From 528072c39363668b528b2bcfaaa6891b92caa8fa Mon Sep 17 00:00:00 2001
From: wolfbeast <mcwerewolf@wolfbeast.com>
Date: Tue, 28 May 2019 18:03:08 +0200
Subject: Improve efficiency of (C++) heap allocations related to
 BytecodeEmitter::code.

While there, also add some sanity checks and clean up code.
---
 js/src/frontend/BytecodeEmitter.cpp | 28 ++++++++++++++++++----------
 js/src/frontend/BytecodeEmitter.h   | 10 ++++++----
 2 files changed, 24 insertions(+), 14 deletions(-)

diff --git a/js/src/frontend/BytecodeEmitter.cpp b/js/src/frontend/BytecodeEmitter.cpp
index b3dd6d777..c524184d6 100644
--- a/js/src/frontend/BytecodeEmitter.cpp
+++ b/js/src/frontend/BytecodeEmitter.cpp
@@ -2260,12 +2260,14 @@ BytecodeEmitter::locationOfNameBoundInFunctionScope(JSAtom* name, EmitterScope*
 bool
 BytecodeEmitter::emitCheck(ptrdiff_t delta, ptrdiff_t* offset)
 {
-    *offset = code().length();
+    size_t oldLength = code().length();
+    *offset = ptrdiff_t(oldLength);
 
-    // Start it off moderately large to avoid repeated resizings early on.
-    // ~98% of cases fit within 1024 bytes.
-    if (code().capacity() == 0 && !code().reserve(1024))
-        return false;
+    size_t newLength = oldLength + size_t(delta);
+    if (MOZ_UNLIKELY(newLength > MaxBytecodeLength)) {
+      ReportAllocationOverflow(cx);
+      return false;
+    }
 
     if (!code().growBy(delta)) {
         ReportOutOfMemory(cx);
@@ -10697,17 +10699,19 @@ BytecodeEmitter::emitTreeInBranch(ParseNode* pn)
 static bool
 AllocSrcNote(ExclusiveContext* cx, SrcNotesVector& notes, unsigned* index)
 {
-    // Start it off moderately large to avoid repeated resizings early on.
-    // ~99% of cases fit within 256 bytes.
-    if (notes.capacity() == 0 && !notes.reserve(256))
-        return false;
+    size_t oldLength = notes.length();
 
+    if (MOZ_UNLIKELY(oldLength + 1 > MaxSrcNotesLength)) {
+        ReportAllocationOverflow(cx);
+        return false;
+    }
+    
     if (!notes.growBy(1)) {
         ReportOutOfMemory(cx);
         return false;
     }
 
-    *index = notes.length() - 1;
+    *index = oldLength;
     return true;
 }
 
@@ -10833,6 +10837,10 @@ BytecodeEmitter::setSrcNoteOffset(unsigned index, unsigned which, ptrdiff_t offs
         /* Maybe this offset was already set to a four-byte value. */
         if (!(*sn & SN_4BYTE_OFFSET_FLAG)) {
             /* Insert three dummy bytes that will be overwritten shortly. */
+            if (MOZ_UNLIKELY(notes.length() + 3 > MaxSrcNotesLength)) {
+                ReportAllocationOverflow(cx);
+                return false;
+            }
             jssrcnote dummy = 0;
             if (!(sn = notes.insert(sn, dummy)) ||
                 !(sn = notes.insert(sn, dummy)) ||
diff --git a/js/src/frontend/BytecodeEmitter.h b/js/src/frontend/BytecodeEmitter.h
index 32668a34c..814fa11d4 100644
--- a/js/src/frontend/BytecodeEmitter.h
+++ b/js/src/frontend/BytecodeEmitter.h
@@ -109,10 +109,12 @@ struct CGYieldOffsetList {
     void finish(YieldOffsetArray& array, uint32_t prologueLength);
 };
 
-// Use zero inline elements because these go on the stack and affect how many
-// nested functions are possible.
-typedef Vector<jsbytecode, 0> BytecodeVector;
-typedef Vector<jssrcnote, 0> SrcNotesVector;
+static size_t MaxBytecodeLength = INT32_MAX;
+static size_t MaxSrcNotesLength = INT32_MAX;
+
+// Have a few inline elements to avoid heap allocation for tiny sequences.
+typedef Vector<jsbytecode, 256> BytecodeVector;
+typedef Vector<jssrcnote, 64> SrcNotesVector;
 
 // Linked list of jump instructions that need to be patched. The linked list is
 // stored in the bytes of the incomplete bytecode that will be patched, so no
-- 
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