From: Chris Lattner <sabre@nondot.org>
Date: Wed, 5 Jan 2005 16:09:16 +0000 (+0000)
Subject: Minor optimization to allocate R8 registers in a better order.
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=0539313fe5b9d9050cc23b245cfe213b927a296b;p=oota-llvm.git

Minor optimization to allocate R8 registers in a better order.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@19289 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Target/X86/X86RegisterInfo.td b/lib/Target/X86/X86RegisterInfo.td
index 1d86c0e6437..f0ab68d4c9b 100644
--- a/lib/Target/X86/X86RegisterInfo.td
+++ b/lib/Target/X86/X86RegisterInfo.td
@@ -61,7 +61,13 @@ let Namespace = "X86" in {
 // top-level register classes.  The order specified in the register list is
 // implicitly defined to be the register allocation order.
 //
-def R8  : RegisterClass<i8,  8, [AL, AH, CL, CH, DL, DH, BL, BH]>;
+
+// List AL,CL,DL before AH,CH,DH, as X86 processors often suffer from false
+// dependences between upper and lower parts of the register.  BL and BH are
+// last because they are call clobbered. Both Athlon and P4 chips suffer this
+// issue.
+def R8  : RegisterClass<i8,  8, [AL, CL, DL, AH, CH, DH, BL, BH]>;
+
 def R16 : RegisterClass<i16, 16, [AX, CX, DX, SI, DI, BX, BP, SP]> {
   let Methods = [{
     iterator allocation_order_end(MachineFunction &MF) const {