[AMDGPU] Compute GISel KnownBits for S_BFE instructions

Author: Pierre-vh

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -16502,12 +16502,57 @@ static void knownBitsForWorkitemID(const GCNSubtarget &ST,
   Known.Zero.setHighBits(llvm::countl_zero(MaxValue));
 }
 
+static void knownBitsForSBFE(const MachineInstr &MI, GISelValueTracking &VT,
+                             KnownBits &Known, const APInt &DemandedElts,
+                             unsigned BFEWidth, bool SExt) {
+  const MachineRegisterInfo &MRI = VT.getMachineFunction().getRegInfo();
+  const MachineOperand &Src1 = MI.getOperand(2);
+
+  unsigned Src1Cst = 0;
+  if (Src1.isImm())
+    Src1Cst = Src1.getImm();
+  else if (Src1.isReg()) {
+    auto Cst = getIConstantVRegValWithLookThrough(Src1.getReg(), MRI);
+    if (!Cst)
+      return;
+    Src1Cst = Cst->Value.getZExtValue();
+  } else
+    return;
+
+  const unsigned Mask = maskTrailingOnes<unsigned>(6);
+  const unsigned Offset = Src1Cst & Mask;
+  const unsigned Width = (Src1Cst >> 16) & Mask;
+
+  VT.computeKnownBitsImpl(MI.getOperand(1).getReg(), Known, DemandedElts);
+
+  const uint64_t WidthMask = maskTrailingOnes<uint64_t>(Width);
+  Known.Zero = Known.Zero.shl(Offset) & WidthMask;
+  Known.One = Known.One.shl(Offset) & WidthMask;
+
+  if (SExt)
+    Known.sextInReg(Width);
+  else
+    Known.Zero |= maskLeadingOnes<unsigned>(BFEWidth - Width);
+}
+
 void SITargetLowering::computeKnownBitsForTargetInstr(
     GISelValueTracking &VT, Register R, KnownBits &Known,
     const APInt &DemandedElts, const MachineRegisterInfo &MRI,
     unsigned Depth) const {
   const MachineInstr *MI = MRI.getVRegDef(R);
   switch (MI->getOpcode()) {
+  case AMDGPU::S_BFE_I32:
+    return knownBitsForSBFE(*MI, VT, Known, DemandedElts, /*Width=*/32,
+                            /*SExt=*/true);
+  case AMDGPU::S_BFE_U32:
+    return knownBitsForSBFE(*MI, VT, Known, DemandedElts, /*Width=*/32,
+                            /*SExt=*/false);
+  case AMDGPU::S_BFE_I64:
+    return knownBitsForSBFE(*MI, VT, Known, DemandedElts, /*Width=*/64,
+                            /*SExt=*/true);
+  case AMDGPU::S_BFE_U64:
+    return knownBitsForSBFE(*MI, VT, Known, DemandedElts, /*Width=*/64,
+                            /*SExt=*/false);
   case AMDGPU::G_INTRINSIC:
   case AMDGPU::G_INTRINSIC_CONVERGENT: {
     Intrinsic::ID IID = cast<GIntrinsic>(MI)->getIntrinsicID();