Bug 1305360 - Part 1: Add a mechanism to allow users to opt out of protection for small buffers. r=jandem

js/src/ds/PageProtectingVector.h

@@ -58,6 +58,13 @@ class PageProtectingVector final
      */
     intptr_t unprotectedBytes;
 
+    /*
+     * The size in bytes that a buffer needs to be before its pages will be
+     * protected. This is intended to reduce churn for small vectors while
+     * still offering protection when they grow large enough.
+     */
+    size_t protectionLowerBound;
+
     bool protectionEnabled;
     bool regionUnprotected;
 
@@ -65,6 +72,8 @@ class PageProtectingVector final
         unprotectedBytes += offsetToPage;
         offsetToPage = (pageSize - (uintptr_t(vector.begin()) & pageMask)) & pageMask;
         unprotectedBytes -= offsetToPage;
+        protectionEnabled = vector.capacity() >= protectionLowerBound &&
+                            vector.capacity() >= pageSize + offsetToPage;
     }
 
     void protect() {
@@ -94,6 +103,10 @@ class PageProtectingVector final
         protect();
     }
 
+    void unprotectOldBuffer() {
+        unprotect();
+    }
+
     bool anyProtected(size_t first, size_t last) {
         return last >= offsetToPage && first < offsetToPage + protectedBytes;
     }
@@ -125,7 +138,7 @@ class PageProtectingVector final
 
         void emplace(PageProtectingVector* holder) {
             vector = holder;
-            vector->unprotect();
+            vector->unprotectOldBuffer();
         }
 
         explicit AutoUnprotect(PageProtectingVector* holder) {
@@ -146,23 +159,22 @@ class PageProtectingVector final
         offsetToPage(0),
         protectedBytes(0),
         unprotectedBytes(0),
+        protectionLowerBound(0),
         protectionEnabled(false),
-        regionUnprotected(false) { updateOffsetToPage(); }
+        regionUnprotected(false) { protectNewBuffer(); }
 
-    ~PageProtectingVector() { unprotect(); }
+    ~PageProtectingVector() { unprotectOldBuffer(); }
 
-    /* Enable protection for the entire buffer. */
-    void enableProtection() {
-        MOZ_ASSERT(!protectionEnabled);
-        protectionEnabled = true;
-        protectNewBuffer();
-    }
-
-    /* Disable protection for the entire buffer. */
-    void disableProtection() {
-        MOZ_ASSERT(protectionEnabled);
-        unprotect();
-        protectionEnabled = false;
+    /*
+     * Sets the lower bound on the size, in bytes, that this vector's underlying
+     * capacity has to be before its used pages will be protected.
+     */
+    void setLowerBoundForProtection(size_t bytes) {
+        if (protectionLowerBound != bytes) {
+            unprotectOldBuffer();
+            protectionLowerBound = bytes;
+            protectNewBuffer();
+        }
     }
 
     /*

js/src/jit/x86-shared/AssemblerBuffer-x86-shared.h

@@ -87,6 +87,8 @@ namespace jit {
         AssemblerBuffer()
             : m_oom(false)
         {
+            // Provide memory protection once the buffer starts to get big.
+            m_buffer.setLowerBoundForProtection(32 * 1024);
         }
 
         void ensureSpace(size_t space)
@@ -139,9 +141,6 @@ namespace jit {
             return m_buffer.begin();
         }
 
-        void enableBufferProtection() { m_buffer.enableProtection(); }
-        void disableBufferProtection() { m_buffer.disableProtection(); }
-
         void unprotectDataRegion(size_t firstByteOffset, size_t lastByteOffset) {
             m_buffer.unprotectRegion(firstByteOffset, lastByteOffset);
         }

js/src/jit/x86-shared/BaseAssembler-x86-shared.h

@@ -3851,9 +3851,6 @@ threeByteOpImmSimd("vblendps", VEX_PD, OP3_BLENDPS_VpsWpsIb, ESCAPE_3A, imm, off
         return m_formatter.append(other.m_formatter.buffer(), other.size());
     }
 
-    void enableBufferProtection() { m_formatter.enableBufferProtection(); }
-    void disableBufferProtection() { m_formatter.disableBufferProtection(); }
-
     void unprotectDataRegion(size_t firstByteOffset, size_t lastByteOffset) {
         m_formatter.unprotectDataRegion(firstByteOffset, lastByteOffset);
     }
@@ -5132,9 +5129,6 @@ threeByteOpImmSimd("vblendps", VEX_PD, OP3_BLENDPS_VpsWpsIb, ESCAPE_3A, imm, off
             return m_buffer.append(values, size);
         }
 
-        void enableBufferProtection() { m_buffer.enableBufferProtection(); }
-        void disableBufferProtection() { m_buffer.disableBufferProtection(); }
-
         void unprotectDataRegion(size_t firstByteOffset, size_t lastByteOffset) {
             m_buffer.unprotectDataRegion(firstByteOffset, lastByteOffset);
         }