[WebAssembly] Elide data segments for .bss sections

Summary:
WebAssembly memories are zero-initialized, so when module does not
import its memory initializing .bss sections is guaranteed to be a
no-op. To reduce binary size and initialization time, .bss sections
are simply not emitted into the final binary unless the memory is
imported.

Reviewers: sbc100

Subscribers: dschuff, jgravelle-google, aheejin, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68965

llvm-svn: 374940

1 files changed, 6 insertions, 0 deletions

diff --git a/lld/wasm/SyntheticSections.cpp b/lld/wasm/SyntheticSections.cpp
index 30c1096e16c..ba04543211c 100644
--- a/lld/wasm/SyntheticSections.cpp
+++ b/lld/wasm/SyntheticSections.cpp
@@ -365,6 +365,12 @@ void ElemSection::writeBody() {
   }
 }
 
+DataCountSection::DataCountSection(ArrayRef<OutputSegment *> segments)
+    : SyntheticSection(llvm::wasm::WASM_SEC_DATACOUNT),
+      numSegments(std::count_if(
+          segments.begin(), segments.end(),
+          [](OutputSegment *const segment) { return !segment->isBss; })) {}
+
 void DataCountSection::writeBody() {
   writeUleb128(bodyOutputStream, numSegments, "data count");
 }