[PATCH] D124655: [IR][CostModel] A scalable vector shuffle can't be an identity shuffle.

[Craig Topper via Phabricator via llvm-commits]

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Even if the minimum number of elements is 1 and the length doesn't change,
we don't know what vscale is so its not guaranteed to be an identity
shuffle. Since the only possible shuffle mask value is 0 or undef, it's
really a broadcast or undefined.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D124655

Files:
  llvm/include/llvm/IR/Instructions.h
  llvm/test/Analysis/CostModel/RISCV/rvv-shuffle.ll


Index: llvm/test/Analysis/CostModel/RISCV/rvv-shuffle.ll
===================================================================
--- llvm/test/Analysis/CostModel/RISCV/rvv-shuffle.ll
+++ llvm/test/Analysis/CostModel/RISCV/rvv-shuffle.ll
@@ -11,7 +11,7 @@
 ; CHECK-NEXT:  Cost Model: Invalid cost for instruction: %3 = shufflevector <vscale x 8 x i16> undef, <vscale x 8 x i16> undef, <vscale x 8 x i32> zeroinitializer
 ; CHECK-NEXT:  Cost Model: Invalid cost for instruction: %4 = shufflevector <vscale x 2 x i32> undef, <vscale x 2 x i32> undef, <vscale x 2 x i32> zeroinitializer
 ; CHECK-NEXT:  Cost Model: Invalid cost for instruction: %5 = shufflevector <vscale x 4 x i32> undef, <vscale x 4 x i32> undef, <vscale x 4 x i32> zeroinitializer
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %6 = shufflevector <vscale x 1 x i64> undef, <vscale x 1 x i64> undef, <vscale x 1 x i32> zeroinitializer
+; CHECK-NEXT:  Cost Model: Invalid cost for instruction: %6 = shufflevector <vscale x 1 x i64> undef, <vscale x 1 x i64> undef, <vscale x 1 x i32> zeroinitializer
 ; CHECK-NEXT:  Cost Model: Invalid cost for instruction: %7 = shufflevector <vscale x 2 x i64> undef, <vscale x 2 x i64> undef, <vscale x 2 x i32> zeroinitializer
 ; CHECK-NEXT:  Cost Model: Invalid cost for instruction: %8 = shufflevector <vscale x 16 x i1> undef, <vscale x 16 x i1> undef, <vscale x 16 x i32> zeroinitializer
 ; CHECK-NEXT:  Cost Model: Invalid cost for instruction: %9 = shufflevector <vscale x 8 x i1> undef, <vscale x 8 x i1> undef, <vscale x 8 x i32> zeroinitializer
Index: llvm/include/llvm/IR/Instructions.h
===================================================================
--- llvm/include/llvm/IR/Instructions.h
+++ llvm/include/llvm/IR/Instructions.h
@@ -2137,6 +2137,12 @@
   static bool isIdentityMask(ArrayRef<int> Mask);
   static bool isIdentityMask(const Constant *Mask) {
     assert(Mask->getType()->isVectorTy() && "Shuffle needs vector constant.");
+
+    // Not possible to express a shuffle mask for a scalable vector for this
+    // case.
+    if (isa<ScalableVectorType>(Mask->getType()))
+      return false;
+
     SmallVector<int, 16> MaskAsInts;
     getShuffleMask(Mask, MaskAsInts);
     return isIdentityMask(MaskAsInts);
@@ -2147,6 +2153,11 @@
   /// from its input vectors.
   /// Example: shufflevector <4 x n> A, <4 x n> B, <4,undef,6,undef>
   bool isIdentity() const {
+    // Not possible to express a shuffle mask for a scalable vector for this
+    // case.
+    if (isa<ScalableVectorType>(getType()))
+      return false;
+
     return !changesLength() && isIdentityMask(ShuffleMask);
   }
 


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