From 662835a88f6f47d5b1451b453046ae80b4b0fc62 Mon Sep 17 00:00:00 2001
From: Alon Zakai <azakai@google.com>
Date: Thu, 25 Jan 2024 13:23:45 -0800
Subject: RemoveUnusedModuleElements: Do not remove unused-but-trapping
 segments (#6242)

An out of bounds active segment traps during startup, which is an effect we must
preserve.

To avoid a regression here, ignore this in TNH mode (where the user assures us
nothing will trap), and also check if a segment will trivially be in bounds and not
trap (if so, it can be removed).

Fixes the remove-unused-module-elements part of #6230

The small change to an existing testcase made a segment there be in bounds,
to avoid this affecting it. Tests for this are in a new file.
---
 src/passes/RemoveUnusedModuleElements.cpp | 53 +++++++++++++++++++++++++++----
 1 file changed, 47 insertions(+), 6 deletions(-)

(limited to 'src')

diff --git a/src/passes/RemoveUnusedModuleElements.cpp b/src/passes/RemoveUnusedModuleElements.cpp
index 6ceab0132..b3874b8dc 100644
--- a/src/passes/RemoveUnusedModuleElements.cpp
+++ b/src/passes/RemoveUnusedModuleElements.cpp
@@ -45,6 +45,7 @@
 #include "ir/subtypes.h"
 #include "ir/utils.h"
 #include "pass.h"
+#include "support/stdckdint.h"
 #include "wasm-builder.h"
 #include "wasm.h"
 
@@ -635,17 +636,57 @@ struct RemoveUnusedModuleElements : public Pass {
     // Active segments that write to imported tables and memories are roots
     // because those writes are externally observable even if the module does
     // not otherwise use the tables or memories.
+    //
+    // Likewise, if traps are possible during startup then just trapping is an
+    // effect (which can happen if the offset is out of bounds).
+    auto maybeRootSegment = [&](ModuleElementKind kind,
+                                Name segmentName,
+                                Index segmentSize,
+                                Expression* offset,
+                                Importable* parent,
+                                Index parentSize) {
+      auto writesToVisible = parent->imported() && segmentSize;
+      auto mayTrap = false;
+      if (!getPassOptions().trapsNeverHappen) {
+        // Check if this might trap. If it is obviously in bounds then it
+        // cannot.
+        auto* c = offset->dynCast<Const>();
+        // Check for overflow in the largest possible space of addresses.
+        using AddressType = Address::address64_t;
+        AddressType maxWritten;
+        // If there is no integer, or if there is and the addition overflows, or
+        // if the addition leads to a too-large value, then we may trap.
+        mayTrap = !c ||
+                  std::ckd_add(&maxWritten,
+                               (AddressType)segmentSize,
+                               (AddressType)c->value.getInteger()) ||
+                  maxWritten > parentSize;
+      }
+      if (writesToVisible || mayTrap) {
+        roots.emplace_back(kind, segmentName);
+      }
+    };
     ModuleUtils::iterActiveDataSegments(*module, [&](DataSegment* segment) {
-      if (module->getMemory(segment->memory)->imported() &&
-          !segment->data.empty()) {
-        roots.emplace_back(ModuleElementKind::DataSegment, segment->name);
+      if (segment->memory.is()) {
+        auto* memory = module->getMemory(segment->memory);
+        maybeRootSegment(ModuleElementKind::DataSegment,
+                         segment->name,
+                         segment->data.size(),
+                         segment->offset,
+                         memory,
+                         memory->initial * Memory::kPageSize);
       }
     });
     ModuleUtils::iterActiveElementSegments(
       *module, [&](ElementSegment* segment) {
-        if (module->getTable(segment->table)->imported() &&
-            !segment->data.empty()) {
-          roots.emplace_back(ModuleElementKind::ElementSegment, segment->name);
+        if (segment->table.is()) {
+          auto* table = module->getTable(segment->table);
+          maybeRootSegment(ModuleElementKind::ElementSegment,
+                           segment->name,
+                           segment->data.size(),
+                           segment->offset,
+                           table,
+                           table->initial * Table::kPageSize);
         }
       });
 
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