[X86] Allow folding of stack reloads when loading a subreg of the spilled reg

We did not support subregs in InlineSpiller:foldMemoryOperand() because targets
may not deal with them correctly.

This adds a target hook to let the spiller know that a target can handle
subregs, and actually enables it for x86 for the case of stack slot reloads.
This fixes PR30832.

Differential Revision: https://reviews.llvm.org/D26521

llvm-svn: 287792

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86InstrInfo.h b/llvm/lib/Target/X86/X86InstrInfo.h
index bc43820a024..a0292bbdaed 100644
--- a/llvm/lib/Target/X86/X86InstrInfo.h
+++ b/llvm/lib/Target/X86/X86InstrInfo.h
@@ -378,6 +378,10 @@ public:
 
   bool expandPostRAPseudo(MachineInstr &MI) const override;
 
+  /// Check whether the target can fold a load that feeds a subreg operand
+  /// (or a subreg operand that feeds a store).
+  bool isSubregFoldable() const override { return true; }
+
   /// foldMemoryOperand - If this target supports it, fold a load or store of
   /// the specified stack slot into the specified machine instruction for the
   /// specified operand(s).  If this is possible, the target should perform the