[LLVMdev] Intel Memory Protection Extensions (and types question)

[David Chisnall]

On 10 Sep 2013, at 10:13, Kostya Serebryany <kcc at google.com> wrote:

> How did you come with 320 bits? 
> 320=64*4+64, which is the size of the metadata table entry plus pointer size,

Sorry, that should have been 192.  The specification allows the metadata to be stored either in look-aside tables or explicitly managed.  The tables impose a very large storage space penalty, so are most likely to be used with C or similar language where it is difficult to modify the data layout.  For languages where there is no requirement to maintain an ABI that interoperates with non-MPX code, the metadata can be stored inline when running in bounds-checked mode.  I forgot that when using it in this mode you needed to store less metadata than when using the bound tables.  

> but why do you call this a fat pointer?

Because that's what it is: a pointer + metadata

> In MPX, the fat pointer never exists as a single entity. 

The pointer and metadata exist in separate registers, but single instructions (loads and stores) operate on the pointer + metadata.  

David