Re: Which POWER implementations need the HWCAP_DFP check?

[Tulio Magno Quites Machado Filho <tuliom at linux dot vnet dot ibm dot com>]

Florian Weimer <fweimer@redhat.com> writes:

> I think this run-time check is unnecessary on POWER6 and later:
>
> 	  double d = (env); \
> 	  if(GLRO(dl_hwcap) & PPC_FEATURE_HAS_DFP) \
> 	    asm volatile (".machine push; " \
> 			  ".machine \"power6\"; " \
> 			  "mtfsf 0xff,%0,1,0; " \
> 			  ".machine pop" : : "f" (d)); \
> 	  else \
> 	    asm volatile ("mtfsf 0xff,%0" : : "f" (d)); \

I think it's still necessary when building glibc for an older processor,
e.g. POWER4, and run it on processors that support DFP, e.g. POWER6.

> In particularly, we do not need it on ppc64le.  Correct?

Agreed, unless the kernel has the right to disable DFP support.

> It's one of the GLIBC_PRIVATE references in libm.so.6, which is why I'm 
> asking.

Would the following code solve this problem?

#include <tls.h>
/* Equivalent to fesetenv, but takes a fenv_t instead of a pointer.  */
#define fesetenv_register(env) \
	do { \
	  double d = (env); \
	  if(THREAD_GET_HWCAP() & PPC_FEATURE_HAS_DFP) \
	    asm volatile (".machine push; " \
			  ".machine \"power6\"; " \
			  "mtfsf 0xff,%0,1,0; " \
			  ".machine pop" : : "f" (d)); \
	  else \
	    asm volatile ("mtfsf 0xff,%0" : : "f" (d)); \
	} while(0)


-- 
Tulio Magno