Add support for dynamic stack realignment when in thumb1 mode.

rdar://10288916

llvm-svn: 142337

1 files changed, 26 insertions, 4 deletions

diff --git a/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp b/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp
index d8481778c0d..ad1edc8dd8f 100644
--- a/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp
+++ b/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp
@@ -155,10 +155,32 @@ void Thumb1FrameLowering::emitPrologue(MachineFunction &MF) const {
   AFI->setGPRCalleeSavedArea2Size(GPRCS2Size);
   AFI->setDPRCalleeSavedAreaSize(DPRCSSize);
 
-  // Thumb1 does not currently support dynamic stack realignment.  Report a
-  // fatal error rather then silently generate bad code.
-  if (RegInfo->needsStackRealignment(MF))
-      report_fatal_error("Dynamic stack realignment not supported for thumb1.");
+  // If we need dynamic stack realignment, do it here. Be paranoid and make
+  // sure if we also have VLAs, we have a base pointer for frame access.
+  if (RegInfo->needsStackRealignment(MF)) {
+    // We cannot use sp as source/dest register here, thus we're emitting the
+    // following sequence:
+    // mov r4, sp
+    // lsrs r4, r4, Log2MaxAlign
+    // lsls r4, r4, Log2MaxAlign
+    // mov sp, r4
+    unsigned MaxAlign = MFI->getMaxAlignment();
+    unsigned Log2MaxAlign = Log2_32(MaxAlign);
+    AddDefaultPred(BuildMI(MBB, MBBI, dl, TII.get(ARM::tMOVr), ARM::R4)
+                   .addReg(ARM::SP, RegState::Kill));
+    AddDefaultPred(AddDefaultT1CC(BuildMI(MBB, MBBI, dl, TII.get(ARM::tLSRri),
+                                          ARM::R4))
+                   .addReg(ARM::R4, RegState::Kill)
+                   .addImm(Log2MaxAlign));
+    AddDefaultPred(AddDefaultT1CC(BuildMI(MBB, MBBI, dl, TII.get(ARM::tLSLri),
+                                          ARM::R4))
+                   .addReg(ARM::R4, RegState::Kill)
+                   .addImm(Log2MaxAlign));
+    AddDefaultPred(BuildMI(MBB, MBBI, dl, TII.get(ARM::tMOVr), ARM::SP)
+                   .addReg(ARM::R4, RegState::Kill));
+
+    AFI->setShouldRestoreSPFromFP(true);    
+  }
 
   // If we need a base pointer, set it up here. It's whatever the value
   // of the stack pointer is at this point. Any variable size objects