[X86] Add a comment about the change in r234075.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234079 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/X86InstrCompiler.td b/lib/Target/X86/X86InstrCompiler.td
index e7de24bca41798bfc9cff0b0a3a4a0afc4d4700a..227f69a9d845ad2ecb15ced2d7eb1c5118e2bd5a 100644 (file)
--- a/lib/Target/X86/X86InstrCompiler.td
+++ b/lib/Target/X86/X86InstrCompiler.td
@@ -1232,6 +1232,8 @@ def : Pat<(store (add (loadi64 addr:$dst), 0x00000000800000000), addr:$dst),
 // with implicit zero-extension instead of a 64-bit and if the immediate has at
 // least 32 bits of leading zeros. If in addition the last 32 bits can be
 // represented with a sign extension of a 8 bit constant, use that.
+// This can also reduce instruction size by eliminating the need for the REX
+// prefix.
 
 let AddedComplexity = 1 in // Give priority over i64immSExt8.
 def : Pat<(and GR64:$src, i64immZExt32SExt8:$imm),