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, 4 insertions, 2 deletions

diff --git a/llvm/lib/MC/MCSectionWasm.cpp b/llvm/lib/MC/MCSectionWasm.cpp
index 81dc432..9faa4ca 100644
--- a/llvm/lib/MC/MCSectionWasm.cpp
+++ b/llvm/lib/MC/MCSectionWasm.cpp
@@ -64,9 +64,11 @@ void MCSectionWasm::PrintSwitchToSection(const MCAsmInfo &MAI, const Triple &T,
   OS << ",\"";
 
   if (IsPassive)
-    OS << "p";
+    OS << 'p';
   if (Group)
-    OS << "G";
+    OS << 'G';
+  if (SegmentFlags & wasm::WASM_SEG_FLAG_STRINGS)
+    OS << 'S';
 
   OS << '"';