Add a -arm-align-constant-islands flag, default off.

Order constant pool entries by descending alignment in the initial
island to ensure packing and correct alignment.  When the command line
flag is set, also align the basic block containing the constant pool
entries.

This is only a partial implementation of constant island alignment. More
to come.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146375 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 35 insertions, 9 deletions

diff --git a/lib/Target/ARM/ARMConstantIslandPass.cpp b/lib/Target/ARM/ARMConstantIslandPass.cpp
index 55a0cc0f44..65abff0b95 100644
--- a/lib/Target/ARM/ARMConstantIslandPass.cpp
+++ b/lib/Target/ARM/ARMConstantIslandPass.cpp
@@ -52,6 +52,10 @@ 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]"));
 
+static cl::opt<bool>
+AlignConstantIslands("arm-align-constant-island", cl::Hidden,
+          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.
@@ -487,8 +491,18 @@ void ARMConstantIslands::DoInitialPlacement(MachineFunction &MF,
   MachineBasicBlock *BB = MF.CreateMachineBasicBlock();
   MF.push_back(BB);
 
-  // Mark the basic block as 4-byte aligned as required by the const-pool.
-  BB->setAlignment(2);
+  // MachineConstantPool measures alignment in bytes. We measure in log2(bytes).
+  unsigned MaxAlign = Log2_32(MF.getConstantPool()->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);
+
+  // Order the entries in BB by descending alignment.  That ensures correct
+  // alignment of all entries as long as BB is sufficiently aligned.  Keep
+  // track of the insertion point for each alignment.  We are going to bucket
+  // sort the entries as they are created.
+  SmallVector<MachineBasicBlock::iterator, 8> InsPoint(MaxAlign + 1, BB->end());
 
   // Add all of the constants from the constant pool to the end block, use an
   // identity mapping of CPI's to CPE's.
@@ -498,23 +512,35 @@ void ARMConstantIslands::DoInitialPlacement(MachineFunction &MF,
   const TargetData &TD = *MF.getTarget().getTargetData();
   for (unsigned i = 0, e = CPs.size(); i != e; ++i) {
     unsigned Size = TD.getTypeAllocSize(CPs[i].getType());
-    // Verify that all constant pool entries are a multiple of 4 bytes.  If not,
-    // we would have to pad them out or something so that instructions stay
-    // aligned.
-    assert((Size & 3) == 0 && "CP Entry not multiple of 4 bytes!");
+    assert(Size >= 4 && "Too small constant pool entry");
+    unsigned Align = CPs[i].getAlignment();
+    assert(isPowerOf2_32(Align) && "Invalid alignment");
+    // Verify that all constant pool entries are a multiple of their alignment.
+    // If not, we would have to pad them out so that instructions stay aligned.
+    assert((Size % Align) == 0 && "CP Entry not multiple of 4 bytes!");
+
+    // Insert CONSTPOOL_ENTRY before entries with a smaller alignment.
+    unsigned LogAlign = Log2_32(Align);
+    MachineBasicBlock::iterator InsAt = InsPoint[LogAlign];
     MachineInstr *CPEMI =
-      BuildMI(BB, DebugLoc(), TII->get(ARM::CONSTPOOL_ENTRY))
+      BuildMI(*BB, InsAt, DebugLoc(), TII->get(ARM::CONSTPOOL_ENTRY))
         .addImm(i).addConstantPoolIndex(i).addImm(Size);
     CPEMIs.push_back(CPEMI);
 
+    // Ensure that future entries with higher alignment get inserted before
+    // CPEMI. This is bucket sort with iterators.
+    for (unsigned a = LogAlign + 1; a < MaxAlign; ++a)
+      if (InsPoint[a] == InsAt)
+        InsPoint[a] = CPEMI;
+
     // Add a new CPEntry, but no corresponding CPUser yet.
     std::vector<CPEntry> CPEs;
     CPEs.push_back(CPEntry(CPEMI, i));
     CPEntries.push_back(CPEs);
     ++NumCPEs;
-    DEBUG(dbgs() << "Moved CPI#" << i << " to end of function as #" << i
-                 << "\n");
+    DEBUG(dbgs() << "Moved CPI#" << i << " to end of function\n");
   }
+  DEBUG(BB->dump());
 }
 
 /// BBHasFallthrough - Return true if the specified basic block can fallthrough