[MBP] Don't outline short optional branches
With the option -outline-optional-branches, LLVM will place optional
branches out of line (more details on r231230).
With this patch, this is not done for short optional branches. A short
optional branch is a branch containing a single block with an
instruction count below a certain threshold (defaulting to 3). Still
everything is guarded under -outline-optional-branches).
Outlining a short branch can't significantly improve code locality. It
can however decrease performance because of the additional jmp and in
cases where the optional branch is hot. This fixes a compile time
regression I have observed in a benchmark.
Review: http://reviews.llvm.org/D8108
llvm-svn: 232802
diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index 06cc7ba..86a4922 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -74,6 +74,12 @@
"post dominator, out of line."),
cl::init(false), cl::Hidden);
+static cl::opt<unsigned> OutlineOptionalThreshold(
+ "outline-optional-threshold",
+ cl::desc("Don't outline optional branches that are a single block with an "
+ "instruction count below this threshold"),
+ cl::init(4), cl::Hidden);
+
namespace {
class BlockChain;
/// \brief Type for our function-wide basic block -> block chain mapping.
@@ -377,8 +383,25 @@
// dominates all terminators of the MachineFunction. If it does, other
// successors must be optional. Don't do this for cold branches.
if (OutlineOptionalBranches && SuccProb > HotProb.getCompl() &&
- UnavoidableBlocks.count(Succ) > 0)
- return Succ;
+ UnavoidableBlocks.count(Succ) > 0) {
+ auto HasShortOptionalBranch = [&]() {
+ for (MachineBasicBlock *Pred : Succ->predecessors()) {
+ // Check whether there is an unplaced optional branch.
+ if (Pred == Succ || (BlockFilter && !BlockFilter->count(Pred)) ||
+ BlockToChain[Pred] == &Chain)
+ continue;
+ // Check whether the optional branch has exactly one BB.
+ if (Pred->pred_size() > 1 || *Pred->pred_begin() != BB)
+ continue;
+ // Check whether the optional branch is small.
+ if (Pred->size() < OutlineOptionalThreshold)
+ return true;
+ }
+ return false;
+ };
+ if (!HasShortOptionalBranch())
+ return Succ;
+ }
// Only consider successors which are either "hot", or wouldn't violate
// any CFG constraints.