[RISCV]Preserve (and X, 0xffff) in targetShrinkDemandedConstant

shrinkdemandedconstant does some optimizations, but is not very friendly to riscv, targetShrinkDemandedConstant to limit the damage.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D134155

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -10041,12 +10041,11 @@ bool RISCVTargetLowering::targetShrinkDemandedConstant(
 
   // And has a few special cases for zext.
   if (Opcode == ISD::AND) {
-    // Preserve (and X, 0xffff) when zext.h is supported.
-    if (Subtarget.hasStdExtZbb() || Subtarget.hasStdExtZbp()) {
-      APInt NewMask = APInt(Mask.getBitWidth(), 0xffff);
-      if (IsLegalMask(NewMask))
-        return UseMask(NewMask);
-    }
+    // Preserve (and X, 0xffff), if zext.h exists use zext.h,
+    // otherwise use SLLI + SRLI.
+    APInt NewMask = APInt(Mask.getBitWidth(), 0xffff);
+    if (IsLegalMask(NewMask))
+      return UseMask(NewMask);
 
     // Try to preserve (and X, 0xffffffff), the (zext_inreg X, i32) pattern.
     if (VT == MVT::i64) {

llvm/test/CodeGen/RISCV/rv64i-demanded-bits.ll

@@ -140,3 +140,16 @@ define signext i32 @andi_srliw(i32 signext %0, ptr %1, i32 signext %2) {
   %6 = add i32 %4, %2
   ret i32 %6
 }
+
+define i32 @and_or(i32 signext %x) {
+; CHECK-LABEL: and_or:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    ori a0, a0, 255
+; CHECK-NEXT:    slli a0, a0, 48
+; CHECK-NEXT:    srli a0, a0, 48
+; CHECK-NEXT:    ret
+entry:
+  %and = and i32 %x, 65280
+  %or = or i32 %and, 255
+  ret i32 %or
+}