From ba3c1f9ce30cf4f8aee5f1961df74d65e11d53bc Mon Sep 17 00:00:00 2001
From: Yuta Saito <kateinoigakukun@gmail.com>
Date: Wed, 21 Feb 2024 03:35:36 +0900
Subject: [WebAssembly] Add segment RETAIN flag to support private retained
 data (#81539)

In WebAssembly, we have `WASM_SYMBOL_NO_STRIP` symbol flag to mark the
referenced content as retained. However, the flag is not enough to
express retained data that is not referenced by any symbol. This patch
adds a new segment flag`WASM_SEG_FLAG_RETAIN` to support "private"
linkage data that is retained by llvm.used.

This kind of data that is not referenced but must be retained is usually
used with encapsulation symbols (__start/__stop). Swift runtime uses
this technique and depends on the fact "all metadata sections in live
objects are retained", which was not guaranteed with `--gc-sections`
before this patch.

This is a revised version of https://reviews.llvm.org/D126950 (has been
reverted) based on @MaskRay's comments
---
 llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp | 29 +++++++++++++++++------
 1 file changed, 22 insertions(+), 7 deletions(-)

(limited to 'llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp')

diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 3cf59bc..654a615 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -2142,7 +2142,7 @@ static const Comdat *getWasmComdat(const GlobalValue *GV) {
   return C;
 }
 
-static unsigned getWasmSectionFlags(SectionKind K) {
+static unsigned getWasmSectionFlags(SectionKind K, bool Retain) {
   unsigned Flags = 0;
 
   if (K.isThreadLocal())
@@ -2151,11 +2151,22 @@ static unsigned getWasmSectionFlags(SectionKind K) {
   if (K.isMergeableCString())
     Flags |= wasm::WASM_SEG_FLAG_STRINGS;
 
+  if (Retain)
+    Flags |= wasm::WASM_SEG_FLAG_RETAIN;
+
   // TODO(sbc): Add suport for K.isMergeableConst()
 
   return Flags;
 }
 
+void TargetLoweringObjectFileWasm::getModuleMetadata(Module &M) {
+  SmallVector<GlobalValue *, 4> Vec;
+  collectUsedGlobalVariables(M, Vec, false);
+  for (GlobalValue *GV : Vec)
+    if (auto *GO = dyn_cast<GlobalObject>(GV))
+      Used.insert(GO);
+}
+
 MCSection *TargetLoweringObjectFileWasm::getExplicitSectionGlobal(
     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
   // We don't support explict section names for functions in the wasm object
@@ -2179,16 +2190,18 @@ MCSection *TargetLoweringObjectFileWasm::getExplicitSectionGlobal(
     Group = C->getName();
   }
 
-  unsigned Flags = getWasmSectionFlags(Kind);
+  unsigned Flags = getWasmSectionFlags(Kind, Used.count(GO));
   MCSectionWasm *Section = getContext().getWasmSection(
       Name, Kind, Flags, Group, MCContext::GenericSectionID);
 
   return Section;
 }
 
-static MCSectionWasm *selectWasmSectionForGlobal(
-    MCContext &Ctx, const GlobalObject *GO, SectionKind Kind, Mangler &Mang,
-    const TargetMachine &TM, bool EmitUniqueSection, unsigned *NextUniqueID) {
+static MCSectionWasm *
+selectWasmSectionForGlobal(MCContext &Ctx, const GlobalObject *GO,
+                           SectionKind Kind, Mangler &Mang,
+                           const TargetMachine &TM, bool EmitUniqueSection,
+                           unsigned *NextUniqueID, bool Retain) {
   StringRef Group = "";
   if (const Comdat *C = getWasmComdat(GO)) {
     Group = C->getName();
@@ -2213,7 +2226,7 @@ static MCSectionWasm *selectWasmSectionForGlobal(
     (*NextUniqueID)++;
   }
 
-  unsigned Flags = getWasmSectionFlags(Kind);
+  unsigned Flags = getWasmSectionFlags(Kind, Retain);
   return Ctx.getWasmSection(Name, Kind, Flags, Group, UniqueID);
 }
 
@@ -2231,9 +2244,11 @@ MCSection *TargetLoweringObjectFileWasm::SelectSectionForGlobal(
   else
     EmitUniqueSection = TM.getDataSections();
   EmitUniqueSection |= GO->hasComdat();
+  bool Retain = Used.count(GO);
+  EmitUniqueSection |= Retain;
 
   return selectWasmSectionForGlobal(getContext(), GO, Kind, getMangler(), TM,
-                                    EmitUniqueSection, &NextUniqueID);
+                                    EmitUniqueSection, &NextUniqueID, Retain);
 }
 
 bool TargetLoweringObjectFileWasm::shouldPutJumpTableInFunctionSection(
-- 
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