I'm not 100% sure
how we were able to eliminate that function and still pass all the
behavior tests, but I'm pretty sure things were still broken without
doing type coercion for every generic function call argument.

After the function prototype is evaluated, it produces a deduplicated
`func_instance` `InternPool.Index` which can then be used for the
generic function call.

Some other nice things made by this simplification are the removal of
`comptime_args_fn_inst` and `preallocated_new_func` from `Sema`, and the
messy logic associated with them.

I have not yet been able to measure the perf of this against master
branch. On one hand, it reduces memory usage and pointer chasing of the
most heavily used `InternPool` Tag - function bodies - but on the other
hand, it does evaluate function prototype expressions more than before.
We will soon find out.

1 files changed, 12 insertions, 8 deletions

diff --git a/src/codegen/spirv.zig b/src/codegen/spirv.zig
index eb697ea94e..9f84781966 100644
--- a/src/codegen/spirv.zig
+++ b/src/codegen/spirv.zig
@@ -238,7 +238,7 @@ pub const DeclGen = struct {
             if (ty.zigTypeTag(mod) == .Fn) {
                 const fn_decl_index = switch (mod.intern_pool.indexToKey(val.ip_index)) {
                     .extern_func => |extern_func| extern_func.decl,
-                    .func => |func| mod.funcPtr(func.index).owner_decl,
+                    .func => |func| func.owner_decl,
                     else => unreachable,
                 };
                 const spv_decl_index = try self.resolveDecl(fn_decl_index);
@@ -255,13 +255,14 @@ pub const DeclGen = struct {
     /// Fetch or allocate a result id for decl index. This function also marks the decl as alive.
     /// Note: Function does not actually generate the decl.
     fn resolveDecl(self: *DeclGen, decl_index: Module.Decl.Index) !SpvModule.Decl.Index {
-        const decl = self.module.declPtr(decl_index);
-        try self.module.markDeclAlive(decl);
+        const mod = self.module;
+        const decl = mod.declPtr(decl_index);
+        try mod.markDeclAlive(decl);
 
         const entry = try self.decl_link.getOrPut(decl_index);
         if (!entry.found_existing) {
             // TODO: Extern fn?
-            const kind: SpvModule.DeclKind = if (decl.val.getFunctionIndex(self.module) != .none)
+            const kind: SpvModule.DeclKind = if (decl.val.isFuncBody(mod))
                 .func
             else
                 .global;
@@ -1268,6 +1269,7 @@ pub const DeclGen = struct {
             },
             .Fn => switch (repr) {
                 .direct => {
+                    const ip = &mod.intern_pool;
                     const fn_info = mod.typeToFunc(ty).?;
                     // TODO: Put this somewhere in Sema.zig
                     if (fn_info.is_var_args)
@@ -1275,8 +1277,8 @@ pub const DeclGen = struct {
 
                     const param_ty_refs = try self.gpa.alloc(CacheRef, fn_info.param_types.len);
                     defer self.gpa.free(param_ty_refs);
-                    for (param_ty_refs, 0..) |*param_type, i| {
-                        param_type.* = try self.resolveType(fn_info.param_types[i].toType(), .direct);
+                    for (param_ty_refs, fn_info.param_types.get(ip)) |*param_type, fn_param_type| {
+                        param_type.* = try self.resolveType(fn_param_type.toType(), .direct);
                     }
                     const return_ty_ref = try self.resolveType(fn_info.return_type.toType(), .direct);
 
@@ -1576,6 +1578,7 @@ pub const DeclGen = struct {
 
     fn genDecl(self: *DeclGen) !void {
         const mod = self.module;
+        const ip = &mod.intern_pool;
         const decl = mod.declPtr(self.decl_index);
         const spv_decl_index = try self.resolveDecl(self.decl_index);
 
@@ -1594,7 +1597,8 @@ pub const DeclGen = struct {
             const fn_info = mod.typeToFunc(decl.ty).?;
 
             try self.args.ensureUnusedCapacity(self.gpa, fn_info.param_types.len);
-            for (fn_info.param_types) |param_type| {
+            for (0..fn_info.param_types.len) |i| {
+                const param_type = fn_info.param_types.get(ip)[i];
                 const param_type_id = try self.resolveTypeId(param_type.toType());
                 const arg_result_id = self.spv.allocId();
                 try self.func.prologue.emit(self.spv.gpa, .OpFunctionParameter, .{
@@ -1621,7 +1625,7 @@ pub const DeclGen = struct {
             try self.func.body.emit(self.spv.gpa, .OpFunctionEnd, {});
             try self.spv.addFunction(spv_decl_index, self.func);
 
-            const fqn = mod.intern_pool.stringToSlice(try decl.getFullyQualifiedName(self.module));
+            const fqn = ip.stringToSlice(try decl.getFullyQualifiedName(self.module));
 
             try self.spv.sections.debug_names.emit(self.gpa, .OpName, .{
                 .target = decl_id,