[llvm-commits] [llvm] r154983 - /llvm/trunk/lib/Transforms/Scalar/LoopStrengthReduce.cpp
Andrew Trick
atrick at apple.com
Tue Apr 17 21:00:10 PDT 2012
Author: atrick
Date: Tue Apr 17 23:00:10 2012
New Revision: 154983
URL: http://llvm.org/viewvc/llvm-project?rev=154983&view=rev
Log:
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.
Modified:
llvm/trunk/lib/Transforms/Scalar/LoopStrengthReduce.cpp
Modified: llvm/trunk/lib/Transforms/Scalar/LoopStrengthReduce.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopStrengthReduce.cpp?rev=154983&r1=154982&r2=154983&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopStrengthReduce.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LoopStrengthReduce.cpp Tue Apr 17 23:00:10 2012
@@ -77,6 +77,12 @@
#include <algorithm>
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 @@
// 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.
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