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Reimplement BranchFolding change to avoid tail merging for a 1 instruction

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common tail, except when the OptimizeForSize function attribute is present.
Radar 7338114.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85441 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bob Wilson
2009-10-28 22:10:20 +00:00
parent e841d2f867
commit 2c04dae715
4 changed files with 18 additions and 16 deletions
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lib/CodeGen/BranchFolding.cpp
+15 -13
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@@ -18,6 +18,7 @@
#define DEBUG_TYPE "branchfolding"
#include "BranchFolding.h"
#include "llvm/Function.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
@@ -465,22 +466,23 @@ unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
CurMPIter!=B && CurMPIter->first==CurHash;
--CurMPIter) {
for (MPIterator I = prior(CurMPIter); I->first==CurHash ; --I) {
unsigned CommonTailLen = ComputeCommonTailLength(
CurMPIter->second,
I->second,
TrialBBI1, TrialBBI2);
unsigned CommonTailLen = ComputeCommonTailLength(CurMPIter->second,
I->second,
TrialBBI1, TrialBBI2);
// If we will have to split a block, there should be at least
// minCommonTailLength instructions in common; if not, at worst
// we will be replacing a fallthrough into the common tail with a
// branch, which at worst breaks even with falling through into
// the duplicated common tail, so 1 instruction in common is enough.
// We will always pick a block we do not have to split as the common
// tail if there is one.
// (Empty blocks will get forwarded and need not be considered.)
// minCommonTailLength instructions in common. Otherwise, if we are
// optimizing for code size, 1 instruction in common is enough. At
// worst we will be replacing a fallthrough into the common tail with a
// branch, which at worst breaks even with falling through into the
// duplicated common tail. We will always pick a block we do not have
// to split as the common tail if there is one. (Empty blocks will get
// forwarded and need not be considered.)
MachineFunction *MF = CurMPIter->second->getParent();
if (CommonTailLen >= minCommonTailLength ||
(CommonTailLen > 0 &&
(TrialBBI1==CurMPIter->second->begin() ||
TrialBBI2==I->second->begin()))) {
MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
(TrialBBI1 == CurMPIter->second->begin() ||
TrialBBI2 == I->second->begin()))) {
if (CommonTailLen > maxCommonTailLength) {
SameTails.clear();
maxCommonTailLength = CommonTailLen;