[lld][WebAssembly] Do not emit relocs against dead symbols (#129346)

When emitting relocs with linked output (i.e. --emit-relocs)
skip relocs against dead symbols (which do not appear in the output)
and do not emit them.

lld/test/wasm/emit-relocs.s

@@ -27,6 +27,12 @@ foo:
   .int32  0
   .size   foo, 4
 
+.section .debug_info,"",@
+.p2align 2
+.int32 unused_function
+.int32 _start
+.int32 0
+
 # CHECK:        - Type:            CODE
 # CHECK-NEXT:     Relocations:
 # CHECK-NEXT:       - Type:            R_WASM_FUNCTION_INDEX_LEB
@@ -42,6 +48,15 @@ foo:
 # CHECK-NEXT:           Value:           1024
 # CHECK-NEXT:         Content:         '00000000'
 
+# There should be a single relocation in this section (just the live symbol)
+# CHECK-NEXT:  - Type:            CUSTOM
+# CHECK-NEXT:    Relocations:
+# CHECK-NEXT:      - Type:            R_WASM_FUNCTION_OFFSET_I32
+# CHECK-NEXT:        Index:           0
+# CHECK-NEXT:        Offset:          0x4
+# CHECK-NEXT:    Name:            .debug_info
+# CHECK-NEXT:    Payload:         FFFFFFFF0200000000000000
+
 # CHECK:        - Type:            CUSTOM
 # CHECK-NEXT:     Name:            linking
 # CHECK-NEXT:     Version:         2

lld/wasm/InputChunks.cpp

@@ -14,6 +14,7 @@
 #include "lld/Common/LLVM.h"
 #include "llvm/Support/LEB128.h"
 #include "llvm/Support/xxhash.h"
+#include <algorithm>
 
 #define DEBUG_TYPE "lld"
 
@@ -167,6 +168,17 @@ void InputChunk::relocate(uint8_t *buf) const {
   }
 }
 
+static bool relocIsLive(const WasmRelocation &rel, ObjFile *file) {
+  return rel.Type == R_WASM_TYPE_INDEX_LEB ||
+         file->getSymbol(rel.Index)->isLive();
+}
+
+size_t InputChunk::getNumLiveRelocations() const {
+  return std::count_if(
+      relocations.begin(), relocations.end(),
+      [this](const WasmRelocation &rel) { return relocIsLive(rel, file); });
+}
+
 // Copy relocation entries to a given output stream.
 // This function is used only when a user passes "-r". For a regular link,
 // we consume relocations instead of copying them to an output file.
@@ -179,6 +191,8 @@ void InputChunk::writeRelocations(raw_ostream &os) const {
                     << " offset=" << Twine(off) << "\n");
 
   for (const WasmRelocation &rel : relocations) {
+    if (!relocIsLive(rel, file))
+      continue;
     writeUleb128(os, rel.Type, "reloc type");
     writeUleb128(os, rel.Offset + off, "reloc offset");
     writeUleb128(os, file->calcNewIndex(rel), "reloc index");

lld/wasm/InputChunks.h

@@ -77,6 +77,7 @@ public:
   uint32_t getInputSectionOffset() const { return inputSectionOffset; }
 
   size_t getNumRelocations() const { return relocations.size(); }
+  size_t getNumLiveRelocations() const;
   void writeRelocations(llvm::raw_ostream &os) const;
   bool generateRelocationCode(raw_ostream &os) const;
 

lld/wasm/OutputSections.cpp

@@ -274,7 +274,7 @@ void CustomSection::writeTo(uint8_t *buf) {
 uint32_t CustomSection::getNumRelocations() const {
   uint32_t count = 0;
   for (const InputChunk *inputSect : inputSections)
-    count += inputSect->getNumRelocations();
+    count += inputSect->getNumLiveRelocations();
   return count;
 }
 

lld/wasm/OutputSections.h

@@ -41,6 +41,7 @@ public:
   virtual void writeTo(uint8_t *buf) = 0;
   virtual void finalizeContents() = 0;
   virtual uint32_t getNumRelocations() const { return 0; }
+  virtual uint32_t getNumLiveRelocations() const { return getNumRelocations(); }
   virtual void writeRelocations(raw_ostream &os) const {}
 
   std::string header;

lld/wasm/Symbols.cpp

@@ -183,7 +183,7 @@ void Symbol::markLive() {
 }
 
 uint32_t Symbol::getOutputSymbolIndex() const {
-  assert(outputSymbolIndex != INVALID_INDEX);
+  assert(outputSymbolIndex != INVALID_INDEX || !isLive());
   return outputSymbolIndex;
 }