parser.c: Optimize json_parse_digits

[byroot]

We can use ctz builtin to get the number of consecutive digits.

== Parsing float parsing (2251051 bytes)
ruby 3.4.6 (2025-09-16 revision dbd83256b1) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after    25.000 i/100ms
Calculating -------------------------------------
               after    231.963 (± 0.0%) i/s    (4.31 ms/i) -      1.175k in   5.065467s

Comparison:
              before:      215.3 i/s
               after:      232.0 i/s - 1.08x  faster

[byroot]

Interestingly, this seem to fail on ubuntu/GCC, but it passes on clang. I'm not yet clear on why.

[tompng]

From a google search result, __builtin_ctzll(0) seems to be an undefined behavior. I'm not sure it's really undefined though.
The trailing_zeros64 fallbacks code when __builtin_ctzll is not defined, actually returns 0 instead of 64 in this case. So at least trailing_zeros64 fallback code should be fixed.

# When this part is commented out:
# #if HAVE_BUILTIN_CTZLL
#     return __builtin_ctzll(input);
# #else

JSON.parse "[12341234]" #=> JSON::ParserError

[byroot]

Yes thank you I just figured this out:

int __builtin_clz (unsigned int x)

Returns the number of leading 0-bits in x, starting at the most significant bit position. If x is 0, the result is undefined.

https://gcc.gnu.org/onlinedocs/gcc/Bit-Operation-Builtins.html

I think GCC assume this case never happens so it removes that branch.

[byroot]

Now I need to review usage of trailing_zeros and trailing_zeros64...

[byroot]

I'm doubting these asserts can actually fail. Not too sure what the compilation flags are on CI, but seems like they are optimized out even with JSON_DEBUG.

I'm unsure what's the best way to add assertions in ruby extensions. Perhaps I should just use RUBY_ASSERT and add a debug ruby in the CI matrix?