From a7be271b6371c5968b77b4227bd3159db558951f Mon Sep 17 00:00:00 2001
From: Alon Zakai <azakai@google.com>
Date: Tue, 14 Jan 2020 08:51:18 -0800
Subject: Omit DWARF debug line ranges starting with 0 (#2587)

Check if an entry starts a new range of addresses. Each range is a set of
related addresses, where in particular, if the first has been zeroed out
by the linker, we must omit the entire range. If we do not, then the
initial range is 0 and the others are offsets relative to it, which will
look like random addresses, perhaps into the middle of instructions, and
perhaps that happen to collide with real ones (a debugger would ignore
those, so we must too; it's easier and better to simply omit them).

See https://bugs.llvm.org/show_bug.cgi?id=44516#c2
---
 src/wasm/wasm-debug.cpp | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

(limited to 'src')

diff --git a/src/wasm/wasm-debug.cpp b/src/wasm/wasm-debug.cpp
index e324d2404..41654f7de 100644
--- a/src/wasm/wasm-debug.cpp
+++ b/src/wasm/wasm-debug.cpp
@@ -225,6 +225,17 @@ struct LineState {
     return false;
   }
 
+  // Checks if this starts a new range of addresses. Each range is a set of
+  // related addresses, where in particular, if the first has been zeroed out
+  // by the linker, we must omit the entire range. (If we do not, then the
+  // initial range is 0 and the others are offsets relative to it, which will
+  // look like random addresses, perhaps into the middle of instructions, and
+  // perhaps that happen to collide with real ones.)
+  bool startsNewRange(llvm::DWARFYAML::LineTableOpcode& opcode) {
+    return opcode.Opcode == 0 &&
+           opcode.SubOpcode == llvm::dwarf::DW_LNE_set_address;
+  }
+
   bool needToEmit() {
     // If any value is 0, can ignore it
     // https://github.com/WebAssembly/debugging/issues/9#issuecomment-567720872
@@ -374,9 +385,23 @@ static void updateDebugLines(const Module& wasm,
     // All the addresses we need to write out.
     std::vector<uint32_t> newAddrs;
     std::unordered_map<uint32_t, LineState> newAddrInfo;
+    // If the address was zeroed out, we must omit the entire range (we could
+    // also leave it unchanged, so that the debugger ignores it based on the
+    // initial zero; but it's easier and better to just not emit it at all).
+    bool omittingRange = false;
     for (auto& opcode : table.Opcodes) {
       // Update the state, and check if we have a new row to emit.
+      if (state.startsNewRange(opcode)) {
+        omittingRange = false;
+      }
       if (state.update(opcode, table)) {
+        if (state.addr == 0) {
+          omittingRange = true;
+        }
+        if (omittingRange) {
+          state = LineState(table);
+          continue;
+        }
         // An expression may not exist for this line table item, if we optimized
         // it away.
         if (auto* expr = oldAddrMap.get(state.addr)) {
-- 
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