[X86] Fix a bug in TEST with immediate creation

[craig.topper]

Summary

This code tries to form a TEST from CMP+AND with an optional
truncate in between. If we looked through the truncate, we may
have extra bits in the AND mask that shouldn't participate in
the checks. Normally SimplifyDemendedBits takes care of this, but
the AND may have another user. So manually mask out any extra bits.

Fixes PR51175.

[pengfei]

I believe we do need a mask here. LGTM.

[RKSimon]

LGTM

llvm/lib/Target/X86/X86ISelDAGToDAG.cpp

 //===- X86ISelDAGToDAG.cpp - A DAG pattern matching inst selector for X86 -===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
case X86ISD::CMP: {
     // use a smaller encoding.
     // Look past the truncate if CMP is the only use of it.
     if (N0.getOpcode() == ISD::AND &&
         N0.getNode()->hasOneUse() &&
         N0.getValueType() != MVT::i8) {
       ConstantSDNode *C = dyn_cast<ConstantSDNode>(N0.getOperand(1));
       if (!C) break;
       uint64_t Mask = C->getZExtValue();
+      // We may have looked through a truncate so mask off any bits that
+      // shouldn't be part of the compare.
+      Mask &= maskTrailingOnes<uint64_t>(CmpVT.getScalarSizeInBits());
 
       // Check if we can replace AND+IMM64 with a shift. This is possible for
       // masks/ like 0xFF000000 or 0x00FFFFFF and if we care only about the zero
       // flag.
       if (CmpVT == MVT::i64 && !isInt<32>(Mask) &&
           onlyUsesZeroFlag(SDValue(Node, 0))) {
         if (isMask_64(~Mask)) {
           unsigned TrailingZeros = countTrailingZeros(Mask);

llvm/test/CodeGen/X86/pr51175.ll

 define i32 @foo(i16 signext %0, i32 %1, i32* nocapture %2) {
 ; CHECK-LABEL: foo:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    incl %edi
 ; CHECK-NEXT:    movl %edi, %eax
 ; CHECK-NEXT:    andl $65527, %eax # imm = 0xFFF7
 ; CHECK-NEXT:    movl %eax, (%rdx)
 ; CHECK-NEXT:    xorl %eax, %eax
-; CHECK-NEXT:    testl $-9, %edi
+; CHECK-NEXT:    testb $-9, %dil
 ; CHECK-NEXT:    cmovel %esi, %eax
 ; CHECK-NEXT:    retq
   %4 = add i16 %0, 1
   %5 = and i16 %4, -9
   %6 = zext i16 %5 to i32
   store i32 %6, i32* %2, align 4
   %7 = trunc i16 %5 to i8
   %8 = icmp eq i8 %7, 0
   %9 = select i1 %8, i32 %1, i32 0
   ret i32 %9
 }