[clang] [clang] Introduce elementwise clz/ctz builtins (PR #131995)

[Fraser Cormack via cfe-commits]

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@@ -847,6 +848,14 @@ of different sizes and signs is forbidden in binary and ternary builtins.
                                                 semantics, see `LangRef
                                                 <http://llvm.org/docs/LangRef.html#llvm-min-intrinsics-comparation>`_
                                                 for the comparison.
+ T __builtin_elementwise_clz(T x[, T y])        return the number of leading 0 bits in the first argument. If          integer types
+                                                the first argument is 0 and an optional second argument is provided,
+                                                the second argument is returned. If the first argument is 0 but only
+                                                one argument is provided, the result is undefined.
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frasercrmck wrote:

I originally had "the behaviour is undefined" as that's what `__builtin_clzg` has, but it was pointed out in another discussion above that we're only returning a `poison` value in this situation and this is narrower in scope than "undefined behaviour". I therefore changed it to "the result is undefined" as the GCC docs say that for `__builtin_clz`.

Is `poison` an "indeterminate" value? I can definitely see "erroneous value" in that `poison` is specified by LLVM as being the result of an erroneous operation.

I'm happy to change it to whatever.

https://github.com/llvm/llvm-project/pull/131995