[llvm] r244503 - x86: Emit LAHF/SAHF instead of PUSHF/POPF

[JF Bastien via llvm-commits]

On Wed, Aug 12, 2015 at 10:53 AM, Reid Kleckner <rnk at google.com> wrote:

> On Mon, Aug 10, 2015 at 5:41 PM, Eli Friedman via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> On Mon, Aug 10, 2015 at 1:59 PM, JF Bastien via llvm-commits <
>> llvm-commits at lists.llvm.org> wrote:
>>
>>> Author: jfb
>>> Date: Mon Aug 10 15:59:36 2015
>>> New Revision: 244503
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=244503&view=rev
>>> Log:
>>> x86: Emit LAHF/SAHF instead of PUSHF/POPF
>>>
>>> NaCl's sandbox doesn't allow PUSHF/POPF out of security concerns
>>> (priviledged emulators have forgotten to mask system bits in the past, and
>>> EFLAGS's DF bit is a constant source of hilarity). Commit r220529 fixed
>>> PR20376 by saving cmpxchg's flags result using EFLAGS, this commit now
>>> generated LAHF/SAHF instead, for all of x86 (not just NaCl) because it
>>> leads to an overall performance gain over PUSHF/POPF.
>>>
>>
>> There's a problem with this: not all x86 CPUs support lahf/sahf in 64-bit
>> mode.
>>
>
> Which CPUs don't support lahf/sahf, old ones or new ones? It's basically
> the only safe way to copy into and out of eflags.
>

Old ones:

"Early AMD64 and Intel 64 CPUs lacked LAHF and SAHF instructions in 64-bit
mode. AMD introduced these instructions (also in 64-bit mode) with their
Athlon 64, Opteron and Turion 64 revision D processors in March 2005[44]
<https://en.wikipedia.org/wiki/X86-64#cite_note-44>[45]
<https://en.wikipedia.org/wiki/X86-64#cite_note-45>[46]
<https://en.wikipedia.org/wiki/X86-64#cite_note-46> while Intel introduced
the instructions with the Pentium 4 G1 stepping in December 2005."
 - https://en.wikipedia.org/wiki/X86-64
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