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Record CodeGen optimization level in the BranchFolding pass so that we can

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use it to control tail merging when there is a tradeoff between performance
and code size.  When there is only 1 instruction in the common tail, we have
been merging.  That can be good for code size but is a definite loss for
performance.  Now we will avoid tail merging in that case when the
optimization level is "Aggressive", i.e., "-O3".  Radar 7338114.

Since the IfConversion pass invokes BranchFolding, it too needs to know
the optimization level.  Note that I removed the RegisterPass instantiation
for IfConversion because it required a default constructor.  If someone
wants to keep that for some reason, we can add a default constructor with
a hard-wired optimization level.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85346 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bob Wilson
2009-10-27 23:49:38 +00:00
parent 03236140fa
commit cd4f04d6bc
6 changed files with 29 additions and 19 deletions
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14
lib/CodeGen/BranchFolding.cpp
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@@ -50,8 +50,9 @@ TailMergeThreshold("tail-merge-threshold",
char BranchFolderPass::ID = 0;
FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
return new BranchFolderPass(DefaultEnableTailMerge);
FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge,
CodeGenOpt::Level OptLevel) {
return new BranchFolderPass(DefaultEnableTailMerge, OptLevel);
}
bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
@@ -63,7 +64,8 @@ bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
BranchFolder::BranchFolder(bool defaultEnableTailMerge) {
BranchFolder::BranchFolder(bool defaultEnableTailMerge, CodeGenOpt::Level OL) {
OptLevel = OL;
switch (FlagEnableTailMerge) {
case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
case cl::BOU_TRUE: EnableTailMerge = true; break;
@@ -470,7 +472,8 @@ unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
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
// minCommonTailLength instructions in common; if not, and if we are not
// optimizing for performance at the expense of code size, 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.
@@ -478,7 +481,8 @@ unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
// tail if there is one.
// (Empty blocks will get forwarded and need not be considered.)
if (CommonTailLen >= minCommonTailLength ||
(CommonTailLen > 0 &&
(OptLevel != CodeGenOpt::Aggressive &&
CommonTailLen > 0 &&
(TrialBBI1==CurMPIter->second->begin() ||
TrialBBI2==I->second->begin()))) {
if (CommonTailLen > maxCommonTailLength) {