[llvm-commits] [llvm] r91220 - /llvm/trunk/lib/Target/X86/X86InstrInfo.cpp

[Evan Cheng]

Author: evancheng
Date: Sat Dec 12 12:55:26 2009
New Revision: 91220

URL: http://llvm.org/viewvc/llvm-project?rev=91220&view=rev
Log:
Add comment about potential partial register stall.

Modified:
    llvm/trunk/lib/Target/X86/X86InstrInfo.cpp

Modified: llvm/trunk/lib/Target/X86/X86InstrInfo.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86InstrInfo.cpp?rev=91220&r1=91219&r2=91220&view=diff

==============================================================================
--- llvm/trunk/lib/Target/X86/X86InstrInfo.cpp (original)
+++ llvm/trunk/lib/Target/X86/X86InstrInfo.cpp Sat Dec 12 12:55:26 2009
@@ -1081,6 +1081,11 @@
             
   // Build and insert into an implicit UNDEF value. This is OK because
   // well be shifting and then extracting the lower 16-bits. 
+  // This has the potential to cause partial stall. e.g.
+  //   movw    (%rbp,%rcx,2), %dx
+  //   leal    -65(%rdx), %esi
+  // But testing has shown this *does* help performance (at least on modern
+  // x86 machines).
   BuildMI(*MFI, MBBI, MI->getDebugLoc(), get(X86::IMPLICIT_DEF), leaInReg);
   MachineInstr *InsMI =
     BuildMI(*MFI, MBBI, MI->getDebugLoc(), get(X86::INSERT_SUBREG),leaInReg)