[IR] Add LLVM IR support for target("aarch64.svcount") type.

The C and C++ Language Extensions for AArch64 SME2 [1] adds a new type called
`svcount_t` which describes a predicate. This is not a predicate vector
mask, but rather a description of a predicate vector mask that can be
expanded into a mask using explicit instructions. The type is a scalable
opaque type.

To implement `svcount_t` type this patch uses the existing Target Extension Type
mechanism, but adds further support so that this type can be a scalable type.

AArch64 CodeGen support will follow in a separate patch.

[1] https://github.com/ARM-software/acle/pull/217

Reviewed By: jcranmer-intel, nikic

Differential Revision: https://reviews.llvm.org/D136861

llvm/docs/AArch64SME.rst

@@ -440,7 +440,38 @@ Committing a lazy-save
 Exception handling and ZA
 -------------------------
 
-4. References
+4. Types
+========
+
+AArch64 Predicate-as-Counter Type
+---------------------------------
+
+:Overview:
+
+The predicate-as-counter type represents the type of a predicate-as-counter
+value held in a AArch64 SVE predicate register. Such a value contains
+information about the number of active lanes, the element width and a bit that
+tells whether the generated mask should be inverted. ACLE intrinsics should be
+used to move the predicate-as-counter value to/from a predicate vector.
+
+There are certain limitations on the type:
+
+* The type can be used for function parameters and return values.
+
+* The supported LLVM operations on this type are limited to ``load``, ``store``,
+  ``phi``, ``select`` and ``alloca`` instructions.
+
+The predicate-as-counter type is a scalable type.
+
+:Syntax:
+
+::
+
+      target("aarch64.svcount")
+
+
+
+5. References
 =============
 
     .. _aarch64_sme_acle:

llvm/lib/IR/Type.cpp

@@ -863,6 +863,11 @@ static TargetTypeInfo getTargetTypeInfo(const TargetExtType *Ty) {
     return TargetTypeInfo(Type::getInt8PtrTy(C, 0), TargetExtType::HasZeroInit,
                           TargetExtType::CanBeGlobal);
   }
+
+  // Opaque types in the AArch64 name space.
+  if (Name == "aarch64.svcount")
+    return TargetTypeInfo(ScalableVectorType::get(Type::getInt1Ty(C), 16));
+
   return TargetTypeInfo(Type::getVoidTy(C));
 }
 

llvm/test/Transforms/InstCombine/AArch64/sme-svcount.ll

@@ -0,0 +1,25 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -passes='instcombine' -S < %s | FileCheck %s
+
+define target("aarch64.svcount") @test_alloca_store_reload(target("aarch64.svcount") %val) nounwind {
+; CHECK-LABEL: @test_alloca_store_reload(
+; CHECK-NEXT:    ret target("aarch64.svcount") [[VAL:%.*]]
+;
+  %ptr = alloca target("aarch64.svcount"), align 1
+  store target("aarch64.svcount") %val, ptr %ptr
+  %res = load target("aarch64.svcount"), ptr %ptr
+  ret target("aarch64.svcount") %res
+}
+
+; Test that instcombine doesn't try to query the (scalable) size of target("aarch64.svcount")
+; in foldSelectInstWithICmp.
+define target("aarch64.svcount") @test_combine_on_select(target("aarch64.svcount") %x, target("aarch64.svcount") %y, i32 %k) {
+; CHECK-LABEL: @test_combine_on_select(
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i32 [[K:%.*]], 42
+; CHECK-NEXT:    [[X_Y:%.*]] = select i1 [[CMP]], target("aarch64.svcount") [[X:%.*]], target("aarch64.svcount") [[Y:%.*]]
+; CHECK-NEXT:    ret target("aarch64.svcount") [[X_Y]]
+;
+  %cmp = icmp sgt i32 %k, 42
+  %x.y = select i1 %cmp, target("aarch64.svcount") %x, target("aarch64.svcount") %y
+  ret target("aarch64.svcount") %x.y
+}

llvm/test/Transforms/SROA/aarch64-sme-svcount.ll

@@ -0,0 +1,12 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -passes='sroa' -S < %s | FileCheck %s
+
+define target("aarch64.svcount") @test_alloca_store_reload(target("aarch64.svcount") %val) nounwind {
+; CHECK-LABEL: @test_alloca_store_reload(
+; CHECK-NEXT:    ret target("aarch64.svcount") [[VAL:%.*]]
+;
+  %ptr = alloca target("aarch64.svcount"), align 1
+  store target("aarch64.svcount") %val, ptr %ptr
+  %res = load target("aarch64.svcount"), ptr %ptr
+  ret target("aarch64.svcount") %res
+}