commit 21143b93a6ee1121b96ee7f4a75ee542580ba57c
author Thomas Lively <tlively@google.com> Thu Sep 19 01:14:59 2019 +0000
committer Thomas Lively <tlively@google.com> Thu Sep 19 01:14:59 2019 +0000
tree c8b1acb01f87d645a6087338ce9276797be60bc6
parent 98a57332ef0e4b859e74a3a43f7f31e9ff1683e0

[WebAssembly] Sort output data sections to place .bss last

Summary:
This was always the intended behavior, but had not been
implemented. This ordering is important for Emscripten when generating
.mem files while compiling to JS, since only zeros at the end of
initialized memory can be dropped.

Fixes https://github.com/emscripten-core/emscripten/issues/8999

Reviewers: sbc100

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67736

llvm-svn: 372284

lld/wasm/Writer.cpp

diff --git a/lld/wasm/Writer.cpp b/lld/wasm/Writer.cpp
index 58d585b..c4a87be 100644
--- a/lld/wasm/Writer.cpp
+++ b/lld/wasm/Writer.cpp
@@ -263,7 +263,6 @@
     memoryPtr += 4;
   }
 
-  // TODO: Add .bss space here.
   if (WasmSym::dataEnd)
     WasmSym::dataEnd->setVirtualAddress(memoryPtr);
 
@@ -667,7 +666,7 @@
       OutputSegment *&s = segmentMap[name];
       if (s == nullptr) {
         LLVM_DEBUG(dbgs() << "new segment: " << name << "\n");
-        s = make<OutputSegment>(name, segments.size());
+        s = make<OutputSegment>(name);
         if (config->sharedMemory || name == ".tdata")
           s->initFlags = WASM_SEGMENT_IS_PASSIVE;
         segments.push_back(s);
@@ -676,6 +675,24 @@
       LLVM_DEBUG(dbgs() << "added data: " << name << ": " << s->size << "\n");
     }
   }
+
+  // Sort segments by type, placing .bss last
+  std::stable_sort(segments.begin(), segments.end(),
+                   [](const OutputSegment *a, const OutputSegment *b) {
+                     auto order = [](StringRef name) {
+                       return StringSwitch<int>(name)
+                           .StartsWith(".rodata", 0)
+                           .StartsWith(".data", 1)
+                           .StartsWith(".tdata", 2)
+                           .StartsWith(".bss", 4)
+                           .Default(3);
+                     };
+                     return order(a->name) < order(b->name);
+                   });
+
+  for (size_t i = 0; i < segments.size(); ++i) {
+    segments[i]->index = i;
+  }
 }
 
 static void createFunction(DefinedFunction *func, StringRef bodyContent) {