Reland: "[lld][WebAssembly] Initial support merging string data"

This change was originally landed in: 5000a1b4b9edeb9e994f2a5b36da8d48599bea49
It was reverted in: 061e071d8c9b98526f35cad55a918a4f1615afd4

This change adds support for a new WASM_SEG_FLAG_STRINGS flag in
the object format which works in a similar fashion to SHF_STRINGS
in the ELF world.

Unlike the ELF linker this support is currently limited:
- No support for SHF_MERGE (non-string merging)
- Always do full tail merging ("lo" can be merged with "hello")
- Only support single byte strings (p2align 0)

Like the ELF linker merging is only performed at `-O1` and above.

This fixes part of https://bugs.llvm.org/show_bug.cgi?id=48828,
although crucially it doesn't not currently support debug sections
because they are not represented by data segments (they are custom
sections)

Differential Revision: https://reviews.llvm.org/D97657

1 files changed, 5 insertions, 3 deletions

diff --git a/llvm/lib/MC/MCContext.cpp b/llvm/lib/MC/MCContext.cpp
index f834575..6a0b7b9 100644
--- a/llvm/lib/MC/MCContext.cpp
+++ b/llvm/lib/MC/MCContext.cpp
@@ -672,7 +672,8 @@ MCSectionCOFF *MCContext::getAssociativeCOFFSection(MCSectionCOFF *Sec,
 }
 
 MCSectionWasm *MCContext::getWasmSection(const Twine &Section, SectionKind K,
-                                         const Twine &Group, unsigned UniqueID,
+                                         unsigned Flags, const Twine &Group,
+                                         unsigned UniqueID,
                                          const char *BeginSymName) {
   MCSymbolWasm *GroupSym = nullptr;
   if (!Group.isTriviallyEmpty() && !Group.str().empty()) {
@@ -680,10 +681,11 @@ MCSectionWasm *MCContext::getWasmSection(const Twine &Section, SectionKind K,
     GroupSym->setComdat(true);
   }
 
-  return getWasmSection(Section, K, GroupSym, UniqueID, BeginSymName);
+  return getWasmSection(Section, K, Flags, GroupSym, UniqueID, BeginSymName);
 }
 
 MCSectionWasm *MCContext::getWasmSection(const Twine &Section, SectionKind Kind,
+                                         unsigned Flags,
                                          const MCSymbolWasm *GroupSym,
                                          unsigned UniqueID,
                                          const char *BeginSymName) {
@@ -704,7 +706,7 @@ MCSectionWasm *MCContext::getWasmSection(const Twine &Section, SectionKind Kind,
   cast<MCSymbolWasm>(Begin)->setType(wasm::WASM_SYMBOL_TYPE_SECTION);
 
   MCSectionWasm *Result = new (WasmAllocator.Allocate())
-      MCSectionWasm(CachedName, Kind, GroupSym, UniqueID, Begin);
+      MCSectionWasm(CachedName, Kind, Flags, GroupSym, UniqueID, Begin);
   Entry.second = Result;
 
   auto *F = new MCDataFragment();