From e70f944e11f6d25d8365363ecd2fdb48a5dbd411 Mon Sep 17 00:00:00 2001 From: Andreas Kling Date: Sat, 26 Nov 2022 19:01:55 +0100 Subject: [PATCH] LibJS: Rearrange ASTNode members so there's a padding hole at the end ASTNode inherits from RefCounted, which has a single 32-bit member. This means that there's a 32-bit padding hole after RefCounted, where we are free to put something (or it will go to waste!) This patch moves ASTNode::m_start_offset into that padding hole, and we now have a 32-bit padding hole at the end of ASTNode instead. This will allow ASTNode subclasses to put things in the ASTNode hole by moving them to the head of the member list. --- Userland/Libraries/LibJS/AST.cpp | 4 ++-- Userland/Libraries/LibJS/AST.h | 4 +++- 2 files changed, 5 insertions(+), 3 deletions(-) diff --git a/Userland/Libraries/LibJS/AST.cpp b/Userland/Libraries/LibJS/AST.cpp index caf127db71..23d7de8836 100644 --- a/Userland/Libraries/LibJS/AST.cpp +++ b/Userland/Libraries/LibJS/AST.cpp @@ -62,8 +62,8 @@ private: }; ASTNode::ASTNode(SourceRange source_range) - : m_source_code(source_range.code) - , m_start_offset(source_range.start.offset) + : m_start_offset(source_range.start.offset) + , m_source_code(source_range.code) , m_end_offset(source_range.end.offset) { } diff --git a/Userland/Libraries/LibJS/AST.h b/Userland/Libraries/LibJS/AST.h index a583f917bd..d757eba77a 100644 --- a/Userland/Libraries/LibJS/AST.h +++ b/Userland/Libraries/LibJS/AST.h @@ -90,8 +90,10 @@ protected: explicit ASTNode(SourceRange); private: - RefPtr m_source_code; + // NOTE: These members are carefully ordered so that `m_start_offset` is packed with the padding after RefCounted::m_ref_count. + // This creates a 4-byte padding hole after `m_end_offset` which is used to pack subclasses better. u32 m_start_offset { 0 }; + RefPtr m_source_code; u32 m_end_offset { 0 }; };