From 19f80c1e7e99f42959cc74892af73e0aad373d20 Mon Sep 17 00:00:00 2001 From: Andrew Trick Date: Wed, 18 Apr 2012 04:00:10 +0000 Subject: [PATCH] loop-reduce: Add an early bailout to catch extremely large loops. This introduces a threshold of 200 IV Users, which is very conservative but should be sufficient to avoid serious compile time sink or stack overflow. The llvm test-suite with LTO never exceeds 190 users per loop. The bug doesn't relate to a specific type of loop. Checking in an arbitrary giant loop as a unit test would be silly. Fixes rdar://11262507. llvm-svn: 154983 --- .../Transforms/Scalar/LoopStrengthReduce.cpp | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp index d57ec22f44ab..fe4700b36e78 100644 --- a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp +++ b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp @@ -77,6 +77,12 @@ #include using namespace llvm; +/// MaxIVUsers is an arbitrary threshold that provides an early opportunitiy for +/// bail out. This threshold is far beyond the number of users that LSR can +/// conceivably solve, so it should not affect generated code, but catches the +/// worst cases before LSR burns too much compile time and stack space. +static const unsigned MaxIVUsers = 200; + // Temporary flag to cleanup congruent phis after LSR phi expansion. // It's currently disabled until we can determine whether it's truly useful or // not. The flag should be removed after the v3.0 release. @@ -4519,6 +4525,17 @@ LSRInstance::LSRInstance(const TargetLowering *tli, Loop *l, Pass *P) // If there's no interesting work to be done, bail early. if (IU.empty()) return; + // If there's too much analysis to be done, bail early. We won't be able to + // model the problem anyway. + unsigned NumUsers = 0; + for (IVUsers::const_iterator UI = IU.begin(), E = IU.end(); UI != E; ++UI) { + if (++NumUsers > MaxIVUsers) { + DEBUG(dbgs() << "LSR skipping loop, too many IV Users in " << *L + << "\n"); + return; + } + } + #ifndef NDEBUG // All dominating loops must have preheaders, or SCEVExpander may not be able // to materialize an AddRecExpr whose Start is an outer AddRecExpr.