X86: replace SUB with CMP if possible

This patch will optimize the following
        movq    %rdi, %rax
        subq    %rsi, %rax
        cmovsq  %rsi, %rdi
        movq    %rdi, %rax
to
        cmpq    %rsi, %rdi
        cmovsq  %rsi, %rdi
        movq    %rdi, %rax

Perform this optimization if the actual result of SUB is not used.

rdar: 11540023


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157755 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 8 insertions, 0 deletions

diff --git a/include/llvm/Target/TargetInstrInfo.h b/include/llvm/Target/TargetInstrInfo.h
index d3c5e20..896c152 100644
--- a/include/llvm/Target/TargetInstrInfo.h
+++ b/include/llvm/Target/TargetInstrInfo.h
@@ -640,6 +640,14 @@ public:
     return false;
   }
 
+  /// OptimizeSubInstr - See if the SUB instruction can be converted into
+  /// something more efficient E.g., on X86, we can replace SUB with CMP
+  /// if the actual result of SUB is not used.
+  virtual bool OptimizeSubInstr(MachineInstr *SubInstr,
+                                const MachineRegisterInfo *MRI) const {
+    return false;
+  }
+
   /// FoldImmediate - 'Reg' is known to be defined by a move immediate
   /// instruction, try to fold the immediate into the use instruction.
   virtual bool FoldImmediate(MachineInstr *UseMI, MachineInstr *DefMI,