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

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

Reviewers: efriedma, sdesmalen, c-rhodes

Reviewed By: sdesmalen

Subscribers: hiraditya, mgorny, tschuett, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78576

CMakeLists.txt

@@ -423,15 +423,16 @@ 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.")

include/llvm/IR/DerivedTypes.h

@@ -417,7 +417,21 @@ public:
   /// 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; }
 

lib/IR/AsmWriter.cpp

@@ -653,10 +653,11 @@ void TypePrinting::print(Type *Ty, raw_ostream &OS) {
   case Type::FixedVectorTyID:
   case Type::ScalableVectorTyID: {
     VectorType *PTy = cast<VectorType>(Ty);
+    ElementCount EC = PTy->getElementCount();
     OS << "<";
-    if (PTy->isScalable())
+    if (EC.Scalable)
       OS << "vscale x ";
-    OS << PTy->getNumElements() << " x ";
+    OS << EC.Min << " x ";
     print(PTy->getElementType(), OS);
     OS << '>';
     return;

lib/IR/Function.cpp

@@ -644,10 +644,10 @@ static std::string getMangledTypeStr(Type* Ty) {
     // Ensure nested function types are distinguishable.
     Result += "f";
   } else if (VectorType* VTy = dyn_cast<VectorType>(Ty)) {
-    if (VTy->isScalable())
+    ElementCount EC = VTy->getElementCount();
+    if (EC.Scalable)
       Result += "nx";
-    Result += "v" + utostr(VTy->getNumElements()) +
-              getMangledTypeStr(VTy->getElementType());
+    Result += "v" + utostr(EC.Min) + getMangledTypeStr(VTy->getElementType());
   } else if (Ty) {
     switch (Ty->getTypeID()) {
     default: llvm_unreachable("Unhandled type");