Random data on formatting integers with Clang with Microsoft CodeGen

[Cleroth]

Put simply, when I write this:

cout << fmt::format("{}", 100u) << "\n";
cout << fmt::format("{}", 100) << "\n";
cout << fmt::format("{}", "aoeu") << "\n";
cout << fmt::format("{}", 2.5) << "\n";
cout << fmt::format("{} - {} - {}", 2.5, 3, "a") << "\n";

I get:

♀õôw100
0¸+
aoeu
2.5
2.5 - ÍשO/☺P§3 - a

The characters seem to be random, varying every time I run the program.
I have ostreams included, if that matters.

Compiler: Visual Studio 2017 - Clang with Microsoft CodeGen (v141_clang_c2)

[Cleroth]

The problem occurs because of FMT_BUILTIN_CLZ. The intrinsic version is wrong.
# define FMT_BUILTIN_CLZ(n) __builtin_clz(n)
should be:
# define FMT_BUILTIN_CLZ(n) (31 - __builtin_clz(n))

And similar for the 64-bit version.

[Cleroth]

Also, I'm concerned about the intrinsic being used by default with some compilers. I'm not sure if all current CPUs actually support it.

[vitaut]

Thanks for the bug report and the PR. As I commented on the PR, a simple workaround is to conditionally compile FMT_BUILTIN_CLZ.

Also, I'm concerned about the intrinsic being used by default with some compilers. I'm not sure if all current CPUs actually support it.

I would expect compilers provide a fallback implementation in the runtime.

[vitaut]

Fixed in 569c5bd.