[CodeGenPrepare] Avoid a scalable-vector crash in ctlz/cttz

[frasercrmck]

Summary

This patch fixes a crash when despeculating ctlz/cttz intrinsics with
scalable-vector types. It is not safe to speculatively get the size of
the vector type in bits in case the vector type is not a fixed-length type. As
it happens this isn't required as vector types are skipped anyway.

[RKSimon]

LGTM

llvm/lib/CodeGen/CodeGenPrepare.cpp

static bool despeculateCountZeros(IntrinsicInst *CountZeros,
   // If it's cheap to speculate, there's nothing to do.
   auto IntrinsicID = CountZeros->getIntrinsicID();
   if ((IntrinsicID == Intrinsic::cttz && TLI->isCheapToSpeculateCttz()) ||
       (IntrinsicID == Intrinsic::ctlz && TLI->isCheapToSpeculateCtlz()))
     return false;
 
   // Only handle legal scalar cases. Anything else requires too much work.
   Type *Ty = CountZeros->getType();
-  unsigned SizeInBits = Ty->getPrimitiveSizeInBits();
+  unsigned SizeInBits = Ty->getScalarSizeInBits();
   if (Ty->isVectorTy() || SizeInBits > DL->getLargestLegalIntTypeSizeInBits())
     return false;
 
   // Bail if the value is never zero.
   if (llvm::isKnownNonZero(CountZeros->getOperand(0), *DL))
     return false;
 
   // The intrinsic will be sunk behind a compare against zero and branch.

llvm/test/Transforms/CodeGenPrepare/RISCV/cttz-ctlz.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -codegenprepare < %s | FileCheck %s
+
+target triple = "riscv64-unknown-unknown"
+
+; Check that despeculating count-zeros intrinsics doesn't crash when those
+; intrinsics use scalable types.
+
+define <vscale x 4 x i64> @cttz_nxv4i64(<vscale x 4 x i64> %x) {
+; CHECK-LABEL: @cttz_nxv4i64(
+; CHECK-NEXT:    [[Z:%.*]] = call <vscale x 4 x i64> @llvm.cttz.nxv4i64(<vscale x 4 x i64> [[X:%.*]], i1 false)
+; CHECK-NEXT:    ret <vscale x 4 x i64> [[Z]]
+;
+  %z = call <vscale x 4 x i64> @llvm.cttz.nxv4i64(<vscale x 4 x i64> %x, i1 false)
+  ret <vscale x 4 x i64> %z
+}
+
+define <vscale x 4 x i64> @ctlz_nxv4i64(<vscale x 4 x i64> %x) {
+; CHECK-LABEL: @ctlz_nxv4i64(
+; CHECK-NEXT:    [[Z:%.*]] = call <vscale x 4 x i64> @llvm.ctlz.nxv4i64(<vscale x 4 x i64> [[X:%.*]], i1 false)
+; CHECK-NEXT:    ret <vscale x 4 x i64> [[Z]]
+;
+  %z = call <vscale x 4 x i64> @llvm.ctlz.nxv4i64(<vscale x 4 x i64> %x, i1 false)
+  ret <vscale x 4 x i64> %z
+}
+
+declare <vscale x 4 x i64> @llvm.cttz.nxv4i64(<vscale x 4 x i64>, i1)
+declare <vscale x 4 x i64> @llvm.ctlz.nxv4i64(<vscale x 4 x i64>, i1)

llvm/test/Transforms/CodeGenPrepare/RISCV/lit.local.cfg

+if not 'RISCV' in config.root.targets:
+    config.unsupported = True