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, 3 insertions, 3 deletions

diff --git a/llvm/lib/MC/WasmObjectWriter.cpp b/llvm/lib/MC/WasmObjectWriter.cpp
index c0faf85..541b464 100644
--- a/llvm/lib/MC/WasmObjectWriter.cpp
+++ b/llvm/lib/MC/WasmObjectWriter.cpp
@@ -67,7 +67,7 @@ struct WasmDataSegment {
   uint32_t InitFlags;
   uint64_t Offset;
   uint32_t Alignment;
-  uint32_t LinkerFlags;
+  uint32_t LinkingFlags;
   SmallVector<char, 4> Data;
 };
 
@@ -1133,7 +1133,7 @@ void WasmObjectWriter::writeLinkingMetaDataSection(
     for (const WasmDataSegment &Segment : DataSegments) {
       writeString(Segment.Name);
       encodeULEB128(Segment.Alignment, W->OS);
-      encodeULEB128(Segment.LinkerFlags, W->OS);
+      encodeULEB128(Segment.LinkingFlags, W->OS);
     }
     endSection(SubSection);
   }
@@ -1440,7 +1440,7 @@ uint64_t WasmObjectWriter::writeOneObject(MCAssembler &Asm,
       Segment.Section = &Section;
       addData(Segment.Data, Section);
       Segment.Alignment = Log2_32(Section.getAlignment());
-      Segment.LinkerFlags = 0;
+      Segment.LinkingFlags = Section.getSegmentFlags();
       DataSize += Segment.Data.size();
       Section.setSegmentIndex(SegmentIndex);