[SVE] Make VectorType::getNumElements() complain for scalable vectors

[ctetreau]

Summary

Piggy-back off of TypeSize's STRICT_FIXED_SIZE_VECTORS flag and:

• if it is defined, assert that the vector is not scalable
• if it is not defined, complain if the vector is scalable

[sdesmalen]

LGTM

[ctetreau]

fix broken tests

[nikic]

llvm/include/llvm/IR/DerivedTypes.h

421

Tiny style nit: This line seems unnecessary, as EC is already a member of VectorType.

llvm/CMakeLists.txt

if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
   option(LLVM_ENABLE_ASSERTIONS "Enable assertions" OFF)
 else()
   option(LLVM_ENABLE_ASSERTIONS "Enable assertions" ON)
 endif()
 
 option(LLVM_ENABLE_EXPENSIVE_CHECKS "Enable expensive checks" OFF)
 
 # While adding scalable vector support to LLVM, we temporarily want to
-# allow an implicit conversion of TypeSize to uint64_t. This CMake flag
-# enables a more strict conversion where it asserts that the type is not
-# a scalable vector type.
+# allow an implicit conversion of TypeSize to uint64_t, and to allow
+# code to get the fixed number of elements from a possibly scalable vector.
+# This CMake flag enables a more strict mode where it asserts that the type
+# is not a scalable vector type.
 #
 # Enabling this flag makes it easier to find cases where the compiler makes
 # assumptions on the size being 'fixed size', when building tests for
 # SVE/SVE2 or other scalable vector architectures.
 option(LLVM_ENABLE_STRICT_FIXED_SIZE_VECTORS
-       "Enable assertions that type is not scalable in implicit conversion from TypeSize to uint64_t" OFF)
+       "Enable assertions that type is not scalable in implicit conversion from TypeSize to uint64_t and calls to getNumElements" OFF)
 
 set(LLVM_ABI_BREAKING_CHECKS "WITH_ASSERTS" CACHE STRING
   "Enable abi-breaking checks.  Can be WITH_ASSERTS, FORCE_ON or FORCE_OFF.")
 
 option(LLVM_FORCE_USE_OLD_TOOLCHAIN
        "Set to ON to force using an old, unsupported host toolchain." OFF)
 
 option(LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN

llvm/include/llvm/IR/DerivedTypes.h

 
 public:
   VectorType(const VectorType &) = delete;
   VectorType &operator=(const VectorType &) = delete;
 
   /// Get the number of elements in this vector. It does not make sense to call
   /// this function on a scalable vector, and this will be moved into
   /// FixedVectorType in a future commit
-  unsigned getNumElements() const { return EC.Min; }
+  unsigned getNumElements() const {
+    ElementCount EC = getElementCount();
+#ifdef STRICT_FIXED_SIZE_VECTORS
+    assert(!EC.Scalable &&
+           "Request for fixed number of elements from scalable vector");
+    return EC.Min;
+#else
+    if (EC.Scalable)
+      WithColor::warning()
+          << "The code that requested the fixed number of elements has made "
+             "the assumption that this vector is not scalable. This assumption "
+             "was not correct, and this may lead to broken code\n";
+    return EC.Min;
+#endif
+  }
 
   Type *getElementType() const { return ContainedType; }
 
   /// This static method is the primary way to construct an VectorType.
   static VectorType *get(Type *ElementType, ElementCount EC);
   static VectorType *get(Type *ElementType, unsigned NumElements,
                          bool Scalable = false) {
     return VectorType::get(ElementType, {NumElements, Scalable});