From 7f2777e8d2d7a8a7cc0c4cade0f590c7882ee9c8 Mon Sep 17 00:00:00 2001
From: Dan Gohman <gohman@apple.com>
Date: Wed, 27 Aug 2008 17:33:15 +0000
Subject: [PATCH] Reinstate the x86-64 portion of r55190. When doing extloads
 into 64-bit registers from 16-bit and smaller memory locations, prefer
 instructions that define the entire 64-bit register, to avoid
 partial-register updates.

llvm-svn: 55422
---
 llvm/lib/Target/X86/X86Instr64bit.td | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/llvm/lib/Target/X86/X86Instr64bit.td b/llvm/lib/Target/X86/X86Instr64bit.td
index a1706a814428..93cd0c498b94 100644
--- a/llvm/lib/Target/X86/X86Instr64bit.td
+++ b/llvm/lib/Target/X86/X86Instr64bit.td
@@ -1254,15 +1254,14 @@ def : Pat<(i64 (zext GR32:$src)),
 def : Pat<(zextloadi64i1 addr:$src), (MOVZX64rm8 addr:$src)>;
 
 // extload
-def : Pat<(extloadi64i1 addr:$src),
-          (INSERT_SUBREG (i64 (IMPLICIT_DEF)), (MOV8rm addr:$src),
-                         x86_subreg_8bit)>;
-def : Pat<(extloadi64i8 addr:$src),
-          (INSERT_SUBREG (i64 (IMPLICIT_DEF)), (MOV8rm addr:$src),
-                         x86_subreg_8bit)>;
-def : Pat<(extloadi64i16 addr:$src),
-          (INSERT_SUBREG (i64 (IMPLICIT_DEF)), (MOV16rm addr:$src),
-                         x86_subreg_16bit)>;
+// When extloading from 16-bit and smaller memory locations into 64-bit registers,
+// use zero-extending loads so that the entire 64-bit register is defined, avoiding
+// partial-register updates.
+def : Pat<(extloadi64i1 addr:$src),  (MOVZX64rm8  addr:$src)>;
+def : Pat<(extloadi64i8 addr:$src),  (MOVZX64rm8  addr:$src)>;
+def : Pat<(extloadi64i16 addr:$src), (MOVZX64rm16 addr:$src)>;
+// For other extloads, use subregs, since the high contents of the register are
+// defined after an extload.
 def : Pat<(extloadi64i32 addr:$src),
           (INSERT_SUBREG (i64 (IMPLICIT_DEF)), (MOV32rm addr:$src),
                          x86_subreg_32bit)>;