[LLVMdev] [PATCH] Do not generate nopl instruction on CPUs that don't support it.
Mikulas Patocka
mikulas at artax.karlin.mff.cuni.cz
Thu Nov 7 15:31:33 PST 2013
On Tue, 5 Nov 2013, Rafael EspĂndola wrote:
> Please include a testcase with the patch.
I'm sending testcase here. Compile it with
"clang -O2 -march=k6-2 -c loop.c"
> gas uses " nopl 0x0(%eax)" for k6_2. Are you sure it is a gas bug?
Yes, it is gas bug. I should report it to binutils maintainers.
Mikulas
> 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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> > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
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>
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