On x86, assert on unbound assembler Labels used as branch targets

Originally reported by Jim Roskind & Wade Hennesey of Amazon. Analysis and patch provided by XIn Liu (xxinliu@amazon.com).

The Label class instances (used to define pseudo-assembly code) can be abused in both the C1 and Interpreter. The most common abuse is being "branched to" but never defined as a location in code via bind(). Adding an assert to catch these caused 106 jtreg/hotspot and 17 jtreg/jdk test failures.

All were caused by the new assertion in the template interpreter for both x86_32 and x86_64. The label backedge_counter_overflow was not bound when UseLoopCounter was True but UseOnStackReplacement was False.

A short reproducer is to invoke the debug version of java containing the new assertion like this:

java -XX:-UseOnStackReplacement

Here is our jdk8u x64 patch. The bug exists in jdk tip too.

diff --git a/src/hotspot/src/cpu/x86/vm/templateTable_x86_64.cpp b/src/hotspot/src/cpu/x86/vm/templateTable_x86_64.cpp
index 301acb44..21814259 100644
--- a/src/hotspot/src/cpu/x86/vm/templateTable_x86_64.cpp
+++ b/src/hotspot/src/cpu/x86/vm/templateTable_x86_64.cpp
@@ -1666,14 +1666,14 @@ void TemplateTable::branch(bool is_jsr, bool is_wide) {
         const Address mdo_backedge_counter(rbx, in_bytes(MethodData::backedge_counter_offset()) +
                                            in_bytes(InvocationCounter::counter_offset()));
         __ increment_mask_and_jump(mdo_backedge_counter, increment, mask,
- rax, false, Assembler::zero, &backedge_counter_overflow);
+ rax, false, Assembler::zero, UseOnStackReplacement ? &backedge_counter_overflow : &dispatch);
         __ jmp(dispatch);
       }
       __ bind(no_mdo);
       // Increment backedge counter in MethodCounters*
       __ movptr(rcx, Address(rcx, Method::method_counters_offset()));
       __ increment_mask_and_jump(Address(rcx, be_offset), increment, mask,
- rax, false, Assembler::zero, &backedge_counter_overflow);
+ rax, false, Assembler::zero, UseOnStackReplacement ? &backedge_counter_overflow : &dispatch);
     } else {
       // increment counter
       __ movptr(rcx, Address(rcx, Method::method_counters_offset()));
diff --git a/src/hotspot/src/share/vm/asm/assembler.hpp b/src/hotspot/src/share/vm/asm/assembler.hpp
index ec8ec5eb..d117e073 100644
--- a/src/hotspot/src/share/vm/asm/assembler.hpp
+++ b/src/hotspot/src/share/vm/asm/assembler.hpp
@@ -169,6 +169,10 @@ class Label VALUE_OBJ_CLASS_SPEC {
   Label() {
     init();
   }
+
+ ~Label() {
+ assert(is_bound() || is_unused(), "Label was never bound to a location, but it was used as a jmp target");
+ }
 };

// A union type for code which has to assemble both constant and