[MachineOutliner] Drop candidates that require fixups if it's beneficial

If it's a bigger code size win to drop candidates that require stack fixups
than to demote every candidate to that variant, the outliner should do that.

This happens if the number of bytes taken by calls to functions that don't
require fixups, plus the number of bytes that'd be left is less than the
number of bytes that it'd take to emit a save + restore for all candidates.

Also add tests for each possible new behaviour.

- machine-outliner-compatible-candidates shows that when we have candidates
that don't use the stack, we can use the default call variant along with the
no save/regsave variant.

- machine-outliner-all-stack shows that when it's better to fix up the stack,
we still will demote all candidates to that case

- machine-outliner-drop-stack shows that we can discard candidates that
require stack fixups when it would be beneficial to do so.

llvm-svn: 348168

1 files changed, 24 insertions, 5 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
index c8d9d2a6d02..56f38aca500 100644
--- a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
@@ -5169,32 +5169,51 @@ AArch64InstrInfo::getOutliningCandidateInfo(
     // We need to decide how to emit calls + frames. We can always emit the same
     // frame if we don't need to save to the stack. If we have to save to the
     // stack, then we need a different frame.
-    unsigned NumNoStackSave = 0;
+    unsigned NumBytesNoStackCalls = 0;
+    std::vector<outliner::Candidate> CandidatesWithoutStackFixups;
 
     for (outliner::Candidate &C : RepeatedSequenceLocs) {
       C.initLRU(TRI);
 
       // Is LR available? If so, we don't need a save.
       if (C.LRU.available(AArch64::LR)) {
+        NumBytesNoStackCalls += 4;
         C.setCallInfo(MachineOutlinerNoLRSave, 4);
-        ++NumNoStackSave;
+        CandidatesWithoutStackFixups.push_back(C);
       }
 
       // Is an unused register available? If so, we won't modify the stack, so
       // we can outline with the same frame type as those that don't save LR.
       else if (findRegisterToSaveLRTo(C)) {
+        NumBytesNoStackCalls += 12;
         C.setCallInfo(MachineOutlinerRegSave, 12);
-        ++NumNoStackSave;
+        CandidatesWithoutStackFixups.push_back(C);
+      }
+
+      // Is SP used in the sequence at all? If not, we don't have to modify
+      // the stack, so we are guaranteed to get the same frame.
+      else if (C.UsedInSequence.available(AArch64::SP)) {
+        NumBytesNoStackCalls += 12;
+        C.setCallInfo(MachineOutlinerDefault, 12);
+        CandidatesWithoutStackFixups.push_back(C);
+      }
+
+      // If we outline this, we need to modify the stack. Pretend we don't
+      // outline this by saving all of its bytes.
+      else {
+        NumBytesNoStackCalls += SequenceSize;
       }
     }
 
     // If there are no places where we have to save LR, then note that we don't
     // have to update the stack. Otherwise, give every candidate the default
     // call type.
-    if (NumNoStackSave == RepeatedSequenceLocs.size())
+    if (NumBytesNoStackCalls <= RepeatedSequenceLocs.size() * 12) {
+      RepeatedSequenceLocs = CandidatesWithoutStackFixups;
       FrameID = MachineOutlinerNoLRSave;
-    else
+    } else {
       SetCandidateCallInfo(MachineOutlinerDefault, 12);
+    }
   }
 
   // Does every candidate's MBB contain a call? If so, then we might have a call