[Bug binutils/27397] CET autodetection fails configure on i486 and i586 targets

[slyfox at inbox dot ru]

https://sourceware.org/bugzilla/show_bug.cgi?id=27397

--- Comment #8 from Sergei Trofimovich <slyfox at inbox dot ru> ---
(In reply to H.J. Lu from comment #7)
> (In reply to Sergei Trofimovich from comment #6)
> > (In reply to H.J. Lu from comment #5)
> > > Created attachment 13220 [details]
> > > An updated patch
> > > 
> > > Try this.
> > 
> > The user reports it's building fine for them on i486:
> > https://bugs.gentoo.org/770061#c26
> > 
> > AFAIU event with the patch ./configure --host=i686-pc-linux-gnu if ran on
> > pre-sse2 CPU, right?
> 
> SSE2 isn't the issue.  The multi-byte NOP is the issue.

Oh, I thought these multibyte NOP come from SSE2 encoding. I'd like to
understand why then these NOPs crash the program with SIGILL.

Looking at endbr32 instruction as an example: "F3 0F 1E FB".

Do I understand correctly it's supposed to be decoded by old CPUs as:
    prefix=F3 (REP) escape="0F 1E" (Reserved-NOP) modrm(?)=FB

Do you know when these 'Reserved-NOP' were introduced as valid instruction to
IA-32? I wonder why K6-III can't handle it: is it a CPU bug (imprecise
implementation of reserved NOPs) or it was introduced later.

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