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, 4 insertions, 3 deletions

diff --git a/src/link/SpirV.zig b/src/link/SpirV.zig
index 5bbd5ebdc0..da5fef7c85 100644
--- a/src/link/SpirV.zig
+++ b/src/link/SpirV.zig
@@ -29,6 +29,7 @@ const assert = std.debug.assert;
 const log = std.log.scoped(.link);
 
 const Module = @import("../Module.zig");
+const InternPool = @import("../InternPool.zig");
 const Compilation = @import("../Compilation.zig");
 const link = @import("../link.zig");
 const codegen = @import("../codegen/spirv.zig");
@@ -103,12 +104,12 @@ pub fn deinit(self: *SpirV) void {
     self.decl_link.deinit();
 }
 
-pub fn updateFunc(self: *SpirV, module: *Module, func_index: Module.Fn.Index, air: Air, liveness: Liveness) !void {
+pub fn updateFunc(self: *SpirV, module: *Module, func_index: InternPool.Index, air: Air, liveness: Liveness) !void {
     if (build_options.skip_non_native) {
         @panic("Attempted to compile for architecture that was disabled by build configuration");
     }
 
-    const func = module.funcPtr(func_index);
+    const func = module.funcInfo(func_index);
 
     var decl_gen = codegen.DeclGen.init(self.base.allocator, module, &self.spv, &self.decl_link);
     defer decl_gen.deinit();
@@ -138,7 +139,7 @@ pub fn updateDeclExports(
     exports: []const *Module.Export,
 ) !void {
     const decl = mod.declPtr(decl_index);
-    if (decl.val.getFunctionIndex(mod) != .none and decl.ty.fnCallingConvention(mod) == .Kernel) {
+    if (decl.val.isFuncBody(mod) and decl.ty.fnCallingConvention(mod) == .Kernel) {
         // TODO: Unify with resolveDecl in spirv.zig.
         const entry = try self.decl_link.getOrPut(decl_index);
         if (!entry.found_existing) {