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loop-reduce: Add an early bailout to catch extremely large loops.

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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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154983 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Andrew Trick 2012-04-18 04:00:10 +00:00
parent 575fcb38de
commit b512263596
1 changed files with 17 additions and 0 deletions
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17
lib/Transforms/Scalar/LoopStrengthReduce.cpp
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@ -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 @@ 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.