[llvmlinux] [PATCH] x86/hweight: LLVMLinux: Fix __arch_hweight{32, 64}() for compilation with clang

[Jan-Simon Moeller]

Am Sonntag, 13. September 2015, 23:14:22 schrieb PaX Team:

> > does not specify that the "call __sw_hweight64" instruction may 
clobber
> > caller saved registers (like %rcx and %rdx).
> 
> actually, the __sw_hweight* functions may *not* clobber anything. the
> trick is in how lib/hweight.o is compiled: CFLAGS contains a set of
> -fcall-saved-REG switches (cf. ARCH_HWEIGHT_CFLAGS) that effectively
> override the function call ABI for the functions defined in this file
> so there's no need for the callers to specify any extra clobbers. this
> way the fast path execution (inlined popcnt insn) doesn't unnecessarily
> perturb the register allocator in the callers.

The issues is that clang does not support these flags. So exactly the
"do not clobber" trick does not happen. 

/js