[PATCH] D41390: [WebAssembly] LLD: Don't write out discarded function symbols

[Nicholas Wilson via Phabricator via llvm-commits]

ncw updated this revision to Diff 129431.
ncw edited the summary of this revision.
ncw added a comment.

Updated to be on top of https://reviews.llvm.org/D40845, and removed non-quite-correct handling of globals/segments.


Repository:
  rLLD LLVM Linker

https://reviews.llvm.org/D41390

Files:
  test/wasm/weak-symbols.ll
  wasm/InputFiles.cpp


Index: wasm/InputFiles.cpp
===================================================================
--- wasm/InputFiles.cpp
+++ wasm/InputFiles.cpp
@@ -14,6 +14,7 @@
 #include "SymbolTable.h"
 #include "lld/Common/ErrorHandler.h"
 #include "lld/Common/Memory.h"
+#include "llvm/ADT/SmallBitVector.h"
 #include "llvm/Object/Binary.h"
 #include "llvm/Object/Wasm.h"
 #include "llvm/Support/raw_ostream.h"
@@ -193,6 +194,7 @@
 
   // Populate `FunctionSymbols` and `GlobalSymbols` based on the WasmSymbols
   // in the object
+  SmallBitVector ReachedFunctions(Funcs.size(), false);
   for (const SymbolRef &Sym : WasmObj->symbols()) {
     const WasmSymbol &WasmSym = WasmObj->getWasmSymbol(Sym.getRawDataRefImpl());
     Symbol *S;
@@ -202,6 +204,8 @@
       if (!isExcludedByComdat(Function)) {
         S = createDefined(WasmSym, Symbol::Kind::DefinedFunctionKind, nullptr,
                           Function);
+        if (S->getFile() == this)
+          ReachedFunctions[WasmSym.ElementIndex - NumFunctionImports] = true;
         break;
       } else {
         Function->Discarded = true;
@@ -252,6 +256,16 @@
         for (size_t I = 0; I < GlobalSymbols.size(); ++I)
             assert(GlobalSymbols[I] != nullptr););
 
+  // This is a crude precursor to garbage collection, but it works really nicely
+  // for discarding multiple copies of weak symbols.  If the function isn't
+  // reachable by any Symbol (because another file has defined the weak symbol),
+  // then it's discardable.  We aren't considering here whether the Symbol is
+  // actually ever called.
+  for (uint32_t I = 0; I < Funcs.size(); ++I) {
+    if (!ReachedFunctions[I])
+      Functions[I]->Discarded = true;
+  }
+
   // Populate `TableSymbols` with all symbols that are called indirectly
   uint32_t SegmentCount = WasmObj->elements().size();
   if (SegmentCount) {
Index: test/wasm/weak-symbols.ll
===================================================================
--- test/wasm/weak-symbols.ll
+++ test/wasm/weak-symbols.ll
@@ -24,7 +24,7 @@
 ; CHECK-NEXT:         ReturnType:      I32
 ; CHECK-NEXT:         ParamTypes:
 ; CHECK-NEXT:   - Type:            FUNCTION
-; CHECK-NEXT:     FunctionTypes:   [ 0, 0, 0, 0, 0 ]
+; CHECK-NEXT:     FunctionTypes:   [ 0, 0, 0, 0 ]
 ; CHECK-NEXT:   - Type:            TABLE
 ; CHECK-NEXT:     Tables:
 ; CHECK-NEXT:       - ElemType:        ANYFUNC
@@ -59,7 +59,7 @@
 ; CHECK-NEXT:         Index:           2
 ; CHECK-NEXT:       - Name:            exportWeak2
 ; CHECK-NEXT:         Kind:            FUNCTION
-; CHECK-NEXT:         Index:           4
+; CHECK-NEXT:         Index:           3
 ; CHECK-NEXT:   - Type:            ELEM
 ; CHECK-NEXT:     Segments:
 ; CHECK-NEXT:       - Offset:
@@ -79,9 +79,6 @@
 ; CHECK-NEXT:         Body:            4181808080000B
 ; CHECK-NEXT:       - Index:           3
 ; CHECK-NEXT:         Locals:
-; CHECK-NEXT:         Body:            41020B
-; CHECK-NEXT:       - Index:           4
-; CHECK-NEXT:         Locals:
 ; CHECK-NEXT:         Body:            4181808080000B
 ; CHECK-NEXT:   - Type:            DATA
 ; CHECK-NEXT:     Segments:
@@ -101,6 +98,6 @@
 ; CHECK-NEXT:         Name:            _start
 ; CHECK-NEXT:       - Index:           2
 ; CHECK-NEXT:         Name:            exportWeak1
-; CHECK-NEXT:       - Index:           4
+; CHECK-NEXT:       - Index:           3
 ; CHECK-NEXT:         Name:            exportWeak2
 ; CHECK-NEXT: ...


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