Re: [RFC PATCH] x86/hweight: Get rid of the special calling convention

From: H. Peter Anvin
Date: Wed May 04 2016 - 15:34:46 EST

On 05/04/2016 12:31 PM, Brian Gerst wrote:
>> - asm (ALTERNATIVE("call __sw_hweight32", POPCNT32, X86_FEATURE_POPCNT)
>> - : "="REG_OUT (res)
>> - : REG_IN (w));
>> + if (likely(static_cpu_has(X86_FEATURE_POPCNT))) {
>> + asm volatile("popcnt %[w], %[res]" : [res] "=r" (res) : [w] "r" (w));
> Do all supported versions of the assembler know of the popcnt
> instruction? That's why is was open coded before. The problem is
> Intel and AMD are constantly adding new instructions and it's a long
> cycle for the user's assembler to get updated.

Most likely not. It would be nice to have some more uniform solution to
that. I'm wondering if we could use the -Wa option to load some kind of
macro package.