On 11.10.2016 16:36, Arsenault, Matthew wrote:
Why does the IR need access to the scratch pointer at all? I don't
understand what problems the explicit IR argument is supposed to solve
or why an intrinsic would be needed. The value is only needed for
lowering the access to scratch, the user program shouldn't need to be
aware it exists
The IR doesn't need access to the scratch pointer, but userdata
registers are a scarce resource and it would be useful to be able to
avoid wasting two full userdata SGPRs on _only_ the scratch pointer when
(a) most shaders don't even need it, and (b) we already have a pair of
SGPRs that points to a table of mostly global data (e.g. tessellation
rings, streamout, polygon stippling mask, ...). It is perfectly natural
to stick the scratch buffer into that table, but the IR needs to be able
to access everything else in the table.
Cheers,
Nicolai
On Tue, Oct 11, 2016 at 9:30 PM +0800, "Nicolai Hähnle"
<nhaeh...@gmail.com <mailto:nhaeh...@gmail.com>> wrote:
On 11.10.2016 15:21, Nicolai Hähnle wrote:
On 11.10.2016 07:36, Dave Airlie wrote:
On 11 October 2016 at 12:13, Dave Airlie <airl...@gmail.com> wrote:
On 11 October 2016 at 11:42, Dave Airlie <airl...@gmail.com> wrote:
On 11 October 2016 at 05:50, Dave Airlie <airl...@gmail.com> wrote:
On 10 October 2016 at 21:45, Arsenault, Matthew
<matthew.arsena...@amd.com> wrote:
I don't like adding explicit IR arguments for ABI arguments,
especially this
one. Adding a special case for the first index feels dirty. The
rest of llvm
also won't be aware of the specialness of the argument. It would be
problematic because bugpoint would eliminate the unused argument
and then
codegen would have to fail in some way when the argument is missing
That's a good point, but is there an alternative without burning two
userdata SGPRs?
One possibility is to define an ABI that says:
1. SGPR0/1 points to an extra data region; it is reserved independently
from the shader arguments.
2. The first 64 bits of that extra data region point to the scratch buffer.
3. The main shader code can retrieve SGPR0/1 using an intrinsic.
This can be made to look somewhat similar to what HSA does.
For radeonsi this might break some of the optimizations that allow LLVM
to move code around: if RW_BUFFERS are now loaded from a pointer
returned by a new intrinsic, then the returned value should have the
same speculability properties as today's argument. That's probably
possible, but we need to keep it in mind.
Nicolai
We should just hardcode the behaviour and switch both radv/radeonsi
over in one go?
I'll try and code up, using the first 64-bits of the first buffer
pointed to by userdata 0/1,
to store things.
I've looked at doing a dword fetch from the first two words of the
0/1 userdata,
It's not optimal for vulkan unfortunately, since the idea I had was
per command
buffer I just allocate one scratch buffer of the size required at
the end, and
patch it in at the start of the command buffer. However in the first
slot I was going
to use the push constants/dynamic buffer to store the value, however
it looks
like I need to keep a list of everyone of these buffers I emit, and
backpatch them
all. It might not be too insane, just a slight bump in the keeping
it simple.
I'm probably losing te plot here, but I'm considering a double
indirection,
we load the 64-bit address from the first two dwords, then load the
64-bits dword
from that address to get the value.
This saves me allocating scratch bo's for secondary command buffers,
and also having to allocating ever increasing scratch bo's as shaders
that
need more scratch get bound to the pipeline.
I'm not sure how much of an effect this should have for GL though.
I've posted a patch to this affect to the llvm phabricator.
It definitely is cleaner for the radv driver.
I still think it would be nice to have the level of indirection or
whatever one wants to call it as a function attribute. This would allow
you to change your mind about e.g. just sticking the scratch pointer
directly into SGPR0/1. radeonsi and radv don't have to be identical in
that regard.
Cheers
Nicolai
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