Documentation: std::intrinsics::cttz does not specify the handling of zero.

[gnzlbg]

The llvm docs for `llvm.cttz.iX(value, flag) specifies that a flag can be passed to control whether zero provides a defined result or not.

Whether std::intrinsics::cttz provides a defined result for zero or not should be specified in the documentation, since if not some code might require if x == 0 { mem::sizeof(x) * 8 } else { std::intrinsics::cttz(x) } to be used (instead of just std::intrinsics::cttz(x)).

See the LLVM documentation: http://llvm.org/docs/LangRef.html#llvm-cttz-intrinsic

[gnzlbg]

From:

rust/src/librustc_typeck/check/intrinsic.rs

Line 264 in 34fd686

"ctpop" | "ctlz" | "cttz" | "bswap" => (1, vec!(param(ccx, 0)), param(ccx, 0)),

It might seem that the intrinsic is always called with a value of 0 (false) but I cannot find where the std::intrinsic::cttz function is implemented to verify this. And the LLVM docs are a bit confusing about what exactly the flag means. The text seems to indicate that true means "consistent handling of zero" but the argument name "is_zero_undef" indicates that true means "inconsistent handling of zero".

[nagisa]

Ah I see now. This intrinsic is used to implement u*::trailing_zeros method, therefore we cannot afford it to be undef for corner-case values. It seems safe to say we pass whatever the flag (which turns out to be false for “0 is defined”) makes 0 input defined for this intrinsic without even checking the implementation.

[gnzlbg]

This intrinsic is used to implement u*::trailing_zeros method, therefore we cannot afford it to be undef for corner-case values.

That makes sense. Having to do x == 0? 8 * CHAR_BITS : __builtin_ctz(x) in C++ is error prone (since this behavior is what is actually desired all of the time).

Where is u*::trailing_zeros / std::intrinsic::cttz implemented? I was trying to find the source to see which argument is passed without any luck.

I guess to improve the docs it would be enough to add a note like:

note: when x == 0 the behavior of std::intrinsic::cttz(x) is defined in all platforms to return 8 * mem::sizeof(x) (that is, the llvm.cttz.iX intrinsic is called with the flag zero_is_undef set to false).

[nagisa]

Where is u*::trailing_zeros / std::intrinsic::cttz implemented? I was trying to find the source to see which argument is passed without any luck.

Easy way to check that is to ask rustc to emit LLVM-IR for your code. If you click LLVM IR on this playpen, you’ll find a %3 = call i64 @llvm.cttz.i64(i64 %2, i1 false), !dbg !25.

Trailing_zeros is implemented here.

[gnzlbg]

Trailing_zeros is implemented here.

Thanks! Where is std::intrinsic::cttz implemented? That should call/generate the llvm.cttz intrinsic and pass the arguments somehow.

If you click LLVM IR on this playpen, you’ll find a %3 = call i64 @llvm.cttz.i64(i64 %2, i1 false), !dbg !25.

Nice, TIL! One can also output the asm directly in the playpen :)

[nagisa]

Where is std::intrinsic::cttz implemented?

Its implemented deep inside the translator and is not you can tweak without modifying the compiler itself. The LLVM intrinsic is emitted here and here.

[frewsxcv]

PR opened #38310