Add a CoveredBySubRegs property to Register descriptions.

When set, this bit indicates that a register is completely defined by
the value of its sub-registers.

Use the CoveredBySubRegs property to infer which super-registers are
call-preserved given a list of callee-saved registers.  For example, the
ARM registers D8-D15 are callee-saved.  This now automatically implies
that Q4-Q7 are call-preserved.

Conversely, Win64 callees save XMM6-XMM15, but the corresponding
YMM6-YMM15 registers are not call-preserved because they are not fully
defined by their sub-registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148363 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 0 deletions

diff --git a/include/llvm/Target/Target.td b/include/llvm/Target/Target.td
index be08b9c9be..f9f30a8bb7 100644
--- a/include/llvm/Target/Target.td
+++ b/include/llvm/Target/Target.td
@@ -86,6 +86,12 @@ class Register<string n, list<string> altNames = []> {
   // This is used by the x86-64 and ARM Thumb targets where some registers
   // require larger instruction encodings.
   int CostPerUse = 0;
+
+  // CoveredBySubRegs - When this bit is set, the value of this register is
+  // completely determined by the value of its sub-registers.  For example, the
+  // x86 register AX is covered by its sub-registers AL and AH, but EAX is not
+  // covered by its sub-register AX.
+  bit CoveredBySubRegs = 0;
 }
 
 // RegisterWithSubRegs - This can be used to define instances of Register which