[llvm] [HashRecognize] Strip excess-TC check (PR #157479)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: Ramkumar Ramachandra (artagnon)

<details>
<summary>Changes</summary>

Checking if trip-count exceeds 256 is no longer necessary, as we have moved away from KnownBits computations to pattern-matching, which is very cheap and independent of TC.

---
Full diff: https://github.com/llvm/llvm-project/pull/157479.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/HashRecognize.cpp (+1-1) 
- (modified) llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll (+4-4) 


``````````diff
diff --git a/llvm/lib/Analysis/HashRecognize.cpp b/llvm/lib/Analysis/HashRecognize.cpp
index 5b3448f5df35d..1b608870d28a8 100644
--- a/llvm/lib/Analysis/HashRecognize.cpp
+++ b/llvm/lib/Analysis/HashRecognize.cpp
@@ -446,7 +446,7 @@ std::variant<PolynomialInfo, StringRef> HashRecognize::recognizeCRC() const {
   if (!Latch || !Exit || !IndVar || L.getNumBlocks() != 1)
     return "Loop not in canonical form";
   unsigned TC = SE.getSmallConstantTripCount(&L);
-  if (!TC || TC > 256 || TC % 8)
+  if (!TC || TC % 8)
     return "Unable to find a small constant byte-multiple trip count";
 
   auto R = getRecurrences(Latch, IndVar, L);
diff --git a/llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll b/llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll
index 432a4d72fafb4..7dec2f8f96906 100644
--- a/llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll
+++ b/llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll
@@ -574,10 +574,10 @@ exit:                                              ; preds = %loop
   ret i16 %crc.next
 }
 
-define i16 @not.crc.tc.limit(i16 %crc.init) {
-; CHECK-LABEL: 'not.crc.tc.limit'
+define i16 @not.crc.tc.exceeds.data.bw(i16 %crc.init) {
+; CHECK-LABEL: 'not.crc.tc.exceeds.data.bw'
 ; CHECK-NEXT:  Did not find a hash algorithm
-; CHECK-NEXT:  Reason: Unable to find a small constant byte-multiple trip count
+; CHECK-NEXT:  Reason: Loop iterations exceed bitwidth of data
 ;
 entry:
   br label %loop
@@ -590,7 +590,7 @@ loop:                                              ; preds = %loop, %entry
   %check.sb = icmp slt i16 %crc, 0
   %crc.next = select i1 %check.sb, i16 %crc.xor, i16 %crc.shl
   %iv.next = add nuw nsw i32 %iv, 1
-  %exit.cond = icmp samesign ult i32 %iv, 512
+  %exit.cond = icmp samesign ult i32 %iv, 511
   br i1 %exit.cond, label %loop, label %exit
 
 exit:                                              ; preds = %loop

``````````

</details>


https://github.com/llvm/llvm-project/pull/157479