[Analysis] Ensure getTypeLegalizationCost returns a simple VT for TypeScalarizeScalableVector

When getTypeConversion returns TypeScalarizeScalableVector we were
sometimes returning a non-simple type from getTypeLegalizationCost.
However, many callers depend upon this being a simple type and will
crash if not. This patch changes getTypeLegalizationCost to ensure
that we always a return sensible simple VT. If the vector type
contains unusual integer types, e.g. <vscale x 2 x i3>, then we just
set the type to MVT::i64 as a reasonable default.

A test has been added here that demonstrates the vectoriser can
correctly calculate the cost of vectorising a "zext i3 to i64"
instruction with a VF=vscale x 1:

  Transforms/LoopVectorize/AArch64/sve-inductions-unusual-types.ll

Differential Revision: https://reviews.llvm.org/D113777

1 files changed, 6 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/TargetLoweringBase.cpp b/llvm/lib/CodeGen/TargetLoweringBase.cpp
index 0dd45be..c0a7eff 100644
--- a/llvm/lib/CodeGen/TargetLoweringBase.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringBase.cpp
@@ -1856,8 +1856,12 @@ TargetLoweringBase::getTypeLegalizationCost(const DataLayout &DL,
   while (true) {
     LegalizeKind LK = getTypeConversion(C, MTy);
 
-    if (LK.first == TypeScalarizeScalableVector)
-      return std::make_pair(InstructionCost::getInvalid(), MVT::getVT(Ty));
+    if (LK.first == TypeScalarizeScalableVector) {
+      // Ensure we return a sensible simple VT here, since many callers of this
+      // function require it.
+      MVT VT = MTy.isSimple() ? MTy.getSimpleVT() : MVT::i64;
+      return std::make_pair(InstructionCost::getInvalid(), VT);
+    }
 
     if (LK.first == TypeLegal)
       return std::make_pair(Cost, MTy.getSimpleVT());