revert "compiler: packed structs cache bit offsets"

This is mostly a revert of a7088fd9a3edb037f0f51bb402a3c557334634f3.
Measurement revealed the commit actually regressed performance.

1 files changed, 10 insertions, 2 deletions

diff --git a/src/type.zig b/src/type.zig
index 83a89b527b..e227487444 100644
--- a/src/type.zig
+++ b/src/type.zig
@@ -3025,8 +3025,16 @@ pub const Type = struct {
         const ip = &mod.intern_pool;
         const struct_type = ip.indexToKey(ty.toIntern()).struct_type;
         assert(struct_type.layout == .Packed);
-        assert(struct_type.haveLayout(ip));
-        return struct_type.offsets.get(ip)[field_index];
+        comptime assert(Type.packed_struct_layout_version == 2);
+
+        var running_bits: u32 = 0;
+        for (struct_type.field_types.get(ip), 0..) |field_ty, i| {
+            if (i == field_index) break;
+            if (!field_ty.toType().hasRuntimeBits(mod)) continue;
+            const field_bits: u32 = @intCast(field_ty.toType().bitSize(mod));
+            running_bits += field_bits;
+        }
+        return running_bits;
     }
 
     pub fn packedStructFieldByteOffset(ty: Type, field_index: usize, mod: *Module) u32 {