Reland "[TargetLowering] Teach DemandedBits about VSCALE"

Reland with a fixup to avoid converting APInts to int64_t which allowed for
overflows (UB) with sufficiently high/low multiplier values.

This allows DemandedBits to see the result of VSCALE will be at most
VScaleMax * some compile-time constant. This relies on the vscale_range()
attribute being present on the function, with a max set. (This is done by
default when clang is targeting AArch64+SVE).

Using this various redundant operations (zexts, sexts, ands, ors, etc)
can be eliminated.

Differential Revision: https://reviews.llvm.org/D138508

1 files changed, 17 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 202178e..a0e7705 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -1125,6 +1125,23 @@ bool TargetLowering::SimplifyDemandedBits(
 
   KnownBits Known2;
   switch (Op.getOpcode()) {
+  case ISD::VSCALE: {
+    Function const &F = TLO.DAG.getMachineFunction().getFunction();
+    Attribute const &Attr = F.getFnAttribute(Attribute::VScaleRange);
+    if (!Attr.isValid())
+      return false;
+    std::optional<unsigned> MaxVScale = Attr.getVScaleRangeMax();
+    if (!MaxVScale.has_value())
+      return false;
+    APInt VScaleResultUpperbound = *MaxVScale * Op.getConstantOperandAPInt(0);
+    bool Negative = VScaleResultUpperbound.isNegative();
+    if (Negative)
+      VScaleResultUpperbound = ~VScaleResultUpperbound;
+    unsigned RequiredBits = VScaleResultUpperbound.getActiveBits();
+    if (RequiredBits < BitWidth)
+      (Negative ? Known.One : Known.Zero).setHighBits(BitWidth - RequiredBits);
+    return false;
+  }
   case ISD::SCALAR_TO_VECTOR: {
     if (VT.isScalableVector())
       return false;