[LLVMdev] [PATCH] Do not generate nopl instruction on CPUs that don't support it.

[Rafael Espíndola]

Please include a testcase with the patch.

gas uses " nopl   0x0(%eax)" for k6_2. Are you sure it is a gas bug?

On 3 November 2013 13:50, Mikulas Patocka
<mikulas at artax.karlin.mff.cuni.cz> wrote:
> Hi
>
> This patch fixes code generation bug - 586-class CPUs don't support the
> nopl instruction and some 686-class CPUs don't support it too.
>
> I created bug 17792 for that.
>
>
> BTW. I think you should also optimize padding on these CPUs - instead of a
> stream of 0x90 nops, you should generate variants of "lea (%esi), %esi"
> instruction like gcc.
>
>
>
> This patch disables generation of nopl instructions on the following CPUs:
>
> i686 - there are i686-class CPUs that don't have nopl: Via c3, Transmeta
>         Crusoe, Microsoft VirtualBox - see
>         https://bbs.archlinux.org/viewtopic.php?pid=775414
> k6, k6-2, k6-3, winchip-c6, winchip2 - these are 586-class CPUs
> via c3 c3-2 - see https://bugs.archlinux.org/task/19733 as a proof that
>         Via c3 and c3-Nehemiah don't have nopl
>
> Mikulas
>
> ---
>  lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> Index: llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
> ===================================================================
> --- llvm.orig/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp     2013-11-03 00:24:20.000000000 +0100
> +++ llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp  2013-11-03 20:57:14.000000000 +0100
> @@ -309,7 +309,10 @@ bool X86AsmBackend::writeNopData(uint64_
>    // This CPU doesnt support long nops. If needed add more.
>    // FIXME: Can we get this from the subtarget somehow?
>    if (CPU == "generic" || CPU == "i386" || CPU == "i486" || CPU == "i586" ||
> -      CPU == "pentium" || CPU == "pentium-mmx" || CPU == "geode") {
> +      CPU == "pentium" || CPU == "pentium-mmx" || CPU == "i686" ||
> +      CPU == "k6" || CPU == "k6-2" || CPU == "k6-3" || CPU == "geode" ||
> +      CPU == "winchip-c6" || CPU == "winchip2" || CPU == "c3" ||
> +      CPU == "c3-2") {
>      for (uint64_t i = 0; i < Count; ++i)
>        OW->Write8(0x90);
>      return true;
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