[RISCV] Prevent infinite loop after D129980.

[craig.topper]

Summary

D129980 converts (seteq (i64 (and X, 0xffffffff)), C1) into
(seteq (i64 (sext_inreg X, i32)), C1). If bit 31 of X is 0, it
will be turned back into an 'and' by SimplifyDemandedBits which
can cause an infinite loop.

To prevent this, check if bit 31 is 0 with computeKnownBits before
doing the transformation.

Fixes PR56905.

[reames]

LGTM

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

if (N0.getOpcode() != ISD::AND || !N0.hasOneUse() ||
       N0.getConstantOperandVal(1) != UINT64_C(0xffffffff))
     return SDValue();
 
   // Looking for an equality compare.
   ISD::CondCode Cond = cast<CondCodeSDNode>(N->getOperand(2))->get();
   if (!isIntEqualitySetCC(Cond))
     return SDValue();
 
+  // Don't do this if the sign bit is provably zero, it will be turned back into
+  // an AND.
+  APInt SignMask = APInt::getOneBitSet(64, 31);
+  if (DAG.MaskedValueIsZero(N0.getOperand(0), SignMask))
+    return SDValue();
+
   const APInt &C1 = N1C->getAPIntValue();
 
   SDLoc dl(N);
   // If the constant is larger than 2^32 - 1 it is impossible for both sides
   // to be equal.
   if (C1.getActiveBits() > 32)
     return DAG.getBoolConstant(Cond == ISD::SETNE, dl, VT, OpVT);
 

llvm/test/CodeGen/RISCV/i64-icmp.ll

 ; RV64I-NEXT:    addi a0, a0, 2
 ; RV64I-NEXT:    snez a0, a0
 ; RV64I-NEXT:    ret
   %1 = and i64 %a, 4294967295
   %2 = icmp ne i64 %1, 4294967294
   %3 = zext i1 %2 to i64
   ret i64 %3
 }
+
+; This used to trigger an infinite loop where we toggled between 'and' and
+; 'sext_inreg'.
+define i64 @icmp_ne_zext_inreg_umin(i64 %a) nounwind {
+; RV64I-LABEL: icmp_ne_zext_inreg_umin:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    lui a1, 30141
+; RV64I-NEXT:    addiw a1, a1, -747
+; RV64I-NEXT:    bltu a0, a1, .LBB66_2
+; RV64I-NEXT:  # %bb.1:
+; RV64I-NEXT:    mv a0, a1
+; RV64I-NEXT:  .LBB66_2:
+; RV64I-NEXT:    addi a0, a0, -123
+; RV64I-NEXT:    snez a0, a0
+; RV64I-NEXT:    ret
+  %1 = call i64 @llvm.umin.i64(i64 %a, i64 123456789)
+  %2 = and i64 %1, 4294967295
+  %3 = icmp ne i64 %2, 123
+  %4 = zext i1 %3 to i64
+  ret i64 %4
+}
+declare i64 @llvm.umin.i64(i64, i64)