Forcing 64-bit long doubles?

Question

I'm building musl-libc statically for a project on an aarch64 (ARM 64-bit) platform. I'd like to avoid any soft floating point libraries such as GCC's soft float library routines. However, these are still appearing in the library archives even when I use -mfloat-abi=hard. As best I can tell, this is because ARM 64-bit platforms define a long double to be 128 bits.

Is there any way to change this behavior? For example, could I force long double to be defined as the same size as a double? I know this is allowed by the C standard, but I'm unsure if there's any way to force Clang (I'm specifically using Clang for this) to compile with such a definition.

Answer 1

I ultimately found a solution, though I can't necessarily recommend it for everyone. It likely will induce bugs elsewhere, but it was good enough for what I needed. It also involves building Clang from scratch (thanks for the suggestion @Art!). Furthermore, the project I'm working on is using LLVM/Clang 3.7.1, so I make no claims for other versions.

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As near as I can tell, the definition of a long double for the AArch64 target occurs in clang/lib/Basic/Targets.cpp:

...
MaxAtomicInlineWidth = 128;
MaxAtomicPromoteWidth = 128;

LongDoubleWidth = LongDoubleAlign = SuitableAlign = 128;
LongDoubleFormat = &llvm::APFloat::IEEEquad;

// {} in inline assembly are neon specifiers, not assembly variant
// specifiers.
...

By modifying the inner 2 lines, I removed any references to the soft-FP routines I mentioned in my question:

LongDoubleWidth = LongDoubleAlign = SuitableAlign = 64;
LongDoubleFormat = &llvm::APFloat::IEEEdouble;

My test programs -- SNU's version of the NASA Parallel Benchmarks -- still validate correctly, so I'm assuming I didn't break anything too badly. Still, this is a non-trivial modification -- I don't recommend it for most individuals (it could lead to breakage elsewhere).