From 6a9a918fbe4adc23dd7d7573c6f1e499f4be074e Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sun, 14 May 2023 20:37:22 -0700
Subject: stage2: encode one-possible-value tuple specially

Anonymous structs and anonymous tuples can be stored via a
only_possible_value tag because their type encodings, by definition,
will have every value specified, which can be used to populate the
fields slice in `Key.Aggregate`.

Also fix `isTupleOrAnonStruct`.
---
 src/InternPool.zig | 44 +++++++++++++++++++++++++++++++++-----------
 1 file changed, 33 insertions(+), 11 deletions(-)

(limited to 'src/InternPool.zig')

diff --git a/src/InternPool.zig b/src/InternPool.zig
index 74cc452176..2435e0ad31 100644
--- a/src/InternPool.zig
+++ b/src/InternPool.zig
@@ -1166,10 +1166,10 @@ pub const Tag = enum(u8) {
     /// Module.Struct object allocated for it.
     /// data is Module.Namespace.Index.
     type_struct_ns,
-    /// An AnonStructType which stores types, names, and values for each field.
+    /// An AnonStructType which stores types, names, and values for fields.
     /// data is extra index of `TypeStructAnon`.
     type_struct_anon,
-    /// An AnonStructType which has only types and values for each field.
+    /// An AnonStructType which has only types and values for fields.
     /// data is extra index of `TypeStructAnon`.
     type_tuple_anon,
     /// A tagged union type.
@@ -1272,7 +1272,8 @@ pub const Tag = enum(u8) {
     /// only one possible value. Not all only-possible-values are encoded this way;
     /// for example structs which have all comptime fields are not encoded this way.
     /// The set of values that are encoded this way is:
-    /// * A struct which has 0 fields.
+    /// * An array or vector which has length 0.
+    /// * A struct which has all fields comptime-known.
     /// data is Index of the type, which is known to be zero bits at runtime.
     only_possible_value,
     /// data is extra index to Key.Union.
@@ -1863,10 +1864,21 @@ pub fn indexToKey(ip: InternPool, index: Index) Key {
         .only_possible_value => {
             const ty = @intToEnum(Index, data);
             return switch (ip.indexToKey(ty)) {
+                // TODO: migrate structs to properly use the InternPool rather
+                // than using the SegmentedList trick, then the struct type will
+                // have a slice of comptime values that can be used here for when
+                // the struct has one possible value due to all fields comptime (same
+                // as the tuple case below).
                 .struct_type => .{ .aggregate = .{
                     .ty = ty,
                     .fields = &.{},
                 } },
+                // There is only one possible value precisely due to the
+                // fact that this values slice is fully populated!
+                .anon_struct_type => |anon_struct_type| .{ .aggregate = .{
+                    .ty = ty,
+                    .fields = anon_struct_type.values,
+                } },
                 else => unreachable,
             };
         },
@@ -2392,12 +2404,6 @@ pub fn get(ip: *InternPool, gpa: Allocator, key: Key) Allocator.Error!Index {
         .aggregate => |aggregate| {
             assert(aggregate.ty != .none);
             for (aggregate.fields) |elem| assert(elem != .none);
-            if (aggregate.fields.len != ip.aggregateTypeLen(aggregate.ty)) {
-                std.debug.print("aggregate fields len = {d}, type len = {d}\n", .{
-                    aggregate.fields.len,
-                    ip.aggregateTypeLen(aggregate.ty),
-                });
-            }
             assert(aggregate.fields.len == ip.aggregateTypeLen(aggregate.ty));
 
             if (aggregate.fields.len == 0) {
@@ -2408,6 +2414,22 @@ pub fn get(ip: *InternPool, gpa: Allocator, key: Key) Allocator.Error!Index {
                 return @intToEnum(Index, ip.items.len - 1);
             }
 
+            switch (ip.indexToKey(aggregate.ty)) {
+                .anon_struct_type => |anon_struct_type| {
+                    if (std.mem.eql(Index, anon_struct_type.values, aggregate.fields)) {
+                        // This encoding works thanks to the fact that, as we just verified,
+                        // the type itself contains a slice of values that can be provided
+                        // in the aggregate fields.
+                        ip.items.appendAssumeCapacity(.{
+                            .tag = .only_possible_value,
+                            .data = @enumToInt(aggregate.ty),
+                        });
+                        return @intToEnum(Index, ip.items.len - 1);
+                    }
+                },
+                else => {},
+            }
+
             try ip.extra.ensureUnusedCapacity(
                 gpa,
                 @typeInfo(Aggregate).Struct.fields.len + aggregate.fields.len,
@@ -3121,8 +3143,8 @@ fn dumpFallible(ip: InternPool, arena: Allocator) anyerror!void {
         }
     };
     counts.sort(SortContext{ .map = &counts });
-    const len = @min(50, counts.count());
-    std.debug.print("  top 50 tags:\n", .{});
+    const len = @min(25, counts.count());
+    std.debug.print("  top 25 tags:\n", .{});
     for (counts.keys()[0..len], counts.values()[0..len]) |tag, stats| {
         std.debug.print("    {s}: {d} occurrences, {d} total bytes\n", .{
             @tagName(tag), stats.count, stats.bytes,
-- 
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