erichkeane added inline comments.
================
Comment at: clang/test/SemaCXX/attr-riscv-rvv-vector-bits.cpp:12
+
+template<typename T> struct S { T var; };
+
----------------
craig.topper wrote:
> aaron.ballman wrote:
> > craig.topper wrote:
> > > aaron.ballman wrote:
> > > > erichkeane wrote:
> > > > > craig.topper wrote:
> > > > > > erichkeane wrote:
> > > > > > > craig.topper wrote:
> > > > > > > > @erichkeane does this cover the dependent case or were you
> > > > > > > > looking for something else?
> > > > > > > >
> > > > > > > > Here are on the only mentions of template I see in SVE tests
> > > > > > > > that use this attribute.
> > > > > > > >
> > > > > > > > ```
> > > > > > > > clang/test$ ack template `ack arm_sve_vector -l`
> > > > > > > > CodeGenCXX/aarch64-mangle-sve-fixed-vectors.cpp
> > > > > > > > 37:template <typename T> struct S {};
> > > > > > > >
> > > > > > > > SemaCXX/attr-arm-sve-vector-bits.cpp
> > > > > > > > 16:template<typename T> struct S { T var; };
> > > > > > > > ```
> > > > > > > >
> > > > > > > > Here is the result for this patch
> > > > > > > >
> > > > > > > > ```
> > > > > > > > clang/test$ ack template `ack riscv_rvv_vector -l`
> > > > > > > > CodeGenCXX/riscv-mangle-rvv-fixed-vectors.cpp
> > > > > > > > 48:template <typename T> struct S {};
> > > > > > > >
> > > > > > > > SemaCXX/attr-riscv-rvv-vector-bits.cpp
> > > > > > > > 12:template<typename T> struct S { T var; };
> > > > > > > > ```
> > > > > > > Thats unfortunate, and I wish I'd thought of it at the time/been
> > > > > > > more active reviewing the SVE stuff then. Really what I'm
> > > > > > > looking for is:
> > > > > > >
> > > > > > > ```
> > > > > > > template<int N>
> > > > > > > struct Whatever {
> > > > > > > using Something = char __attribute((riscv_rvv_vector_bits(N)));
> > > > > > > };
> > > > > > >
> > > > > > > void Func(Whatever<5>::Something MyVar){}
> > > > > > >
> > > > > > > ```
> > > > > > That does not appear to work.
> > > > > >
> > > > > > ```
> > > > > > $ ./bin/clang test.cpp --target=riscv64 -march=rv64gcv
> > > > > > -mrvv-vector-bits=zvl
> > > > > > test.cpp:3:41: error: 'riscv_rvv_vector_bits' attribute requires an
> > > > > > integer constant
> > > > > > using Something = char __attribute((riscv_rvv_vector_bits(N)));
> > > > > > ```
> > > > > >
> > > > > > It's not very useful as a template parameter. There's only one
> > > > > > value that works and that's whatever __RISCV_RVV_VLEN_BITS is set
> > > > > > to.
> > > > > Thats really unfortunate, but it makes me wonder what
> > > > > `DependentVectorType ` is for in this case, or the handling of said
> > > > > things. Because I would expect:
> > > > >
> > > > > ```
> > > > > template<typename T, int Size>
> > > > > using RiscvVector = T __attribute__((risv_rvv_vector_bits(Size)));
> > > > >
> > > > > RiscvVector<char, <TheRightAnswer>> Foo;
> > > > > ```
> > > > > to be useful. Even if not, I'd expect:
> > > > > ```
> > > > > template<typename T>
> > > > > using RiscvVector = T
> > > > > __attribute__((risv_rvv_vector_bits(TheRightAnswer)));
> > > > > RiscvVector<char> Foo;
> > > > > ```
> > > > > to both work.
> > > > >
> > > > > >>It's not very useful as a template parameter. There's only one
> > > > > >>value that works and that's whatever __RISCV_RVV_VLEN_BITS is set
> > > > > >>to.
> > > > > This makes me wonder why this attribute takes an integer constant
> > > > > anyway, if it is just a 'guess what the right answer is!' sorta
> > > > > thing. Seems to me this never should have taken a parameter.
> > > > > It's not very useful as a template parameter. There's only one value
> > > > > that works and that's whatever __RISCV_RVV_VLEN_BITS is set to.
> > > >
> > > > Can you help me understand why the argument exists then?
> > > >
> > > > We're pretty inconsistent about attribute arguments properly handling
> > > > things like constant expressions vs integer literals, but the trend
> > > > lately is to accept a constant expression rather than only a literal
> > > > because of how often users like to give names to literals and how much
> > > > more constexpr code we're seeing in the wild.
> > > This is what's in ARM's ACLE documentation:
> > >
> > >
> > >
> > > > The ACLE only defines the effect of the attribute if all of the
> > > > following are true:
> > > > 1. the attribute is attached to a single SVE vector type (such as
> > > > svint32_t) or to the SVE predicate
> > > > type svbool_t;
> > > > 2. the arguments “…” consist of a single nonzero integer constant
> > > > expression (referred to as N below); and
> > > > 3. N==__ARM_FEATURE_SVE_BITS.
> > > > In other cases the implementation must do one of the following:
> > > > • ignore the attribute; a warning would then be appropriate, but is not
> > > > required
> > > > • reject the program with a diagnostic
> > > > • extend requirement (3) above to support other values of N besides
> > > > __ARM_FEATURE_SVE_BITS
> > > > • process the attribute in accordance with a later revision of the ACLE
> > >
> > >
> > >
> > >
> > >
> > > So there's a bullet in there that allows an implementation to support
> > > other values, but it is not required.
> > Thank you, the current design makes more sense to me now. I'm less
> > concerned about whether we support dependent values for this attribute
> > argument. If we start to support values of `N` other than
> > `__ARM_FEATURE_SVE_BITS` then it might make sense to care about it at that
> > point. But I don't think users are going to do stuff like:
> > ```
> > template <int N>
> > using fixed_int8m1_t __attribute__((riscv_rvv_vector_bits(N))) = vint8m1_t;
> >
> > fixed_int8m1_t<__ARM_FEATURE_SVE_BITS> foo;
> > ```
> > However, it is still important to test that the type attribute works in a
> > situation like:
> > ```
> > template <typename Ty>
> > using Something = Ty
> > __attribute__((riscv_rvv_vector_bits(__ARM_FEATURE_SVE_BITS)));
> >
> > // Ensure that Something is correctly attributed, that the underlying type
> > for Ty is valid for the attribute, etc
> > ```
> >
> It looks like it doesn't work for that case.
THAT is super unfortunate, and really should work in this case. The SVE
implementers could probably help out here.
Repository:
rG LLVM Github Monorepo
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https://reviews.llvm.org/D145088/new/
https://reviews.llvm.org/D145088
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