RFR: 8289046: Undefined Behaviour in x86 class Assembler

[Quan Anh Mai]

On Thu, 23 Jun 2022 14:52:54 GMT, Andrew Haley <aph at openjdk.org> wrote:

> 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);

I believe there are some compiler directives somewhere to silent the compiler of `nullptr` dereference, should we delete those also?

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

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