[SelectionDAG] Prevent warnings when extracting fixed length vector from scalable.

ComputeNumSignBits and computeKnownBits both trigger "Scalable flag
may be dropped" warnings when a fixed length vector is extracted
from a scalable vector.  This patch assumes nothing about the
demanded elements thus matching the behaviour when extracting a
scalable vector from a scalable vector.

Differential Revision: https://reviews.llvm.org/D83642

Author: paulwalker-arm

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -2718,6 +2718,9 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
   case ISD::EXTRACT_SUBVECTOR: {
     // Offset the demanded elts by the subvector index.
     SDValue Src = Op.getOperand(0);
+    // Bail until we can represent demanded elements for scalable vectors.
+    if (Src.getValueType().isScalableVector())
+      break;
     uint64_t Idx = Op.getConstantOperandVal(1);
     unsigned NumSrcElts = Src.getValueType().getVectorNumElements();
     APInt DemandedSrcElts = DemandedElts.zextOrSelf(NumSrcElts).shl(Idx);
@@ -3973,6 +3976,9 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
   case ISD::EXTRACT_SUBVECTOR: {
     // Offset the demanded elts by the subvector index.
     SDValue Src = Op.getOperand(0);
+    // Bail until we can represent demanded elements for scalable vectors.
+    if (Src.getValueType().isScalableVector())
+      break;
     uint64_t Idx = Op.getConstantOperandVal(1);
     unsigned NumSrcElts = Src.getValueType().getVectorNumElements();
     APInt DemandedSrcElts = DemandedElts.zextOrSelf(NumSrcElts).shl(Idx);

llvm/test/CodeGen/AArch64/sve-fixed-length-subvector.ll

@@ -13,7 +13,10 @@
 ; RUN: llc -aarch64-sve-vector-bits-min=1664 -aarch64-enable-atomic-cfg-tidy=false < %s | FileCheck %s -check-prefixes=CHECK,VBITS_GE_512,VBITS_GE_1024
 ; RUN: llc -aarch64-sve-vector-bits-min=1792 -aarch64-enable-atomic-cfg-tidy=false < %s | FileCheck %s -check-prefixes=CHECK,VBITS_GE_512,VBITS_GE_1024
 ; RUN: llc -aarch64-sve-vector-bits-min=1920 -aarch64-enable-atomic-cfg-tidy=false < %s | FileCheck %s -check-prefixes=CHECK,VBITS_GE_512,VBITS_GE_1024
-; RUN: llc -aarch64-sve-vector-bits-min=2048 -aarch64-enable-atomic-cfg-tidy=false < %s | FileCheck %s -check-prefixes=CHECK,VBITS_GE_512,VBITS_GE_1024,VBITS_GE_2048
+; RUN: llc -aarch64-sve-vector-bits-min=2048 -aarch64-enable-atomic-cfg-tidy=false < %s 2>%t | FileCheck %s -check-prefixes=CHECK,VBITS_GE_512,VBITS_GE_1024,VBITS_GE_2048
+; RUN: FileCheck --check-prefix=WARN --allow-empty %s <%t
+
+; WARN-NOT: warning
 
 ; Test we can code generater patterns of the form:
 ;   fixed_length_vector = ISD::EXTRACT_SUBVECTOR scalable_vector, 0
@@ -85,4 +88,19 @@ bb1:
   ret void
 }
 
+;
+define <8 x i1> @no_warn_dropped_scalable(<8 x i32>* %in) #0 {
+; CHECK-LABEL: no_warn_dropped_scalable:
+; CHECK: ptrue [[PG:p[0-9]+]].s, vl8
+; CHECK: ld1w { z{{[0-9]+}}.s }, [[PG]]/z, [x0]
+; CHECK-COUNT-8: cmp w{{[0-9]+}}, #0
+; CHECK: ret
+  %a = load <8 x i32>, <8 x i32>* %in
+  br label %bb1
+
+bb1:
+  %cond = icmp sgt <8 x i32> %a, zeroinitializer
+  ret <8 x i1> %cond
+}
+
 attributes #0 = { "target-features"="+sve" }