JDK-7079626 : x64 emits unnecessary REX prefix
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: compiler
  • Affected Version: hs22
  • Priority: P4
  • Status: Closed
  • Resolution: Fixed
  • OS: generic
  • CPU: generic
  • Submitted: 2011-08-16
  • Updated: 2012-01-10
  • Resolved: 2011-11-25
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JDK 7 Other
7u2Fixed hs22Fixed
Description
While investigating some other bug we found out that on x64 we sometimes emit unnecessary REX prefixes like:

  0xfffffd7ffa65f00f: rex mov    0x10(%rsi),%edi  ;*getfield offset
                                                ; - java.lang.String::hashCode@17 (line 1497)
  0xfffffd7ffa65f013: rex mov    0xc(%rsi),%ebp  ;*getfield value
                                                ; - java.lang.String::hashCode@22 (line 1498)
Comments
EVALUATION See main CR
12-09-2011
EVALUATION http://hg.openjdk.java.net/hsx/hotspot-rt/hotspot/rev/381bf869f784
23-08-2011
EVALUATION http://hg.openjdk.java.net/hsx/hotspot-comp/hotspot/rev/381bf869f784
17-08-2011
SUGGESTED FIX diff -r cfcf2ba8f3eb src/cpu/x86/vm/assembler_x86.cpp --- a/src/cpu/x86/vm/assembler_x86.cpp +++ b/src/cpu/x86/vm/assembler_x86.cpp @@ -3674,7 +3674,7 @@ void Assembler::prefix(Address adr, Regi } else { if (adr.index_needs_rex()) { prefix(REX_X); - } else if (reg->encoding() >= 4 ) { + } else if (byteinst && reg->encoding() >= 4 ) { prefix(REX); } }
16-08-2011
EVALUATION In Assembler::prefix we don't check byteinst argument as we do in Assembler::prefix_and_encode.
16-08-2011