Re: __builtin_add_overflow on clang

[Paul Eggert]

Mattias Engdegård wrote:
> __builtin_mul_overflow(long long) does expand to a call to __mulodi4 when 
> building for 32-bit x86 even with current Clang; either this function is absent 
> from libgcc and these platforms do not link to compiler-rt, or their 
> compiler-rt is somehow built without it. The former seems more likely.
> Did the bug report include any Clang/LLVM version information?

No, he built with lots of clangs on lots of platforms, many with clang locally 
installed instead of being packaged by the vendor, and he typically didn't give 
version numbers.

He did mention that openSUSE 13 had the relevant library here:

/usr/local/ashare/llvm/llvm-20140716/lib/clang/3.5.0/lib/linux/libclang_rt.builtins-x86_64.a

but it wasn't clear whether this library was from an optional package as opposed 
to the barebones Clang package. He also said that this particular library "is 
absent from some later clang releases" - not sure whether that means absent from 
that package in openSUSE, or something else.

In many of the builds he could get clang to work by configuring with CC=clang 
LDFLAGS=--rtlib=compiler-rt. However, this wasn't always enough. Eventually he 
gave up on using clang to compile GNU sed because of the __builtin_mul_overflow 
hassle.

> We can safely assume that macOS will use compiler-rt by default rather than 
> libgcc

In that case I installed the attached, which uses __builtin_mul_overflow with 
Clang on macOS.

0001-Speed-up-INT_MULTIPLY_WRAPV-on-macOS.patch
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