RFR: 8276563: Undefined Behaviour in class Assembler [v9]

[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.

Andrew Haley has updated the pull request incrementally with one additional commit since the last revision:

  Tweako stuff.

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6280/files
  - new: https://git.openjdk.java.net/jdk/pull/6280/files/c7ec4ca9..0ce81fd2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6280&range=08
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6280&range=07-08

  Stats: 7 lines in 1 file changed: 2 ins; 0 del; 5 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