[PING][RFC] Support for changing vector lengths

[Alan Hayward <Alan dot Hayward at arm dot com>]

Ping. Anyone have an opinion on this?

(Not expecting a review on the variable_sve2 branch at this point - just
If it’s worth continuing in that direction or worth switching over to the
xml version).

Alan.


> On 7 Nov 2018, at 15:35, Alan Hayward <Alan.Hayward@arm.com> wrote:
> 
> I’ve been rethinking the work I’ve been doing for supporting the Aarch64 vector
> length changing whilst a process is running, and am considering a different
> approach, as my current version has some problems. Below is a summary of the
> current state of play, a description of the patches I’ve been writing, the
> holes in that approach, and the outline for an alternative version.
> Does anyone have any thoughts on this?
> 
> 
> Current state of HEAD for Aarch64 SVE:
> Upon startup read the current vector length, then create a target description,
> where all the registers are a fixed size based off the current vector length.
> If vector length changes, then the vector registers will be shown with
> incorrect lengths.
> 
> 
> The need to support vector lengths changing at runtime:
> In the general case, it’s not expected that Aarch64 SVE applications will
> change the vector length during runtime. However, Linux does provide an
> interface for changing per thread and specific use cases include:
> * Specific hardware is known to work faster at a given length for a given loop
> * For debugging purposes
> * Ultra cautious application restricts the vector length due to the vendor
>  being unable to test on larger vector lengths.
> In addition we should take into account future variable architectures, such
> as RISC-V where the vector length is expected to be changed before every
> vector loop and the maximum possible size of a register is not fixed by the
> architecture spec.
> 
> 
> My original variable plan:
> “Target descriptions are fixed. Obtain a new target description when the vector
> length changes.”
> 
> I’ve uploaded a set of patches to the branch users/ahayward/variable_sve2.
> When the inferior stops, GDB makes a call to get the gdbarch. By overloading
> thread_architecture(), we re-read the vector length and if it has changed, then
> get a different target description either by creating a new one or grabbing it
> from a cache.
> Similar functionality is added to gdbserver.
> The problem is, target descriptions are per process not per thread. I’m not
> sure what would be needed to change it. Every access to the target description
> would now require the thread id? I’m not sure of the impact of this across
> large threaded applications.
> This also currently causes the regcache to get recreated each switch.
> 
> 
> Alternative plan:
> “Target descriptions understand variable registers.”
> 
> I’ve not yet coded any of this, and it exists just as an idea.
> The ability for a register to be variable sized registers is added to target
> descriptions.
> We know the maximum size of the registers at runtime (by asking the kernel).
> Code would create a vector register then pass that in when creating a variable
> register:
>  tdesc_reg *vg_reg = tdesc_create_reg (feature, "vg", regnum++, 1, NULL, 64, "int");
>  tdesc_create_variable_reg (feature, "z0", regnum++, 1, NULL, 128, "svev”, vg_reg, max_vg_value);
> In xml:
>  <reg name=“z0” bitsize="128" type="aarch64v" variable=“yes” scalar_reg=“vg” maxbitsize=“2048” />
> Size of variable reg = min (*scalar_reg * bitsize, maxbitsize)
> Similar support needs adding to the register types.
> 
> The register cache can use the maximum size of each register for allocating the
> cache. This is perfectly fine for SVE (I also suspect most programs will be
> running using the maximum length allowed). Reading/writing a register needs to
> limit to the current size. Printing/Setting a register will have to read the
> associated variable register and then scale accordingly.
> Additional work would be required for RISC-V as the maximum register sized is
> not fixed; I suspect that is mostly in the regcache handling.
> 
> 
> As a side note, it's probably worth mentioning SVE ACLE ( Currently in beta, see
> https://static.docs.arm.com/100987/0000/acle_sve_100987_0000_00_en.pdf ).
> These are C extensions which add variable size types such as svint32_t (a vector
> containing 32bit integers).  There are clear uses cases for a user writing code
> using this and I suspect it would require extending the type system in GDB to
> allow for variable sized types.  This would be similar to the changes made to GCC
> to support compiling to SVE, which was a large piece of work.  There is no support
> (yet) for SVE ACLE in GCC, but it's something to think about.
> 
> 
> Thanks,
> Alan.
>