[SVE] Take constant fold fast path for splatted vscale vectors

This should be a perfectly reasonable operation for scalable vectors.
Currently, it only works for zeroinitializer values of
ScalableVectorType, but the fundamental operation is sound and it should
be possible to make it work for other splats

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D77442

Author: christetreault-llvm

llvm/lib/IR/ConstantFold.cpp

@@ -2042,18 +2042,18 @@ Constant *llvm::ConstantFoldCompareInstruction(unsigned short pred,
     }
   } else if (auto *C1VTy = dyn_cast<VectorType>(C1->getType())) {
 
-    // Do not iterate on scalable vector. The number of elements is unknown at
-    // compile-time.
-    if (isa<ScalableVectorType>(C1VTy))
-      return nullptr;
-
     // Fast path for splatted constants.
     if (Constant *C1Splat = C1->getSplatValue())
       if (Constant *C2Splat = C2->getSplatValue())
         return ConstantVector::getSplat(
             C1VTy->getElementCount(),
             ConstantExpr::getCompare(pred, C1Splat, C2Splat));
 
+    // Do not iterate on scalable vector. The number of elements is unknown at
+    // compile-time.
+    if (isa<ScalableVectorType>(C1VTy))
+      return nullptr;
+
     // If we can constant fold the comparison of each element, constant fold
     // the whole vector comparison.
     SmallVector<Constant*, 4> ResElts;

llvm/test/Transforms/InstSimplify/ConstProp/vscale.ll

@@ -208,7 +208,7 @@ define <vscale x 4 x i32> @shufflevector() {
 
 define <vscale x 2 x double> @load() {
 ; CHECK-LABEL: @load(
-; CHECK-NEXT:    [[R:%.*]] = load <vscale x 2 x double>, <vscale x 2 x double>* getelementptr (<vscale x 2 x double>, <vscale x 2 x double>* null, i64 1)
+; CHECK-NEXT:    [[R:%.*]] = load <vscale x 2 x double>, <vscale x 2 x double>* getelementptr (<vscale x 2 x double>, <vscale x 2 x double>* null, i64 1), align 16
 ; CHECK-NEXT:    ret <vscale x 2 x double> [[R]]
 ;
   %r = load <vscale x 2 x double>, <vscale x 2 x double>* getelementptr (<vscale x 2 x double>, <vscale x 2 x double>* null, i64 1)
@@ -262,7 +262,7 @@ define <vscale x 4 x i1> @icmp_undef() {
 
 define <vscale x 4 x i1> @icmp_zero() {
 ; CHECK-LABEL: @icmp_zero(
-; CHECK-NEXT:    ret <vscale x 4 x i1> icmp eq (<vscale x 4 x i32> zeroinitializer, <vscale x 4 x i32> zeroinitializer)
+; CHECK-NEXT:    ret <vscale x 4 x i1> shufflevector (<vscale x 4 x i1> insertelement (<vscale x 4 x i1> undef, i1 true, i32 0), <vscale x 4 x i1> undef, <vscale x 4 x i32> zeroinitializer)
 ;
   %r = icmp eq <vscale x 4 x i32> zeroinitializer, zeroinitializer
   ret <vscale x 4 x i1> %r