RFR: 8289046: Undefined Behaviour in x86 class Assembler [v5]

[Andrew Haley]

> All instances of type Register exhibit UB in the form of wild pointer (including null pointer) dereferences. This isn't very hard to fix: we should make Registers pointers to something rather than aliases of small integers. 
> 
> Here's an example of what was happening:
> 
>    ` rax->encoding();`
> 
> Where rax is defined as `(Register *)0`.
> 
> This patch things so that rax is now defined as a pointer to the start of a static array of RegisterImpl.
> 
> 
> typedef const RegisterImpl* Register;
> extern RegisterImpl all_Registers[RegisterImpl::number_of_declared_registers + 1] ;
> inline constexpr Register RegisterImpl::first() { return all_Registers + 1; };
> inline constexpr Register as_Register(int encoding) { return RegisterImpl::first() + encoding; }
> constexpr Register rax = as_register(0);

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

  Delete changes to hotspot/shared.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9261/files
  - new: https://git.openjdk.org/jdk/pull/9261/files/8f965c9f..df457ba6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9261&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9261&range=03-04

  Stats: 29 lines in 2 files changed: 3 ins; 15 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/9261.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9261/head:pull/9261

PR: https://git.openjdk.org/jdk/pull/9261