[LTO][MC] Discard non-prevailing defined symbols in module-level assembly

This is the alternative approach to D96931.

In LTO, for each module with inlineasm block, prepend directive ".lto_discard <sym>, <sym>*" to the beginning of the inline
asm.  ".lto_discard" is both a module inlineasm block marker and (optionally) provides a list of symbols to be discarded.

In MC while emitting for inlineasm, discard symbol binding & symbol
definitions according to ".lto_disard".

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D98762

1 files changed, 29 insertions, 1 deletions

diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index 8bcb160..3cd8c78 100644
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -11,7 +11,9 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/LTO/LTO.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/StringExtras.h"
 #include "llvm/Analysis/OptimizationRemarkEmitter.h"
 #include "llvm/Analysis/StackSafetyAnalysis.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
@@ -752,6 +754,7 @@ LTO::addRegularLTO(BitcodeModule BM, ArrayRef<InputFile::Symbol> Syms,
   Skip();
 
   std::set<const Comdat *> NonPrevailingComdats;
+  SmallSet<StringRef, 2> NonPrevailingAsmSymbols;
   for (const InputFile::Symbol &Sym : Syms) {
     assert(ResI != ResE);
     SymbolResolution Res = *ResI++;
@@ -798,7 +801,14 @@ LTO::addRegularLTO(BitcodeModule BM, ArrayRef<InputFile::Symbol> Syms,
           GV->setDLLStorageClass(GlobalValue::DLLStorageClassTypes::
                                  DefaultStorageClass);
       }
+    } else if (auto *AS = Msym.dyn_cast<ModuleSymbolTable::AsmSymbol *>()) {
+      // Collect non-prevailing symbols.
+      if (!Res.Prevailing)
+        NonPrevailingAsmSymbols.insert(AS->first);
+    } else {
+      llvm_unreachable("unknown symbol type");
     }
+
     // Common resolution: collect the maximum size/alignment over all commons.
     // We also record if we see an instance of a common as prevailing, so that
     // if none is prevailing we can ignore it later.
@@ -812,11 +822,29 @@ LTO::addRegularLTO(BitcodeModule BM, ArrayRef<InputFile::Symbol> Syms,
         CommonRes.Align = max(*SymAlign, CommonRes.Align);
       CommonRes.Prevailing |= Res.Prevailing;
     }
-
   }
+
   if (!M.getComdatSymbolTable().empty())
     for (GlobalValue &GV : M.global_values())
       handleNonPrevailingComdat(GV, NonPrevailingComdats);
+
+  // Prepend ".lto_discard <sym>, <sym>*" directive to each module inline asm
+  // block.
+  if (!M.getModuleInlineAsm().empty()) {
+    std::string NewIA = ".lto_discard";
+    if (!NonPrevailingAsmSymbols.empty()) {
+      // Don't dicard a symbol if there is a live .symver for it.
+      ModuleSymbolTable::CollectAsmSymvers(
+          M, [&](StringRef Name, StringRef Alias) {
+            if (!NonPrevailingAsmSymbols.count(Alias))
+              NonPrevailingAsmSymbols.erase(Name);
+          });
+      NewIA += " " + llvm::join(NonPrevailingAsmSymbols, ", ");
+    }
+    NewIA += "\n";
+    M.setModuleInlineAsm(NewIA + M.getModuleInlineAsm());
+  }
+
   assert(MsymI == MsymE);
   return std::move(Mod);
 }