r221064 - Avoid undefined behavior in the x86 lzcnt header file by explicitly checking for 0 before calling __builtin_clz. Without this the optimizers may take advantage of the undefined behavior and produce incorrect results. LLVM itself still needs

[Robinson, Paul]

Thanks.  I've been trying to do this today but having trouble getting to
my work systems from home.

Are there __builtin_ctz* calls (in bmiintrin.h or whatever) that assume they
have tzcnt, and are also missing guards?
--paulr

> -----Original Message-----
> From: cfe-commits-bounces at cs.uiuc.edu [mailto:cfe-commits-
> bounces at cs.uiuc.edu] On Behalf Of Craig Topper
> Sent: Saturday, November 01, 2014 3:25 PM
> To: cfe-commits at cs.uiuc.edu
> Subject: r221064 - Avoid undefined behavior in the x86 lzcnt header file
> by explicitly checking for 0 before calling __builtin_clz. Without this
> the optimizers may take advantage of the undefined behavior and produce
> incorrect results. LLVM itself still needs t...
> 
> Author: ctopper
> Date: Sat Nov  1 17:25:23 2014
> New Revision: 221064
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=221064&view=rev
> Log:
> Avoid undefined behavior in the x86 lzcnt header file by explicitly
> checking for 0 before calling __builtin_clz. Without this the optimizers
> may take advantage of the undefined behavior and produce incorrect
> results. LLVM itself still needs to be taught to merge the zero check into
> the llvm.ctlz with defined zero behavior.
> 
> Modified:
>     cfe/trunk/lib/Headers/lzcntintrin.h
> 
> Modified: cfe/trunk/lib/Headers/lzcntintrin.h
> URL: http://llvm.org/viewvc/llvm-
> project/cfe/trunk/lib/Headers/lzcntintrin.h?rev=221064&r1=221063&r2=221064
> &view=diff
> ==========================================================================
> ====
> --- cfe/trunk/lib/Headers/lzcntintrin.h (original)
> +++ cfe/trunk/lib/Headers/lzcntintrin.h Sat Nov  1 17:25:23 2014
> @@ -35,20 +35,20 @@
>  static __inline__ unsigned short __attribute__((__always_inline__,
> __nodebug__))
>  __lzcnt16(unsigned short __X)
>  {
> -  return __builtin_clzs(__X);
> +  return __X ? __builtin_clzs(__X) : 16;
>  }
> 
>  static __inline__ unsigned int __attribute__((__always_inline__,
> __nodebug__))
>  __lzcnt32(unsigned int __X)
>  {
> -  return __builtin_clz(__X);
> +  return __X ? __builtin_clz(__X) : 32;
>  }
> 
>  #ifdef __x86_64__
>  static __inline__ unsigned long long __attribute__((__always_inline__,
> __nodebug__))
>  __lzcnt64(unsigned long long __X)
>  {
> -  return __builtin_clzll(__X);
> +  return __X ? __builtin_clzll(__X) : 64;
>  }
>  #endif
> 
> 
> 
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