[SelectionDAG] Codesize: don't expand SHIFT to SHIFT_PARTS

And instead just generate a libcall. My motivating example on ARM was a simple:
  
  shl i64 %A, %B

for which the code bloat is quite significant. For other targets that also
accept __int128/i128 such as AArch64 and X86, it is also beneficial for these
cases to generate a libcall when optimising for minsize. On these 64-bit targets,
the 64-bits shifts are of course unaffected because the SHIFT/SHIFT_PARTS
lowering operation action is not set to custom/expand.

Differential Revision: https://reviews.llvm.org/D57386



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352736 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 7 insertions, 0 deletions

diff --git a/include/llvm/CodeGen/TargetLowering.h b/include/llvm/CodeGen/TargetLowering.h
index c21eb7911a1..72535c568a1 100644
--- a/include/llvm/CodeGen/TargetLowering.h
+++ b/include/llvm/CodeGen/TargetLowering.h
@@ -642,6 +642,13 @@ public:
     return RepRegClassCostForVT[VT.SimpleTy];
   }
 
+  /// Return true if SHIFT instructions should be expanded to SHIFT_PARTS
+  /// instructions, and false if a library call is preferred (e.g for code-size
+  /// reasons).
+  virtual bool shouldExpandShift(SelectionDAG &DAG, SDNode *N) const {
+    return true;
+  }
+
   /// Return true if the target has native support for the specified value type.
   /// This means that it has a register that directly holds it without
   /// promotions or expansions.