On 03/24/2014 03:08 PM, Tom Stellard wrote:
On Mon, Mar 24, 2014 at 03:01:14PM -0400, Justin Holewinski wrote:
On 03/24/2014 02:53 PM, Tom Stellard wrote:
On Mon, Mar 24, 2014 at 02:46:06PM -0400, Justin Holewinski wrote:
I don't have anything against making this a target-independent
IR-level, as long as no-one complains about it being a core pass.
Perhaps the pass could only execute if a target explicitly enables a
flag. Something like "preferNonGenericPointers". The default could
be 'false', and the pass would only modify the IR if the target sets
it to 'true'. Of course, this also assumes address space 0 is
generic. This is currently true for the in-tree targets and
CUDA/OpenCL support in Clang, but I don't believe its a set rule
anywhere.
What do mean by 'generic' address space here?
In this context, 'generic' means an address space that encompasses
all other address spaces. This is roughly equivalent to the OpenCL
2.0 generic address space. Is addrspace(0) not a generic address
space for R600?
No, it's not generic. It's what we use for OpenCL's private address space.
Also, I missed the beginning of this discussion, which target-independent pass
does this impact?
None yet :)
The question is whether a pass that converts generic address space usage
to non-generic address space usage should be target-independent, or
specific to a particular back-end. An example would be an IR sequence
like the following:
%ptr = ...
%val = load i32* %ptr
In this case, %ptr is a generic address space pointer (assuming an
address space mapping where 0 is generic). But if an analysis can prove
that the pointer %ptr was originally addrspacecast'd from a specific
address space (or some other mechanism through which the pointer's
specific address space can be determined), it may be beneficial to
explicitly convert the IR to something like:
%ptr = ...
%ptr.0 = addrspacecast i32* to i32 addrspace(3)*
%val = load i32 addrspace(3)* %ptr.0
Such a translation may generate better code for some targets.
-Tom
-Tom
On 03/24/2014 02:28 PM, Jingyue Wu wrote:
I agree with your concern. However, both CUDA and OpenCL (two most
popular users of addrspacecast I believe) support generic address
space, and could benefit from this optimization Would we end up
with duplicated code (at least one for CUDA one for opencl) if we
put it in the back-end?
Jingyue
On Mon, Mar 24, 2014 at 11:22 AM, Justin Holewinski
<[email protected] <mailto:[email protected]>> wrote:
The hard part would be making this optimization general enough to
be target-independent. Optimizing to non-zero address spaces may
not make sense for all targets (or even all future versions of
PTX). I agree that there should be an IR-level optimization for
this, but perhaps its too target-specific and should actually live
in the back-end.
On 03/24/2014 01:05 PM, Jingyue Wu wrote:
Right. We are aware of this issue, and think it should be
addressed in the IR optimizer (similar to InstCombineLoadCast and
InstCombineStoreToCast) instead of clang. Do you think this is an
appropriate approach? Is this optimization general enough to stay
in the IR optimizer or target-dependent?
Jingyue
On Mon, Mar 24, 2014 at 4:54 AM, Justin Holewinski
<[email protected]
<mailto:[email protected]>> wrote:
Hi Jingyue,
I committed the addrspacecast isel patterns to NVPTX. Also,
I wanted to point out that your changes in the last test case
in this patch (address-spaces.cu <http://address-spaces.cu>)
represent changes that may lead to performance degradation.
Specific address spaces should be used whenever possible for
loads/stores. Casting everything to a generic address is
still correct, but may lead to additional indirections for
the hardware.
On Fri, Mar 21, 2014 at 2:25 PM, Justin Holewinski
<[email protected] <mailto:[email protected]>> wrote:
addrspacecast support in NVPTX is on my todo list. I'll
try to put something together in the next few days.
On 3/21/14, 2:20 PM, Jingyue Wu wrote:
Hi,
Static local variables in CUDA can be declared with
address space qualifiers, such as __shared__. Therefore,
the codegen needs to potentially addrspacecast a static
local variable to the type expected by its declaration.
Peter did something similar for global variables in
r157167.
All clang tests passed.
Justin: The NVPTX backend support for addrspacecast
seems not complete. We can send you follow-up patches
once this one gets in.
Jingyue
-- Thanks,
Justin Holewinski
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Thanks,
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