Record CodeGen optimization level in the BranchFolding pass so that we can
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
diff --git a/lib/CodeGen/BranchFolding.cpp b/lib/CodeGen/BranchFolding.cpp
index 66c5aa5..11777d5 100644
--- a/lib/CodeGen/BranchFolding.cpp
+++ b/lib/CodeGen/BranchFolding.cpp
@@ -50,8 +50,9 @@
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 @@
-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 @@
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 @@
// 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) {