RFR: 8276563: Undefined Behaviour in class Assembler

[Andrew Haley]

The HotSpot code base contains a number of instances of Undefined Behavior, which can cause all manner of unpleasant surprises.
The UB to which this patch relates is in class `Assembler`, in which instances are pointers to (nonexistent) objects defined as, for example,


  typedef RegisterImpl *Register;
  const Register r10 = ((Register)10);


Registers have accessors, e.g.:

` int RegisterImpl::encoding() const { return (intptr_t)this; }`

This works by an accident of implementation: it is not legal C++.

The most obvious way to this UB bug is to make instances of `Register` point to something, and to use pointer subtraction to find the encoding: (simplified for clarity)


  extern RegisterImpl all_Registers[num_Registers];
  int RegisterImpl::encoding() const   { return this - all_Registers; }


After this patch there is slightly more work to be done when assembling code but it's merely the subtraction of a constant in `encoding()` and the difference in execution time is so small (and the startup variance so large) that I have been unable to measure it, even after averaging 100 runs. It does lead to an increase of about 1% in the size of the stripped libjvm.so, but I think that can be recovered by a subsequent patch.

An alternative way to implement this would be to make the encoding a byte-wide field in `RegisterImpl` and define encoding() this way:

`  int RegisterImpl::encoding() const   { return _encoding; }`

This would result in smaller code, but I suspect slower.

If this change is accepted, I propose that all instances of this pattern in HotSpot be treated similarly.

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Commit messages:
 - 8276563: Undefined Behaviour in class Assembler

Changes: https://git.openjdk.java.net/jdk/pull/6280/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6280&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276563
  Stats: 116 lines in 5 files changed: 67 ins; 22 del; 27 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6280.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6280/head:pull/6280

PR: https://git.openjdk.java.net/jdk/pull/6280