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Avoid duplicating loop header which leads to unnatural loops (and just seem like general badness to me, likely to cause code explosion).

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Patch by Florian Brandner.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52223 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Evan Cheng 2008-06-11 19:07:54 +00:00
parent 4468440a2a
commit bb318c073e
2 changed files with 45 additions and 0 deletions
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18
lib/Transforms/Scalar/TailDuplication.cpp
View File

@ -33,6 +33,7 @@
#include "llvm/Support/Debug.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Analysis/LoopInfo.h"
#include <map>
using namespace llvm;
@ -50,10 +51,12 @@ namespace {
static char ID; // Pass identification, replacement for typeid
TailDup() : FunctionPass((intptr_t)&ID) {}
virtual void getAnalysisUsage(AnalysisUsage &AU) const;
private:
inline bool shouldEliminateUnconditionalBranch(TerminatorInst *, unsigned);
inline void eliminateUnconditionalBranch(BranchInst *BI);
SmallPtrSet<BasicBlock*, 4> CycleDetector;
LoopInfo *LI; // The current loop information
};
}
@ -69,6 +72,9 @@ FunctionPass *llvm::createTailDuplicationPass() { return new TailDup(); }
/// a place it already pointed to earlier; see PR 2323.
bool TailDup::runOnFunction(Function &F) {
bool Changed = false;
LI = &getAnalysis<LoopInfo>();
CycleDetector.clear();
for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
if (shouldEliminateUnconditionalBranch(I->getTerminator(),
@ -83,6 +89,10 @@ bool TailDup::runOnFunction(Function &F) {
return Changed;
}
void TailDup::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<LoopInfo>();
}
/// shouldEliminateUnconditionalBranch - Return true if this branch looks
/// attractive to eliminate. We eliminate the branch if the destination basic
/// block has <= 5 instructions in it, not counting PHI nodes. In practice,
@ -186,6 +196,14 @@ bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI,
if (!CycleDetector.insert(Dest))
return false;
// Avoid non-natural loops:
// If a loop header is duplicated, the former natural loop will contain two
// paths into the loop --> the loop it not natural anymore. We want to avoid
// this, because other optimizaions may fail to improve the loop because of
// this.
if (LI->isLoopHeader(Dest))
return false;
return true;
}

27
test/Transforms/TailDup/2008-06-11-AvoidDupLoopHeader.ll Normal file
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@ -0,0 +1,27 @@
; RUN: llvm-as < %s | opt -tailduplicate -taildup-threshold=3 -stats -disable-output |&
; RUN: not grep tailduplicate
define i32 @foo(i32 %l) nounwind {
entry:
%cond = icmp eq i32 %l, 1 ; <i1> [#uses=1]
br i1 %cond, label %bb, label %bb9
bb: ; preds = %entry
br label %bb9
bb5: ; preds = %bb9
%tmp7 = call i32 (...)* @bar( i32 %x.0 ) nounwind ; <i32> [#uses=1]
br label %bb9
bb9: ; preds = %bb5, %bb, %entry
%x.0 = phi i32 [ 0, %entry ], [ %tmp7, %bb5 ], [ 1525, %bb ] ; <i32> [#uses=2]
%l_addr.0 = phi i32 [ %l, %entry ], [ %tmp11, %bb5 ], [ %l, %bb ] ; <i32> [#uses=1]
%tmp11 = add i32 %l_addr.0, -1 ; <i32> [#uses=2]
%tmp13 = icmp eq i32 %tmp11, -1 ; <i1> [#uses=1]
br i1 %tmp13, label %bb15, label %bb5
bb15: ; preds = %bb9
ret i32 %x.0
}
declare i32 @bar(...)