[cfe-dev] [RFC] Ternary operator with vector type predicate
Min-Yih Hsu via cfe-dev
cfe-dev at lists.llvm.org
Thu May 21 09:04:08 PDT 2020
Thanks for everyone’s feedback.
I think the bottom line is that the originally proposal works under OpenCL and gcc-style vector extension. Should we support it for ext_vector_type under non-OpenCL mode?
Surprisingly the bug Anastasia pointed to was also asking the same question. Most of the comments there agreed with the approach and I didn’t see any comments against it in this thread. So I’ll cook a simple patch to turn on this switch for non-OpenCL mode for everyone’s review.
> On May 20, 2020, at 2:49 PM, Anastasia Stulova <Anastasia.Stulova at arm.com> wrote:
>
> Right, If OpenCL mode compilation flag is removed from the example that Alexey has given, the compilation fails!
>
> FYI, I just came across this old bug discussing similar issue:
> https://bugs.llvm.org/show_bug.cgi?id=33103 <https://bugs.llvm.org/show_bug.cgi?id=33103>
>
> From: Bader, Alexey <alexey.bader at intel.com>
> Sent: 19 May 2020 20:31
> To: Finkel, Hal J. <hfinkel at anl.gov>; Anastasia Stulova <Anastasia.Stulova at arm.com>; Keane, Erich <erich.keane at intel.com>; Min-Yih Hsu <minyihh at uci.edu>; cfe-dev (cfe-dev at lists.llvm.org) <cfe-dev at lists.llvm.org>
> Cc: nd <nd at arm.com>
> Subject: RE: [cfe-dev] [RFC] Ternary operator with vector type predicate
>
> I never used this extensions in non-OpenCL mode, so I can’t predict what is the impact of the proposed changes.
>
> I just note that code snippet from the original email works in OpenCL mode as described in RFC with one very minor note:
>
> I’m proposing to support vector type values as predicates in the ternary operator: If the predicate value has vector type, each of its element would be converted to bool before being used to select corresponding elements from the second or third operands. Note that this is exactly what GCC is doing right now [1] in their vector extensions.
>
> In OpenCL mode the comparison operator returns bitmask to enable bitwise operations (https://godbolt.org/z/F6ynx6):
>
> float4 foo(float4 a, float4 b, int4 *c) {
> int4 x = {1, 2, 3, 4};
> *c = (a < b) & x;
> return a > b? a : b;
> }
>
> and select is defined as
>
> gentype select(gentype a, gentype b, igentype c) gentype select(gentype a, gentype b, ugentype c)
> For each component of a vector type,
>
> result[i] = if MSB of c[i] is set ? b[i] : a[i].
>
> For a scalar type, result = c ? b : a.
>
> igentype and ugentype must have the same number of elements and bits as gentype36.
>
> I guess, if we convert predicate elements to bool, we should get the same behavior.
>
> From: Finkel, Hal J. <hfinkel at anl.gov>
> Sent: Tuesday, May 19, 2020 9:50 PM
> To: Bader, Alexey <alexey.bader at intel.com>; Anastasia Stulova <Anastasia.Stulova at arm.com>; Keane, Erich <erich.keane at intel.com>; Min-Yih Hsu <minyihh at uci.edu>; cfe-dev (cfe-dev at lists.llvm.org) <cfe-dev at lists.llvm.org>
> Cc: nd <nd at arm.com>
> Subject: Re: [cfe-dev] [RFC] Ternary operator with vector type predicate
>
> Thanks, Alexey. In that case, do you have any thoughts on the original email in this thread?
>
> -Hal
>
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
> From: Bader, Alexey <alexey.bader at intel.com <mailto:alexey.bader at intel.com>>
> Sent: Tuesday, May 19, 2020 1:48 PM
> To: Finkel, Hal J. <hfinkel at anl.gov <mailto:hfinkel at anl.gov>>; Anastasia Stulova <Anastasia.Stulova at arm.com <mailto:Anastasia.Stulova at arm.com>>; Keane, Erich <erich.keane at intel.com <mailto:erich.keane at intel.com>>; Min-Yih Hsu <minyihh at uci.edu <mailto:minyihh at uci.edu>>; cfe-dev (cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>) <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>>
> Cc: nd <nd at arm.com <mailto:nd at arm.com>>
> Subject: RE: [cfe-dev] [RFC] Ternary operator with vector type predicate
>
> Yes, we do implement these rules for OpenCL and they are applied to all OpenCL versions.
> https://godbolt.org/z/YH6ExF <https://godbolt.org/z/YH6ExF>
>
>
> From: cfe-dev <cfe-dev-bounces at lists.llvm.org <mailto:cfe-dev-bounces at lists.llvm.org>> On Behalf Of Finkel, Hal J. via cfe-dev
> Sent: Tuesday, May 19, 2020 8:36 PM
> To: Anastasia Stulova <Anastasia.Stulova at arm.com <mailto:Anastasia.Stulova at arm.com>>; Keane, Erich <erich.keane at intel.com <mailto:erich.keane at intel.com>>; Min-Yih Hsu <minyihh at uci.edu <mailto:minyihh at uci.edu>>; clang developer list <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>>
> Cc: nd <nd at arm.com <mailto:nd at arm.com>>
> Subject: Re: [cfe-dev] [RFC] Ternary operator with vector type predicate
>
> Thanks, Anastasia. I have certainly used the ext_vector_type types outside of OpenCL mode in the past. Do we currently implement the OpenCL 2 ternary operator rules for these vectors?
>
> -Hal
>
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
> From: Anastasia Stulova <Anastasia.Stulova at arm.com <mailto:Anastasia.Stulova at arm.com>>
> Sent: Tuesday, May 19, 2020 12:30 PM
> To: Keane, Erich <erich.keane at intel.com <mailto:erich.keane at intel.com>>; Min-Yih Hsu <minyihh at uci.edu <mailto:minyihh at uci.edu>>; Finkel, Hal J. <hfinkel at anl.gov <mailto:hfinkel at anl.gov>>; clang developer list <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>>
> Cc: nd <nd at arm.com <mailto:nd at arm.com>>
> Subject: Re: [cfe-dev] [RFC] Ternary operator with vector type predicate
>
> I see ext_vector_type as a Clang extension that implements OpenCL rules under OpenCL mode. I believe there are areas where it behaves slightly differently if it's used in non-OpenCL code. I am not aware of details, but I believe it is used outside of OpenCL mode too.
>
> Anastasia
> From: cfe-dev <cfe-dev-bounces at lists.llvm.org <mailto:cfe-dev-bounces at lists.llvm.org>> on behalf of Finkel, Hal J. via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>>
> Sent: 19 May 2020 14:22
> To: Keane, Erich <erich.keane at intel.com <mailto:erich.keane at intel.com>>; Min-Yih Hsu <minyihh at uci.edu <mailto:minyihh at uci.edu>>
> Cc: cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>>
> Subject: Re: [cfe-dev] [RFC] Ternary operator with vector type predicate
>
>
> From: Keane, Erich <erich.keane at intel.com <mailto:erich.keane at intel.com>>
> Sent: Tuesday, May 19, 2020 8:10 AM
> To: Finkel, Hal J. <hfinkel at anl.gov <mailto:hfinkel at anl.gov>>; Min-Yih Hsu <minyihh at uci.edu <mailto:minyihh at uci.edu>>
> Cc: cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>>
> Subject: RE: [cfe-dev] [RFC] Ternary operator with vector type predicate
>
> What is the intent with the ext-vector-types?
>
>
> That's a really good question. I've always thought of them as a slightly-more-convenient version of the vector types, although I suppose that they're really supposed to be OpenCL vectors. The Clang extension docs say, "OpenCL vector types are created using the ext_vector_type attribute. It supports the V.xyzw syntax and other tidbits as seen in OpenCL." Thus, for OpenCL, I suppose that it is supposed to do this: https://www.khronos.org/registry/OpenCL/specs/2.2/html/OpenCL_C.html#operators-ternary-selection <https://www.khronos.org/registry/OpenCL/specs/2.2/html/OpenCL_C.html#operators-ternary-selection>
>
> -Hal
>
>
>
>
>
>
> It seems to me that it is an incompatible extension without anyone really maintaining them, and slightly different SEMA rules (for seemingly no reason).
>
> The conditional operator for the vector_size value was implemented as a GCC compatibility option, but no such motivation existed for ext-vetctor-types.
>
> That said, the semantic analysis for this in ext-vector-types is sufficiently different that it wasn’t possible to combine implementations. The rules had diverged significantly at one point, so there wasn’t really a way to do so compatibly.
>
> From: Finkel, Hal J. <hfinkel at anl.gov <mailto:hfinkel at anl.gov>>
> Sent: Tuesday, May 19, 2020 6:05 AM
> To: Keane, Erich <erich.keane at intel.com <mailto:erich.keane at intel.com>>; Min-Yih Hsu <minyihh at uci.edu <mailto:minyihh at uci.edu>>
> Cc: cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
> Subject: Re: [cfe-dev] [RFC] Ternary operator with vector type predicate
>
> In general, I don't like to see features supported by not-ext-vector types that are not supported by ext-vector types.
>
> That having been said, is this related to our discussion on bool vectors:
>
> http://lists.llvm.org/pipermail/cfe-dev/2020-May/065434.html <http://lists.llvm.org/pipermail/cfe-dev/2020-May/065434.html>
>
> -Hal
>
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
> From: cfe-dev <cfe-dev-bounces at lists.llvm.org <mailto:cfe-dev-bounces at lists.llvm.org>> on behalf of Min-Yih Hsu via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>>
> Sent: Friday, May 8, 2020 1:17 PM
> To: Keane, Erich <erich.keane at intel.com <mailto:erich.keane at intel.com>>
> Cc: cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>>
> Subject: Re: [cfe-dev] [RFC] Ternary operator with vector type predicate
>
> Interesting….then I think the question simply becomes whether we should support it in ext_vector_type as well.
>
> Thanks for the pointer
>
> -Min
>
> On May 8, 2020, at 11:04 AM, Keane, Erich <erich.keane at intel.com <mailto:erich.keane at intel.com>> wrote:
>
> Note that this was implemented for non-ext-vector types here:
> https://github.com/llvm/llvm-project/commit/349636d2bfc39a5c81a835a95d203a42d9f9301a <https://github.com/llvm/llvm-project/commit/349636d2bfc39a5c81a835a95d203a42d9f9301a>
>
> Which matches the GCC version.
> The attribute is vector_size, not ext_vector_type however.
>
> Otherwise, it does everything you’re proposing (like I said, just with a more portable vector-type definition).
>
>
>
> From: cfe-dev <cfe-dev-bounces at lists.llvm.org <mailto:cfe-dev-bounces at lists.llvm.org>> On Behalf Of Min-Yih Hsu via cfe-dev
> Sent: Friday, May 8, 2020 10:58 AM
> To: cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
> Subject: [cfe-dev] [RFC] Ternary operator with vector type predicate
>
> Hi folks,
>
> In clang’s vector extension, the ternary operator (?:) is able to select from two vector type values based on the boolean type predicate. For example:
> ```
> typedef float float4 __attribute__((ext_vector_type(4)));
>
> float4 foo(bool cond, float4 a, float4 b) {
> return cond? a : b;
> }
> ```
> However, clang currently doesn’t support vector type values as the predicates. So code shown below won’t compile:
> ```
> float4 foo(float4 a, float4 b) {
> return a > b? a : b;
> }
> ```
> Which is essentially just a max function that can be implemented by hardware instructions (vmaxps in Intel AVX for example) efficiently.
>
> I’m proposing to support vector type values as predicates in the ternary operator: If the predicate value has vector type, each of its element would be converted to bool before being used to select corresponding elements from the second or third operands. Note that this is exactly what GCC is doing right now [1] in their vector extensions.
>
> On the (IR) codegen side, since the IR `select`, `icmp`, and `fcmp` instruction all support vector type values as the operands, I don’t think there are much trouble in it.
>
> Here are the reasons and some advantages:
> 1. It provides more flexibility for programmers to do conditional selections on vectors. Especially for cases that can be optimized by hardware instructions like the motivated example above.
>
> 2. It will not break the current usages of ternary operator: If the predicate is a scalar value, it’s still sticking to the current model.
>
> 3. If the goal of our extensions is aiming to “support a broad range of GCC extensions”. Then the behavior should also be as consistent as possible.
>
> I’ll try to come up with a patch for preview as soon as possible.
> Also a slightly off-topic thing: In our document [2], the ternary operator has always been mis-typed as ‘:?’ Maybe we should fix it some times.
>
> Thanks for the feedbacks in advance.
> -Min
>
> [1]: https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html <https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html>
> [2]: https://clang.llvm.org/docs/LanguageExtensions.html#vectors-and-extended-vectors <https://clang.llvm.org/docs/LanguageExtensions.html#vectors-and-extended-vectors>
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