[cfe-dev] [libcxx] Win32 port must not use __popcnt intrinsect

[Howard Hinnant]

Committed revision 145684.

Sorry it took me so long to review this.

Howard

On Nov 24, 2011, at 7:25 AM, Jean-Daniel Dupas wrote:

> 
> Le 24 nov. 2011 à 12:42, Ruben Van Boxem a écrit :
> 
>> 2011/11/24 Jean-Daniel Dupas <devlists at shadowlab.org>:
>>> Hello,
>>> While browsing win32 support headers of the libcxx, I found these macros:
>>> #define __builtin_popcount __popcnt
>>> #define __builtin_popcountl __popcnt
>>> #define __builtin_popcountll(__i) static_cast<int>(__popcnt64(__i))
>>> While using _BitScanReverse and other built-in is fine, using  __popcnt is
>>> not safe at all. Whatever the cpu target is, the compiler will generate a
>>> __popcnt instruction, even if it does not support it.
>>> From MSDN documentation:
>>> To determine hardware support for the popcnt instruction, call
>>> the __cpuid intrinsic with InfoType=0x00000001 and check bit 23 ofCPUInfo[2]
>>> (ECX). This bit is 1 if the instruction is supported, and 0 otherwise. If
>>> you run code that uses this intrinsic on hardware that does not support
>>> the popcnt instruction, the results are unpredictable.
>>> Where "unpredictable" generally means raising an "invalid instruction"
>>> exception.
>>> This macros should be either removed if there are not used, or replaced by a
>>> safe implementation.
>> 
>> That's my fault. After further investigation, you are completely
>> correct: I thought I found that all x86 processors supported these
>> instructions, but that was only valid for the _BitScan* functions). I
>> don't know how to fix this (I'm just a humble hobbyist), so patches
>> are welcome.
>> 
>> The problem is only theoretical though, as libc++ is unusable with
>> current (and near-future) versions of MSVC, as their C++11 support is
>> not good enough for the rest of the library as far as I can tell.
>> There's problems with decltype and overload resolution (which is
>> really to compiler-level to work around). The intrinsics in question
>> are only used for MSVC/non-Clang/non-GCC compiles.
>> 
> 
> You're right. I recently read about the new feature of VS 2011, and it look like we may have to wait for a long time before it append.
> I just sketched a patch with a standard implementation of popcnt, but as I didn't try to compile it. 
> But as this code cannot really be tested until the MS compiler is able to compile libc++, may commenting out these macros is enough (with a comment explaining why).
> 
>> I'm working (or rather, as working) on using a MSVC-based Clang to
>> further enhance libc++ compatibility with Visual Studio headers, but
>> haven't gotten to useful parts yet. Help is welcome!
> 
> As many people, I really lack of spare time to do all the things I'd like, and hacking on clang and libc++ on windows is part of this things.
> So if I find time to help, be sure I will let you know ;-)
> 
>> Thanks for the interest,
>> 
>> Ruben
> 
> -- Jean-Daniel
> 
> 
> 
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