[mips] Optimize stack pointer adjustments.

Instead of always using addu to adjust the stack pointer when the
size out is of the range of an addiu instruction, use subu so that
a smaller constant can be generated.

This can give savings of ~3 instructions whenever a function has a
a stack frame whose size is out of range of an addiu instruction.

This change may break some naive stack unwinders.

Partially resolves PR/26291.

Thanks to David Chisnall for reporting the issue.

Reviewers: dsanders, vkalintiris

Differential Review: http://reviews.llvm.org/D21321

llvm-svn: 272666

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/Target/Mips/MCTargetDesc/MipsABIInfo.h b/llvm/lib/Target/Mips/MCTargetDesc/MipsABIInfo.h
index 4ea45e56702..9372a3c2bb1 100644
--- a/llvm/lib/Target/Mips/MCTargetDesc/MipsABIInfo.h
+++ b/llvm/lib/Target/Mips/MCTargetDesc/MipsABIInfo.h
@@ -69,6 +69,7 @@ public:
   unsigned GetZeroReg() const;
   unsigned GetPtrAdduOp() const;
   unsigned GetPtrAddiuOp() const;
+  unsigned GetPtrSubuOp() const;
   unsigned GetPtrAndOp() const;
   unsigned GetGPRMoveOp() const;
   inline bool ArePtrs64bit() const { return IsN64(); }