InternPool: store alignment of anon decls

Commit 5393e56500d499753dbc39704c0161b47d1e4d5c has a flaw pointed out
by @mlugg: the `ty` field of pointer values changes when comptime values
are pointer-casted. This commit introduces a new encoding which
additionally stores the "original pointer type" which is used to store
the alignment of the anonymous decl, and potentially other information
in the future such as section and pointer address space. However, this
new encoding is only used when the original pointer type differs from
the casted pointer type in a meaningful way.

I was able to make the LLVM backend and the C backend lower anonymous
decls with the appropriate alignment, however I will need some help
figuring out how to do this for the backends that lower anonymous decls
via src/codegen.zig and the wasm backend.

1 files changed, 19 insertions, 2 deletions

diff --git a/src/codegen/c.zig b/src/codegen/c.zig
index 94598985c0..897d0ea047 100644
--- a/src/codegen/c.zig
+++ b/src/codegen/c.zig
@@ -531,6 +531,7 @@ pub const DeclGen = struct {
     /// Keeps track of anonymous decls that need to be rendered before this
     /// (named) Decl in the output C code.
     anon_decl_deps: std.AutoArrayHashMapUnmanaged(InternPool.Index, C.DeclBlock),
+    aligned_anon_decls: std.AutoArrayHashMapUnmanaged(InternPool.Index, Alignment),
 
     fn fail(dg: *DeclGen, comptime format: []const u8, args: anytype) error{ AnalysisFail, OutOfMemory } {
         @setCold(true);
@@ -548,11 +549,12 @@ pub const DeclGen = struct {
         writer: anytype,
         ty: Type,
         ptr_val: Value,
-        decl_val: InternPool.Index,
+        anon_decl: InternPool.Key.Ptr.Addr.AnonDecl,
         location: ValueRenderLocation,
     ) error{ OutOfMemory, AnalysisFail }!void {
         const mod = dg.module;
         const ip = &mod.intern_pool;
+        const decl_val = anon_decl.val;
         const decl_ty = ip.typeOf(decl_val).toType();
 
         // Render an undefined pointer if we have a pointer to a zero-bit or comptime type.
@@ -592,8 +594,23 @@ pub const DeclGen = struct {
 
         // Indicate that the anon decl should be rendered to the output so that
         // our reference above is not undefined.
+        const ptr_type = ip.indexToKey(anon_decl.orig_ty).ptr_type;
         const gop = try dg.anon_decl_deps.getOrPut(dg.gpa, decl_val);
         if (!gop.found_existing) gop.value_ptr.* = .{};
+
+        // Only insert an alignment entry if the alignment is greater than ABI
+        // alignment. If there is already an entry, keep the greater alignment.
+        const explicit_alignment = ptr_type.flags.alignment;
+        if (explicit_alignment != .none) {
+            const abi_alignment = ptr_type.child.toType().abiAlignment(mod);
+            if (explicit_alignment.compareStrict(.gt, abi_alignment)) {
+                const aligned_gop = try dg.aligned_anon_decls.getOrPut(dg.gpa, decl_val);
+                aligned_gop.value_ptr.* = if (aligned_gop.found_existing)
+                    aligned_gop.value_ptr.maxStrict(explicit_alignment)
+                else
+                    explicit_alignment;
+            }
+        }
     }
 
     fn renderDeclValue(
@@ -651,7 +668,7 @@ pub const DeclGen = struct {
         switch (ptr.addr) {
             .decl => |d| try dg.renderDeclValue(writer, ptr_ty, ptr_val.toValue(), d, location),
             .mut_decl => |md| try dg.renderDeclValue(writer, ptr_ty, ptr_val.toValue(), md.decl, location),
-            .anon_decl => |decl_val| try dg.renderAnonDeclValue(writer, ptr_ty, ptr_val.toValue(), decl_val, location),
+            .anon_decl => |anon_decl| try dg.renderAnonDeclValue(writer, ptr_ty, ptr_val.toValue(), anon_decl, location),
             .int => |int| {
                 try writer.writeByte('(');
                 try dg.renderCType(writer, ptr_cty);