[lld] b0211de - [ELF] Change Symbol::verdefIndex from uint32_t to uint16_t

[Fangrui Song via llvm-commits]

Author: Fangrui Song
Date: 2021-12-15T17:59:30-08:00
New Revision: b0211de5e30a92aef68ceaf43e5042f610d7333f

URL: https://github.com/llvm/llvm-project/commit/b0211de5e30a92aef68ceaf43e5042f610d7333f
DIFF: https://github.com/llvm/llvm-project/commit/b0211de5e30a92aef68ceaf43e5042f610d7333f.diff

LOG: [ELF] Change Symbol::verdefIndex from uint32_t to uint16_t

The SHT_GNU_version index is 16-bit, so the 32-bit value is a waste.
Technically non-default version index 0x7fff uses version index 0xffff,
but it is impossible in practice.

This change decreases sizeof(SymbolUnion) from 80 to 72 on ELF64 platforms.
Memory usage decreases by 1% when linking a large executable.

Added: 
    

Modified: 
    lld/ELF/SymbolTable.cpp
    lld/ELF/Symbols.h

Removed: 
    


################################################################################
diff  --git a/lld/ELF/SymbolTable.cpp b/lld/ELF/SymbolTable.cpp
index 9bb2e7ddaf51..ce77f9233fea 100644
--- a/lld/ELF/SymbolTable.cpp
+++ b/lld/ELF/SymbolTable.cpp
@@ -230,7 +230,7 @@ bool SymbolTable::assignExactVersion(SymbolVersion ver, uint16_t versionId,
 
     // If the version has not been assigned, verdefIndex is -1. Use an arbitrary
     // number (0) to indicate the version has been assigned.
-    if (sym->verdefIndex == UINT32_C(-1)) {
+    if (sym->verdefIndex == uint16_t(-1)) {
       sym->verdefIndex = 0;
       sym->versionId = versionId;
     }
@@ -249,7 +249,7 @@ void SymbolTable::assignWildcardVersion(SymbolVersion ver, uint16_t versionId,
   // so we set a version to a symbol only if no version has been assigned
   // to the symbol. This behavior is compatible with GNU.
   for (Symbol *sym : findAllByVersion(ver, includeNonDefault))
-    if (sym->verdefIndex == UINT32_C(-1)) {
+    if (sym->verdefIndex == uint16_t(-1)) {
       sym->verdefIndex = 0;
       sym->versionId = versionId;
     }

diff  --git a/lld/ELF/Symbols.h b/lld/ELF/Symbols.h
index 5569bd8880c2..b0b7832135a7 100644
--- a/lld/ELF/Symbols.h
+++ b/lld/ELF/Symbols.h
@@ -86,7 +86,7 @@ class Symbol {
   uint32_t globalDynIndex = -1;
 
   // This field is a index to the symbol's version definition.
-  uint32_t verdefIndex = -1;
+  uint16_t verdefIndex = -1;
 
   // Version definition index.
   uint16_t versionId;
@@ -365,7 +365,7 @@ class SharedSymbol : public Symbol {
 
   SharedSymbol(InputFile &file, StringRef name, uint8_t binding,
                uint8_t stOther, uint8_t type, uint64_t value, uint64_t size,
-               uint32_t alignment, uint32_t verdefIndex)
+               uint32_t alignment, uint16_t verdefIndex)
       : Symbol(SharedKind, &file, name, binding, stOther, type), value(value),
         size(size), alignment(alignment) {
     this->verdefIndex = verdefIndex;
@@ -487,9 +487,9 @@ union SymbolUnion {
 };
 
 // It is important to keep the size of SymbolUnion small for performance and
-// memory usage reasons. 80 bytes is a soft limit based on the size of Defined
+// memory usage reasons. 72 bytes is a soft limit based on the size of Defined
 // on a 64-bit system.
-static_assert(sizeof(SymbolUnion) <= 80, "SymbolUnion too large");
+static_assert(sizeof(SymbolUnion) <= 72, "SymbolUnion too large");
 
 template <typename T> struct AssertSymbol {
   static_assert(std::is_trivially_destructible<T>(),