[LoopInfo] Limit the iterations to check whether a loop has dedicated exits

for extreme large case.

We had a case that a single loop which has 4000 exits and the average number
of predecessors of each exit is > 1000, and we found compiling the case spent
a significant amount of time on checking whether a loop has dedicated exits.
This patch adds a limit for the iterations to the check. With the patch, the
time to compile our testcase reduced from 1000s to 200s (clang release build).

Differential Revision: https://reviews.llvm.org/D67359

llvm-svn: 372990

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/LoopInfo.cpp b/llvm/lib/Analysis/LoopInfo.cpp
index 419f013c0e7..5d481a4131a 100644
--- a/llvm/lib/Analysis/LoopInfo.cpp
+++ b/llvm/lib/Analysis/LoopInfo.cpp
@@ -54,6 +54,13 @@ static cl::opt<bool, true>
     VerifyLoopInfoX("verify-loop-info", cl::location(VerifyLoopInfo),
                     cl::Hidden, cl::desc("Verify loop info (time consuming)"));
 
+namespace llvm {
+cl::opt<uint64_t> MaxDedicateExitIterations(
+    "max-dedicate-exit-iterations", cl::Hidden, cl::init(100000),
+    cl::desc("Max number of loop iterations to check whether a loop is a "
+             "dedicated exit loop. "));
+}
+
 //===----------------------------------------------------------------------===//
 // Loop implementation
 //