[HashRecognize] Check TC against bitwidth of LHSAux

The trip-count of a CRC algorithm can legitimiately be greater than the
bitwidth of the result: what it cannot exceed is the bitwidth of the
data, or LHSAux. crc8.le.tc16 is now successfully recognized as a CRC
algorithm.

Author: artagnon

llvm/lib/Analysis/HashRecognize.cpp

@@ -247,16 +247,10 @@ KnownBits ValueEvolution::compute(const Value *V) {
 }
 
 bool ValueEvolution::computeEvolutions(ArrayRef<PhiStepPair> PhiEvolutions) {
-  for (unsigned I = 0; I < TripCount; ++I) {
-    for (auto [Phi, Step] : PhiEvolutions) {
-      KnownBits KnownAtIter = computeInstr(Step);
-      if (KnownAtIter.getBitWidth() < I + 1) {
-        ErrStr = "Loop iterations exceed bitwidth of result";
-        return false;
-      }
-      KnownPhis.emplace_or_assign(Phi, KnownAtIter);
-    }
-  }
+  for (unsigned I = 0; I < TripCount; ++I)
+    for (auto [Phi, Step] : PhiEvolutions)
+      KnownPhis.emplace_or_assign(Phi, computeInstr(Step));
+
   return ErrStr.empty();
 }
 
@@ -605,6 +599,13 @@ HashRecognize::recognizeCRC() const {
       return "Recurrences not intertwined with XOR";
   }
 
+  // Make sure that the TC doesn't exceed the bitwidth of LHSAux, or LHS.
+  Value *LHS = ConditionalRecurrence.Start;
+  Value *LHSAux = SimpleRecurrence ? SimpleRecurrence.Start : nullptr;
+  if (TC > (LHSAux ? LHSAux->getType()->getIntegerBitWidth()
+                   : LHS->getType()->getIntegerBitWidth()))
+    return "Loop iterations exceed bitwidth of data";
+
   // Make sure that the computed value is used in the exit block: this should be
   // true even if it is only really used in an outer loop's exit block, since
   // the loop is in LCSSA form.
@@ -633,13 +634,13 @@ HashRecognize::recognizeCRC() const {
     return VE.getError();
   KnownBits ResultBits = VE.KnownPhis.at(ConditionalRecurrence.Phi);
 
+  unsigned N = std::min(TC, ResultBits.getBitWidth());
   auto IsZero = [](const KnownBits &K) { return K.isZero(); };
-  if (!checkExtractBits(ResultBits, TC, IsZero, *ByteOrderSwapped))
+  if (!checkExtractBits(ResultBits, N, IsZero, *ByteOrderSwapped))
     return ErrBits(ResultBits, TC, *ByteOrderSwapped);
 
-  Value *LHSAux = SimpleRecurrence ? SimpleRecurrence.Start : nullptr;
-  return PolynomialInfo(TC, ConditionalRecurrence.Start, GenPoly, ComputedValue,
-                        *ByteOrderSwapped, LHSAux);
+  return PolynomialInfo(TC, LHS, GenPoly, ComputedValue, *ByteOrderSwapped,
+                        LHSAux);
 }
 
 void CRCTable::print(raw_ostream &OS) const {

llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll

@@ -146,8 +146,28 @@ exit:                                              ; preds = %loop
 
 define i8 @crc8.le.tc16(i16 %msg, i8 %checksum) {
 ; CHECK-LABEL: 'crc8.le.tc16'
-; CHECK-NEXT:  Did not find a hash algorithm
-; CHECK-NEXT:  Reason: Loop iterations exceed bitwidth of result
+; CHECK-NEXT:  Found little-endian CRC-8 loop with trip count 16
+; CHECK-NEXT:    Initial CRC: i8 %checksum
+; CHECK-NEXT:    Generating polynomial: 29
+; CHECK-NEXT:    Computed CRC: %crc.next = select i1 %check.sb, i8 %crc.lshr, i8 %crc.xor
+; CHECK-NEXT:    Auxiliary data: i16 %msg
+; CHECK-NEXT:    Computed CRC lookup table:
+; CHECK-NEXT:  0 9 18 27 31 22 13 4 5 12 23 30 26 19 8 1
+; CHECK-NEXT:  10 3 24 17 21 28 7 14 15 6 29 20 16 25 2 11
+; CHECK-NEXT:  20 29 6 15 11 2 25 16 17 24 3 10 14 7 28 21
+; CHECK-NEXT:  30 23 12 5 1 8 19 26 27 18 9 0 4 13 22 31
+; CHECK-NEXT:  19 26 1 8 12 5 30 23 22 31 4 13 9 0 27 18
+; CHECK-NEXT:  25 16 11 2 6 15 20 29 28 21 14 7 3 10 17 24
+; CHECK-NEXT:  7 14 21 28 24 17 10 3 2 11 16 25 29 20 15 6
+; CHECK-NEXT:  13 4 31 22 18 27 0 9 8 1 26 19 23 30 5 12
+; CHECK-NEXT:  29 20 15 6 2 11 16 25 24 17 10 3 7 14 21 28
+; CHECK-NEXT:  23 30 5 12 8 1 26 19 18 27 0 9 13 4 31 22
+; CHECK-NEXT:  9 0 27 18 22 31 4 13 12 5 30 23 19 26 1 8
+; CHECK-NEXT:  3 10 17 24 28 21 14 7 6 15 20 29 25 16 11 2
+; CHECK-NEXT:  14 7 28 21 17 24 3 10 11 2 25 16 20 29 6 15
+; CHECK-NEXT:  4 13 22 31 27 18 9 0 1 8 19 26 30 23 12 5
+; CHECK-NEXT:  26 19 8 1 5 12 23 30 31 22 13 4 0 9 18 27
+; CHECK-NEXT:  16 25 2 11 15 6 29 20 21 28 7 14 10 3 24 17
 ;
 entry:
   br label %loop
@@ -679,7 +699,7 @@ exit:                                              ; preds = %loop
 define i16 @not.crc.excess.tc(i16 %msg, i16 %checksum) {
 ; CHECK-LABEL: 'not.crc.excess.tc'
 ; CHECK-NEXT:  Did not find a hash algorithm
-; CHECK-NEXT:  Reason: Loop iterations exceed bitwidth of result
+; CHECK-NEXT:  Reason: Loop iterations exceed bitwidth of data
 ;
 entry:
   br label %loop