[llvm-bugs] [Bug 38045] New: LLVM fails to generate REX.W prefix

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38045

            Bug ID: 38045
           Summary: LLVM fails to generate REX.W prefix
           Product: libraries
           Version: 6.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: bjornpagen at gmail.com
                CC: llvm-bugs at lists.llvm.org

https://godbolt.org/g/fkRRsE

llc fails to compile this for all clang >= 3.5:

------------------------------------------------------------
; ModuleID = 'rexw.c'
source_filename = "rexw.c"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

module asm "rex.w sysexit"

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{!"clang version 6.0.0 (tags/RELEASE_600/final)"}
------------------------------------------------------------

with output:

------------------------------------------------------------
<inline asm>:1:1: error: invalid instruction mnemonic 'rex.w'
rex.w sysexit
^~~~~
------------------------------------------------------------

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