[LLVMbugs] [Bug 22965] New: [x86] Less optimized multi-byte nop instructions are generated

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=22965

            Bug ID: 22965
           Summary: [x86] Less optimized multi-byte nop instructions are
                    generated
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: hjl.tools at gmail.com
                CC: llvmbugs at cs.uiuc.edu, michael.m.kuperstein at intel.com
    Classification: Unclassified

Created attachment 14080
  --> https://llvm.org/bugs/attachment.cgi?id=14080&action=edit
A testcase

llvm generates multi-byte nop instructions greater than 10 bytes,
which have performance on x86 processors. GNU assembler has been
changed to limit multi-byte nop instructions to 10 bytes:

https://sourceware.org/ml/binutils/2015-03/msg00260.html

to avoid nops like:

66 66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00     data16 data16 data16 data16
data16 nopw %cs:0x0(%rax,%rax,1)
66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00     data16 data16 data16 data16
data16 nopw %cs:0x0(%rax,%rax,1)
66 66 66 66 2e 0f 1f 84 00 00 00 00 00     data16 data16 data16 nopw
%cs:0x0(%rax,%rax,1)
66 66 66 2e 0f 1f 84 00 00 00 00 00     data16 data16 nopw %cs:0x0(%rax,%rax,1)
66 66 2e 0f 1f 84 00 00 00 00 00     data16 nopw %cs:0x0(%rax,%rax,1)

llvm should also be updated.

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