[llvm] [LoopUnroll] Introduce UnrollMaxIterations as a hard cap on how many iterations we try to unroll (PR #78648)

[Nikita Popov via llvm-commits]

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@@ -453,6 +457,14 @@ LoopUnrollResult llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
     }
   }
 
+  // Certain cases with UBSAN can cause trip count to be calculated as INT_MAX,
+  // Block unrolling at a reasonable limit so that the compiler doesn't hang
+  // trying to unroll the loop. See PR77842
+  if (ULO.Count > UnrollMaxIterations) {
+    LLVM_DEBUG(dbgs() << "Won't unroll; trip count is too large\n");
+    return LoopUnrollResult::Unmodified;
+  }
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nikic wrote:

This check should be in Scalar/LoopUnrollPass, not Utils/LoopUnroll. That's where all the other unroll count thresholds are implemented.

https://github.com/llvm/llvm-project/pull/78648