[llvm-commits] CVS: llvm/lib/CodeGen/BranchFolding.cpp
Chris Lattner
sabre at nondot.org
Wed Nov 1 11:36:43 PST 2006
Changes in directory llvm/lib/CodeGen:
BranchFolding.cpp updated: 1.29 -> 1.30
---
Log message:
give branch folding a simple heuristic to decide which block to split so that
it inserts an uncond branch where it is less likely to cause a problem. This
fixes some perf issues on ppc.
---
Diffs of the changes: (+49 -6)
BranchFolding.cpp | 55 ++++++++++++++++++++++++++++++++++++++++++++++++------
1 files changed, 49 insertions(+), 6 deletions(-)
Index: llvm/lib/CodeGen/BranchFolding.cpp
diff -u llvm/lib/CodeGen/BranchFolding.cpp:1.29 llvm/lib/CodeGen/BranchFolding.cpp:1.30
--- llvm/lib/CodeGen/BranchFolding.cpp:1.29 Tue Oct 31 19:16:12 2006
+++ llvm/lib/CodeGen/BranchFolding.cpp Wed Nov 1 13:36:29 2006
@@ -283,6 +283,44 @@
return NewMBB;
}
+/// EstimateRuntime - Make a rough estimate for how long it will take to run
+/// the specified code.
+static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
+ MachineBasicBlock::iterator E,
+ const TargetInstrInfo *TII) {
+ unsigned Time = 0;
+ for (; I != E; ++I) {
+ const TargetInstrDescriptor &TID = TII->get(I->getOpcode());
+ if (TID.Flags & M_CALL_FLAG)
+ Time += 10;
+ else if (TID.Flags & (M_LOAD_FLAG|M_STORE_FLAG))
+ Time += 2;
+ else
+ ++Time;
+ }
+ return Time;
+}
+
+/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at
+/// MBB2I and then insert an unconditional branch in the other block. Determine
+/// which is the best to split
+static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1,
+ MachineBasicBlock::iterator MBB1I,
+ MachineBasicBlock *MBB2,
+ MachineBasicBlock::iterator MBB2I,
+ const TargetInstrInfo *TII) {
+ // TODO: if we had some notion of which block was hotter, we could split
+ // the hot block, so it is the fall-through. Since we don't have profile info
+ // make a decision based on which will hurt most to split.
+ unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I, TII);
+ unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I, TII);
+
+ // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the
+ // MBB1 block so it falls through. This will penalize the MBB2 path, but will
+ // have a lower overall impact on the program execution.
+ return MBB1Time < MBB2Time;
+}
+
bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
MadeChange = false;
@@ -355,12 +393,17 @@
MergePotentials.pop_back();
continue;
}
- // TODO: if we had some notion of which block was hotter, we could split
- // the hot block, so it is the fall-through. For now, just split the
- // second block.
- MBB2 = SplitMBBAt(*MBB2, BBI2);
- BBI2 = MBB2->begin();
- (MergePotentials.end()-2)->second = MBB2;
+
+ // Decide whether we want to split CurMBB or MBB2.
+ if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII)) {
+ CurMBB = SplitMBBAt(*CurMBB, BBI1);
+ BBI1 = CurMBB->begin();
+ MergePotentials.back().second = CurMBB;
+ } else {
+ MBB2 = SplitMBBAt(*MBB2, BBI2);
+ BBI2 = MBB2->begin();
+ (MergePotentials.end()-2)->second = MBB2;
+ }
}
if (MBB2->begin() == BBI2) {
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