[Stack realignment] Handling of aligned allocas.

This patch implements dynamic realignment of stack objects for targets
with a non-realigned stack pointer. Behaviour in FunctionLoweringInfo
is changed so that for a target that has StackRealignable set to
false, over-aligned static allocas are considered to be variable-sized
objects and are handled with DYNAMIC_STACKALLOC nodes.

It would be good to group aligned allocas into a single big alloca as
an optimization, but this is yet todo.

SystemZ benefits from this, due to its stack frame layout.

New tests SystemZ/alloca-03.ll for aligned allocas, and
SystemZ/alloca-04.ll for "no-realign-stack" attribute on functions.

Review and help from Ulrich Weigand and Hal Finkel.

llvm-svn: 254227

1 files changed, 15 insertions, 13 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp b/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
index 8d42dcef140..ff0ccd415db 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
@@ -87,6 +87,7 @@ void FunctionLoweringInfo::set(const Function &fn, MachineFunction &mf,
   TLI = MF->getSubtarget().getTargetLowering();
   RegInfo = &MF->getRegInfo();
   MachineModuleInfo &MMI = MF->getMMI();
+  const TargetFrameLowering *TFI = MF->getSubtarget().getFrameLowering();
 
   // Check whether the function can return without sret-demotion.
   SmallVector<ISD::OutputArg, 4> Outs;
@@ -103,28 +104,29 @@ void FunctionLoweringInfo::set(const Function &fn, MachineFunction &mf,
     for (BasicBlock::const_iterator I = BB->begin(), E = BB->end();
          I != E; ++I) {
       if (const AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
-        // Static allocas can be folded into the initial stack frame adjustment.
-        if (AI->isStaticAlloca()) {
+        Type *Ty = AI->getAllocatedType();
+        unsigned Align =
+          std::max((unsigned)MF->getDataLayout().getPrefTypeAlignment(Ty),
+                   AI->getAlignment());
+        unsigned StackAlign = TFI->getStackAlignment();
+
+        // Static allocas can be folded into the initial stack frame
+        // adjustment. For targets that don't realign the stack, don't
+        // do this if there is an extra alignment requirement.
+        if (AI->isStaticAlloca() && 
+            (TFI->isStackRealignable() || (Align <= StackAlign))) {
           const ConstantInt *CUI = cast<ConstantInt>(AI->getArraySize());
-          Type *Ty = AI->getAllocatedType();
           uint64_t TySize = MF->getDataLayout().getTypeAllocSize(Ty);
-          unsigned Align =
-              std::max((unsigned)MF->getDataLayout().getPrefTypeAlignment(Ty),
-                       AI->getAlignment());
 
           TySize *= CUI->getZExtValue();   // Get total allocated size.
           if (TySize == 0) TySize = 1; // Don't create zero-sized stack objects.
 
           StaticAllocaMap[AI] =
             MF->getFrameInfo()->CreateStackObject(TySize, Align, false, AI);
-
         } else {
-          unsigned Align =
-              std::max((unsigned)MF->getDataLayout().getPrefTypeAlignment(
-                           AI->getAllocatedType()),
-                       AI->getAlignment());
-          unsigned StackAlign =
-              MF->getSubtarget().getFrameLowering()->getStackAlignment();
+          // FIXME: Overaligned static allocas should be grouped into
+          // a single dynamic allocation instead of using a separate
+          // stack allocation for each one.
           if (Align <= StackAlign)
             Align = 0;
           // Inform the Frame Information that we have variable-sized objects.