[HashRecognize] Tighten pre-conditions for analysis

Exit early if the TC is not a byte-multiple, as optimization works by
dividing TC by 8. Also delay the SCEV TC query.

Author: artagnon

llvm/lib/Analysis/HashRecognize.cpp

@@ -561,14 +561,14 @@ std::variant<PolynomialInfo, ErrBits, StringRef>
 HashRecognize::recognizeCRC() const {
   if (!L.isInnermost())
     return "Loop is not innermost";
-  unsigned TC = SE.getSmallConstantMaxTripCount(&L);
-  if (!TC || TC > 256)
-    return "Unable to find a small constant trip count";
   BasicBlock *Latch = L.getLoopLatch();
   BasicBlock *Exit = L.getExitBlock();
   const PHINode *IndVar = L.getCanonicalInductionVariable();
-  if (!Latch || !Exit || !IndVar)
+  if (!Latch || !Exit || !IndVar || L.getNumBlocks() != 1)
     return "Loop not in canonical form";
+  unsigned TC = SE.getSmallConstantTripCount(&L);
+  if (!TC || TC > 256 || TC % 8)
+    return "Unable to find a small constant byte-multiple trip count";
 
   auto R = getRecurrences(Latch, IndVar, L);
   if (!R)

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

@@ -384,7 +384,7 @@ exit:                                              ; preds = %loop
 define i16 @not.crc.non.const.tc(i16 %crc.init, i32 %loop.limit) {
 ; CHECK-LABEL: 'not.crc.non.const.tc'
 ; CHECK-NEXT:  Did not find a hash algorithm
-; CHECK-NEXT:  Reason: Unable to find a small constant trip count
+; CHECK-NEXT:  Reason: Unable to find a small constant byte-multiple trip count
 ;
 entry:
   br label %loop
@@ -430,7 +430,7 @@ exit:                                              ; preds = %loop
 define i16 @not.crc.tc.limit(i16 %crc.init) {
 ; CHECK-LABEL: 'not.crc.tc.limit'
 ; CHECK-NEXT:  Did not find a hash algorithm
-; CHECK-NEXT:  Reason: Unable to find a small constant trip count
+; CHECK-NEXT:  Reason: Unable to find a small constant byte-multiple trip count
 ;
 entry:
   br label %loop
@@ -617,7 +617,7 @@ loop:                                              ; preds = %loop, %entry
   %crc.xor = xor i16 %crc.lshr, -24575
   %crc.next = select i1 %check.sb, i16 %crc.lshr, i16 %crc.xor
   %iv.next = add nuw nsw i8 %iv, 1
-  %exit.cond = icmp samesign ult i8 %iv, 20
+  %exit.cond = icmp samesign ult i8 %iv, 31
   br i1 %exit.cond, label %loop, label %exit
 
 exit:                                              ; preds = %loop