ARM: Align functions containing Thumb-2 jump tables to 4 bytes.

[oota-llvm.git] / lib / Target / ARM / ARMConstantIslandPass.cpp

diff --git a/lib/Target/ARM/ARMConstantIslandPass.cpp b/lib/Target/ARM/ARMConstantIslandPass.cpp
index 5d295317c556efc3bf26c103425794d08270ec63..0a47400c490d97cad8bdc7b5aa49b97fd1c84472 100644 (file)
--- a/lib/Target/ARM/ARMConstantIslandPass.cpp
+++ b/lib/Target/ARM/ARMConstantIslandPass.cpp
@@ -53,11 +53,6 @@ static cl::opt<bool>
 AdjustJumpTableBlocks("arm-adjust-jump-tables", cl::Hidden, cl::init(true),
           cl::desc("Adjust basic block layout to better use TB[BH]"));
 
-// FIXME: This option should be removed once it has received sufficient testing.
-static cl::opt<bool>
-AlignConstantIslands("arm-align-constant-islands", cl::Hidden, cl::init(true),
-          cl::desc("Align constant islands in code"));
-
 /// UnknownPadding - Return the worst case padding that could result from
 /// unknown offset bits.  This does not include alignment padding caused by
 /// known offset bits.
@@ -383,11 +378,9 @@ bool ARMConstantIslands::runOnMachineFunction(MachineFunction &mf) {
                << MCP->getConstants().size() << " CP entries, aligned to "
                << MCP->getConstantPoolAlignment() << " bytes *****\n");
 
-  TII = (const ARMBaseInstrInfo *)MF->getTarget()
-            .getSubtargetImpl()
-            ->getInstrInfo();
+  STI = &static_cast<const ARMSubtarget &>(MF->getSubtarget());
+  TII = STI->getInstrInfo();
   AFI = MF->getInfo<ARMFunctionInfo>();
-  STI = &MF->getTarget().getSubtarget<ARMSubtarget>();
 
   isThumb = AFI->isThumbFunction();
   isThumb1 = AFI->isThumb1OnlyFunction();
@@ -414,13 +407,6 @@ bool ARMConstantIslands::runOnMachineFunction(MachineFunction &mf) {
     MF->RenumberBlocks();
   }
 
-  // Thumb1 functions containing constant pools get 4-byte alignment.
-  // This is so we can keep exact track of where the alignment padding goes.
-
-  // ARM and Thumb2 functions need to be 4-byte aligned.
-  if (!isThumb1)
-    MF->ensureAlignment(2);  // 2 = log2(4)
-
   // Perform the initial placement of the constant pool entries.  To start with,
   // we put them all at the end of the function.
   std::vector<MachineInstr*> CPEMIs;
@@ -437,6 +423,10 @@ bool ARMConstantIslands::runOnMachineFunction(MachineFunction &mf) {
   CPEMIs.clear();
   DEBUG(dumpBBs());
 
+  // Functions with jump tables need an alignment of 4 because they use the ADR
+  // instruction, which aligns the PC to 4 bytes before adding an offset.
+  if (!T2JumpTables.empty())
+    MF->ensureAlignment(2);
 
   /// Remove dead constant pool entries.
   MadeChange |= removeUnusedCPEntries();
@@ -515,8 +505,7 @@ ARMConstantIslands::doInitialPlacement(std::vector<MachineInstr*> &CPEMIs) {
   unsigned MaxAlign = Log2_32(MCP->getConstantPoolAlignment());
 
   // Mark the basic block as required by the const-pool.
-  // If AlignConstantIslands isn't set, use 4-byte alignment for everything.
-  BB->setAlignment(AlignConstantIslands ? MaxAlign : 2);
+  BB->setAlignment(MaxAlign);
 
   // The function needs to be as aligned as the basic blocks. The linker may
   // move functions around based on their alignment.
@@ -532,7 +521,7 @@ ARMConstantIslands::doInitialPlacement(std::vector<MachineInstr*> &CPEMIs) {
   // identity mapping of CPI's to CPE's.
   const std::vector<MachineConstantPoolEntry> &CPs = MCP->getConstants();
 
-  const DataLayout &TD = *MF->getSubtarget().getDataLayout();
+  const DataLayout &TD = *MF->getTarget().getDataLayout();
   for (unsigned i = 0, e = CPs.size(); i != e; ++i) {
     unsigned Size = TD.getTypeAllocSize(CPs[i].getType());
     assert(Size >= 4 && "Too small constant pool entry");
@@ -606,10 +595,6 @@ ARMConstantIslands::CPEntry
 unsigned ARMConstantIslands::getCPELogAlign(const MachineInstr *CPEMI) {
   assert(CPEMI && CPEMI->getOpcode() == ARM::CONSTPOOL_ENTRY);
 
-  // Everything is 4-byte aligned unless AlignConstantIslands is set.
-  if (!AlignConstantIslands)
-    return 2;
-
   unsigned CPI = CPEMI->getOperand(1).getIndex();
   assert(CPI < MCP->getConstants().size() && "Invalid constant pool index.");
   unsigned Align = MCP->getConstants()[CPI].getAlignment();
@@ -1764,8 +1749,13 @@ bool ARMConstantIslands::optimizeThumb2Instructions() {
 bool ARMConstantIslands::optimizeThumb2Branches() {
   bool MadeChange = false;
 
-  for (unsigned i = 0, e = ImmBranches.size(); i != e; ++i) {
-    ImmBranch &Br = ImmBranches[i];
+  // The order in which branches appear in ImmBranches is approximately their
+  // order within the function body. By visiting later branches first, we reduce
+  // the distance between earlier forward branches and their targets, making it
+  // more likely that the cbn?z optimization, which can only apply to forward
+  // branches, will succeed.
+  for (unsigned i = ImmBranches.size(); i != 0; --i) {
+    ImmBranch &Br = ImmBranches[i-1];
     unsigned Opcode = Br.MI->getOpcode();
     unsigned NewOpc = 0;
     unsigned Scale = 1;