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

[Jean-Daniel Dupas]

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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