[PATCH] D75723: [X86] Make intrinsics _BitScan* not limited to Windows

[Kan Shengchen via Phabricator via cfe-commits]

skan marked an inline comment as done.
skan added inline comments.


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Comment at: clang/include/clang/Basic/BuiltinsX86.def:1904
+// BITSCAN
+TARGET_BUILTIN(_BitScanForward, "UcUNi*UNi", "n", "")
+TARGET_BUILTIN(_BitScanReverse, "UcUNi*UNi", "n", "")
----------------
rnk wrote:
> craig.topper wrote:
> > skan wrote:
> > > craig.topper wrote:
> > > > The N specifier here is sort of MSVC mode specific. I need to think about this.
> > > > 
> > > > This also makes this available without including a header file which isn't good if it doesn't start with __builtin.
> > > I think we can define a new builtin `__builtin_ia32_BitScanForward` in BuiltinsX86.def. And when macro `_MSC_VER` is not defined, define `_BitScanForward` in ia32intrin.h by calling the `__builtin_ia32_BitScanForward`. Then we can avoid the N specifier and the name issue. Do you think it's appropriate?
> > Instead of new builtins, can we use  __builtin_clz, __builtin_clzl, __builin_ctz, __builtin_ctzl?
> Right, even though _BitScan* is in the implementers namespace, we want to be careful about adding builtins that don't start with `__builtin_`. Unless we set some dramatically new direction of making all the implementer's namespace MSVC builtins available everywhere, I don't see why we would do this when we already have equivalent builtins.
> 
> Is there some particular motivation as to why you want to make these available everywhere? `-fms-extensions` is kind of already available everywhere. This compiles on Linux with `-fms-extensions`:
> ```
> extern unsigned char _BitScanReverse64(unsigned int *, unsigned long long);
> unsigned char test_BitScanReverse64(unsigned int *index, unsigned long long mask) {
>   return _BitScanReverse64(index, mask);
> }
> ```
My thought is that the page [[ https://software.intel.com/sites/landingpage/IntrinsicsGuide/#expand=408,410,404,460,404,405,408&text=BitScan | intel intrinsic guide ]] says we can use the intrinsics _BitScan* as long as we include the header file  immintrin.h, so we need support them on linux for user convenience even though similar intrinsics _bit_scan_* are available.

Since the purpose is to support **intrinsics** _BitScan* on linux rather than **builtin** _BitScan*, I think we can wrap the intrinsiscs with existing builtin as craig suggested. 


Repository:
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  https://reviews.llvm.org/D75723/new/

https://reviews.llvm.org/D75723