Add explanatory comment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@138042 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 5 insertions, 0 deletions

diff --git a/lib/Target/ARM/ARMInstrThumb.td b/lib/Target/ARM/ARMInstrThumb.td
index 199691f6c3..a6e0ed737b 100644
--- a/lib/Target/ARM/ARMInstrThumb.td
+++ b/lib/Target/ARM/ARMInstrThumb.td
@@ -125,6 +125,11 @@ def t_addrmode_rr : Operand<i32>,
 
 // t_addrmode_rrs := reg + reg
 //
+// We use separate scaled versions because the Select* functions need
+// to explicitly check for a matching constant and return false here so that
+// the reg+imm forms will match instead. This is a horrible way to do that,
+// as it forces tight coupling between the methods, but it's how selectiondag
+// currently works.
 def t_addrmode_rrs1 : Operand<i32>,
                       ComplexPattern<i32, 2, "SelectThumbAddrModeRI5S1", []> {
   let EncoderMethod = "getThumbAddrModeRegRegOpValue";