Bug 906964 - ARM: leave some head-room in the double pools to help avoid bailing out which causes compilation failure for asm.js code. r=mjrosenb

js/src/jit/arm/Assembler-arm.h

@@ -1303,12 +1303,17 @@ class Assembler
     void initWithAllocator() {
         m_buffer.initWithAllocator();
 
+        // Note that the sizes for the double pools are set to 1020 rather than 1024 to
+        // work around a rare edge case that would otherwise bail out - which is not
+        // possible for Asm.js code and causes a compilation failure.  See the comment at
+        // the fail_bail call within IonAssemberBufferWithConstantPools.h: finishPool().
+
         // Set up the backwards double region
-        new (&pools_[2]) Pool (1024, 8, 4, 8, 8, m_buffer.LifoAlloc_, true);
+        new (&pools_[2]) Pool (1020, 8, 4, 8, 8, m_buffer.LifoAlloc_, true);
         // Set up the backwards 32 bit region
         new (&pools_[3]) Pool (4096, 4, 4, 8, 4, m_buffer.LifoAlloc_, true, true);
         // Set up the forwards double region
-        new (doublePool) Pool (1024, 8, 4, 8, 8, m_buffer.LifoAlloc_, false, false, &pools_[2]);
+        new (doublePool) Pool (1020, 8, 4, 8, 8, m_buffer.LifoAlloc_, false, false, &pools_[2]);
         // Set up the forwards 32 bit region
         new (int32Pool) Pool (4096, 4, 4, 8, 4, m_buffer.LifoAlloc_, false, true, &pools_[3]);
         for (int i = 0; i < 4; i++) {