Re: Is there an intrinsic similar to _BitScanForward on Linux?

[LIU Hao <lh_mouse@126.com>]

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在 2022/11/30 15:23, bootmgr--- via Gcc-help 写道:
> 
> And I want to port it to Linux, so I am looking for any functions like _BitScanForward. I trying to use
> __builtin_ctz instead _BitScanForward, but it is undefined when x is 0 and it can't
> determine whether x is 0 or not. Then I found __bsfd in x86intrin.h, but it's the same as
> __builtin_ctz, which can't determine x is 0.
> 
> 
> So is there any solution, preferably cross-platform?
> 
> 
	
```
__attribute__((__gnu_inline__, __nothrow__, __pure__))
extern __inline__
bool
_BitScanForward(uint32_t* index, uint32_t value)
   {
     bool zf;
     __asm__ (
       "{ bsfl %2, %1 | bsf %1, %2 }"
       : "=@ccz"(zf), "=r"(*index) : "rm"(value)
     );
     return !zf;
   }
```

Godbolt:  https://gcc.godbolt.org/z/vzEjGj11e


-- 
Best regards,
LIU Hao


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