[PATCH] Add a RequireStructuredCFG Field to TargetMachine.

[Vincent Lejeune]

  Now prevents the tailduplication pass.

http://llvm-reviews.chandlerc.com/D2260

CHANGE SINCE LAST DIFF
  http://llvm-reviews.chandlerc.com/D2260?vs=5766&id=5858#toc

Files:
  docs/WritingAnLLVMBackend.rst
  include/llvm/Target/TargetMachine.h
  lib/CodeGen/BranchFolding.cpp
  lib/CodeGen/MachineBasicBlock.cpp
  lib/CodeGen/Passes.cpp
  lib/Target/R600/AMDGPUTargetMachine.cpp
  lib/Target/TargetMachine.cpp

Index: docs/WritingAnLLVMBackend.rst
===================================================================
--- docs/WritingAnLLVMBackend.rst
+++ docs/WritingAnLLVMBackend.rst
@@ -238,6 +238,12 @@
 * ``getTargetLowering()``
 * ``getJITInfo()``
 
+Some architectures, such as GPUs, do not support jumping to an arbitrary
+program location and implement branching using masked execution and loop using
+special instructions around the loop body. In order to avoid CFG modifications
+that introduce irreducible control flow not handled by such hardware, a target
+must call `setRequiresStructuredCFG(true)` when being initialized.
+
 In addition, the ``XXXTargetMachine`` constructor should specify a
 ``TargetDescription`` string that determines the data layout for the target
 machine, including characteristics such as pointer size, alignment, and
Index: include/llvm/Target/TargetMachine.h
===================================================================
--- include/llvm/Target/TargetMachine.h
+++ include/llvm/Target/TargetMachine.h
@@ -88,6 +88,7 @@
   unsigned MCUseLoc : 1;
   unsigned MCUseCFI : 1;
   unsigned MCUseDwarfDirectory : 1;
+  unsigned RequireStructuredCFG : 1;
 
 public:
   virtual ~TargetMachine();
@@ -156,6 +157,9 @@
     return 0;
   }
 
+  bool requiresStructuredCFG() const { return RequireStructuredCFG; }
+  void setRequiresStructuredCFG(bool Value) { RequireStructuredCFG = Value; }
+
   /// hasMCRelaxAll - Check whether all machine code instructions should be
   /// relaxed.
   bool hasMCRelaxAll() const { return MCRelaxAll; }
Index: lib/CodeGen/BranchFolding.cpp
===================================================================
--- lib/CodeGen/BranchFolding.cpp
+++ lib/CodeGen/BranchFolding.cpp
@@ -83,7 +83,11 @@
 
 bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
   TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
-  BranchFolder Folder(PassConfig->getEnableTailMerge(), /*CommonHoist=*/true);
+  // TailMerge can create jump into if branches that make CFG irreducible for
+  // HW that requires structurized CFG.
+  bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
+      PassConfig->getEnableTailMerge();
+  BranchFolder Folder(EnableTailMerge, /*CommonHoist=*/true);
   return Folder.OptimizeFunction(MF,
                                  MF.getTarget().getInstrInfo(),
                                  MF.getTarget().getRegisterInfo(),
Index: lib/CodeGen/MachineBasicBlock.cpp
===================================================================
--- lib/CodeGen/MachineBasicBlock.cpp
+++ lib/CodeGen/MachineBasicBlock.cpp
@@ -677,6 +677,11 @@
   MachineFunction *MF = getParent();
   DebugLoc dl;  // FIXME: this is nowhere
 
+  // Performances might be harmed by HW implementing branching using exec mask
+  // and always executes both side of the branches anyway.
+  if (MF->getTarget().requiresStructuredCFG())
+    return NULL;
+
   // We may need to update this's terminator, but we can't do that if
   // AnalyzeBranch fails. If this uses a jump table, we won't touch it.
   const TargetInstrInfo *TII = MF->getTarget().getInstrInfo();
Index: lib/CodeGen/Passes.cpp
===================================================================
--- lib/CodeGen/Passes.cpp
+++ lib/CodeGen/Passes.cpp
@@ -725,7 +725,10 @@
     printAndVerify("After BranchFolding");
 
   // Tail duplication.
-  if (addPass(&TailDuplicateID))
+  // Note that duplicating tail just increase code size and only degrades
+  // performance in case of target that requires Structured Control Flow.
+  // In addition it can also make CFG irreducible. Thus we disable it.
+  if (!TM->requiresStructuredCFG() && addPass(&TailDuplicateID))
     printAndVerify("After TailDuplicate");
 
   // Copy propagation.
Index: lib/Target/R600/AMDGPUTargetMachine.cpp
===================================================================
--- lib/Target/R600/AMDGPUTargetMachine.cpp
+++ lib/Target/R600/AMDGPUTargetMachine.cpp
@@ -72,6 +72,7 @@
     InstrInfo.reset(new SIInstrInfo(*this));
     TLInfo.reset(new SITargetLowering(*this));
   }
+  setRequiresStructuredCFG(true);
   initAsmInfo();
 }
 
Index: lib/Target/TargetMachine.cpp
===================================================================
--- lib/Target/TargetMachine.cpp
+++ lib/Target/TargetMachine.cpp
@@ -55,6 +55,7 @@
     MCUseLoc(true),
     MCUseCFI(true),
     MCUseDwarfDirectory(false),
+    RequireStructuredCFG(false),
     Options(Options) {
 }
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