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Don't attempt to split subloops out of a loop with a huge number of backedges.

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Not only will this take huge amounts of compile time, the resultant loop nests
won't be useful for optimization.  This reduces loopsimplify time on
Transforms/LoopSimplify/2006-08-11-LoopSimplifyLongTime.ll from ~32s to ~0.4s
with a debug build of llvm on a 2.7Ghz G5.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29647 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2006-08-12 05:25:00 +00:00
parent 9289b637a4
commit 3bb4657488
1 changed files with 19 additions and 8 deletions
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27
lib/Transforms/Utils/LoopSimplify.cpp
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@ -168,6 +168,8 @@ bool LoopSimplify::runOnFunction(Function &F) {
///
bool LoopSimplify::ProcessLoop(Loop *L) {
bool Changed = false;
ReprocessLoop:
// Canonicalize inner loops before outer loops. Inner loop canonicalization
// can provide work for the outer loop to canonicalize.
for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
@ -208,16 +210,25 @@ bool LoopSimplify::ProcessLoop(Loop *L) {
// If the header has more than two predecessors at this point (from the
// preheader and from multiple backedges), we must adjust the loop.
if (L->getNumBackEdges() != 1) {
// If this is really a nested loop, rip it out into a child loop.
if (Loop *NL = SeparateNestedLoop(L)) {
++NumNested;
// This is a big restructuring change, reprocess the whole loop.
ProcessLoop(NL);
return true;
unsigned NumBackedges = L->getNumBackEdges();
if (NumBackedges != 1) {
// If this is really a nested loop, rip it out into a child loop. Don't do
// this for loops with a giant number of backedges, just factor them into a
// common backedge instead.
if (NumBackedges < 8) {
if (Loop *NL = SeparateNestedLoop(L)) {
++NumNested;
// This is a big restructuring change, reprocess the whole loop.
ProcessLoop(NL);
Changed = true;
// GCC doesn't tail recursion eliminate this.
goto ReprocessLoop;
}
}
// If we either couldn't, or didn't want to, identify nesting of the loops,
// insert a new block that all backedges target, then make it jump to the
// loop header.
InsertUniqueBackedgeBlock(L);
NumInserted++;
Changed = true;