[InstCombine] Fold zext-of-icmp with no shift

This regression triggers after commit f400daa to fix infinite loop issue.

In this case, we can known the shift count is 0, so it will not be
triggered by the form of (iN (~X) u>> (N - 1)) in commit 21d3871, of
which N indicates the data type bitwidth of X.

Fixes #68465

Author: vfdff

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -904,19 +904,18 @@ Instruction *InstCombinerImpl::transformZExtICmp(ICmpInst *Cmp,
     // zext (X == 0) to i32 --> (X>>1)^1 iff X has only the 2nd bit set.
     // zext (X != 0) to i32 --> X        iff X has only the low bit set.
     // zext (X != 0) to i32 --> X>>1     iff X has only the 2nd bit set.
-    if (Op1CV->isZero() && Cmp->isEquality() &&
-        (Cmp->getOperand(0)->getType() == Zext.getType() ||
-         Cmp->getPredicate() == ICmpInst::ICMP_NE)) {
-      // If Op1C some other power of two, convert:
-      KnownBits Known = computeKnownBits(Cmp->getOperand(0), 0, &Zext);
 
+    if (Op1CV->isZero() && Cmp->isEquality()) {
       // Exactly 1 possible 1? But not the high-bit because that is
       // canonicalized to this form.
+      KnownBits Known = computeKnownBits(Cmp->getOperand(0), 0, &Zext);
       APInt KnownZeroMask(~Known.Zero);
-      if (KnownZeroMask.isPowerOf2() &&
-          (Zext.getType()->getScalarSizeInBits() !=
-           KnownZeroMask.logBase2() + 1)) {
-        uint32_t ShAmt = KnownZeroMask.logBase2();
+      uint32_t ShAmt = KnownZeroMask.logBase2();
+      bool IsExpectShAmt = KnownZeroMask.isPowerOf2() &&
+                           (Zext.getType()->getScalarSizeInBits() != ShAmt + 1);
+      if (IsExpectShAmt &&
+          (Cmp->getOperand(0)->getType() == Zext.getType() ||
+           Cmp->getPredicate() == ICmpInst::ICMP_NE || ShAmt == 0)) {
         Value *In = Cmp->getOperand(0);
         if (ShAmt) {
           // Perform a logical shr by shiftamt.

llvm/test/Transforms/InstCombine/zext.ll

@@ -749,10 +749,11 @@ define i64 @zext_icmp_ne_bool_1(ptr %ptr) {
   ret i64 %len
 }
 
+; https://alive2.llvm.org/ce/z/k7qosS
 define i32  @zext_icmp_eq0_no_shift(ptr %ptr ) {
 ; CHECK-LABEL: @zext_icmp_eq0_no_shift(
 ; CHECK-NEXT:    [[X:%.*]] = load i8, ptr [[PTR:%.*]], align 1, !range [[RNG1:![0-9]+]]
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i8 [[X]], 0
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i8 [[X]], 1
 ; CHECK-NEXT:    [[RES:%.*]] = zext i8 [[TMP1]] to i32
 ; CHECK-NEXT:    ret i32 [[RES]]
 ;