[PATCH] D24044: CodeGen: Tail-duplicate longer blocks that end with indirect branches.

[Kyle Butt via llvm-commits]

iteratee created this revision.
iteratee added a reviewer: davidxl.
iteratee added subscribers: llvm-commits, echristo, timshen.
iteratee set the repository for this revision to rL LLVM.

Specifically, this is currently done for tail-duplication before
register allocation, and is now done for tail-duplication during layout.

A similar change was necessary for tail-merging during layout to prevent
the two from being in conflict.


Repository:
  rL LLVM

https://reviews.llvm.org/D24044

Files:
  lib/CodeGen/BranchFolding.cpp
  lib/CodeGen/TailDuplicator.cpp

Index: lib/CodeGen/TailDuplicator.cpp
===================================================================
--- lib/CodeGen/TailDuplicator.cpp
+++ lib/CodeGen/TailDuplicator.cpp
@@ -560,11 +560,16 @@
   // to allow undoing the effects of tail merging and other optimizations
   // that rearrange the predecessors of the indirect branch.
 
-  bool HasIndirectbr = false;
-  if (!TailBB.empty())
-    HasIndirectbr = TailBB.back().isIndirectBranch();
+  bool HasLocalIndirectbr = false;
+  if (!TailBB.empty()) {
+    MachineInstr &back = TailBB.back();
+    // On some platforms return instructions are marked as indirect branches and
+    // not as return instructions. What we're actually looking for is indirect
+    // branches that occur within a function, so we check for local successors.
+    HasLocalIndirectbr = back.isIndirectBranch() && TailBB.succ_size() != 0;
+  }
 
-  if (HasIndirectbr && PreRegAlloc)
+  if (HasLocalIndirectbr && (PreRegAlloc || LayoutMode))
     MaxDuplicateCount = TailDupIndirectBranchSize;
 
   // Check the instructions in the block to determine whether tail-duplication
@@ -620,7 +625,7 @@
     }
   }
 
-  if (HasIndirectbr && PreRegAlloc)
+  if (HasLocalIndirectbr && PreRegAlloc)
     return true;
 
   if (IsSimple)
Index: lib/CodeGen/BranchFolding.cpp
===================================================================
--- lib/CodeGen/BranchFolding.cpp
+++ lib/CodeGen/BranchFolding.cpp
@@ -31,6 +31,7 @@
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/TailDuplicator.h"
 #include "llvm/CodeGen/TargetPassConfig.h"
 #include "llvm/IR/Function.h"
 #include "llvm/Support/CommandLine.h"
@@ -701,8 +702,19 @@
        CurMPIter != B && CurMPIter->getHash() == CurHash; --CurMPIter) {
     for (MPIterator I = std::prev(CurMPIter); I->getHash() == CurHash; --I) {
       unsigned CommonTailLen;
+      unsigned MinCommonTailLengthInside = MinCommonTailLength;
+      // Prefer to leave indirect branches duplicated after placement.
+      if (AfterBlockPlacement) {
+        MachineBasicBlock &Block1 = *CurMPIter->getBlock();
+        if (Block1.empty())
+          // Look for indirect local branches, and try to avoid merging them
+          // when merging during layout. Having multiple copies of these
+          // branches allows for better branch prediction.
+          if (Block1.back().isIndirectBranch() && Block1.succ_size() != 0)
+            MinCommonTailLengthInside = TailDupIndirectBranchSize + 1;
+      }
       if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
-                            MinCommonTailLength,
+                            MinCommonTailLengthInside,
                             CommonTailLen, TrialBBI1, TrialBBI2,
                             SuccBB, PredBB,
                             FuncletMembership,


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