Patchwork [4/4,powerpc] fegetenv_status: simplify instruction generation

login
register
mail settings
Submitter Paul Clarke
Date Aug. 20, 2019, 9:19 p.m.
Message ID <1566335985-14601-5-git-send-email-pc@us.ibm.com>
Download mbox | patch
Permalink /patch/34200/
State New
Headers show

Comments

Paul Clarke - Aug. 20, 2019, 9:19 p.m.
From: "Paul A. Clarke" <pc@us.ibm.com>

fegetenv_status() wants to use the lighter weight instruction 'mffsl'
for reading the Floating-Point Status and Control Register (FPSCR).
It currently will use it directly if compiled '-mcpu=power9', and will
perform a runtime check (cpu_supports("arch_3_00")) otherwise.

Nicely, it turns out that the 'mffsl' instruction will decode to
'mffs' on architectures older than "arch_3_00" because the additional
bits set for 'mffsl' are "don't care" for 'mffs'.  'mffs' is a superset
of 'mffsl'.

So, just generate 'mffsl'.

2019-08-20  Paul A. Clarke  <pc@us.ibm.com>

	* sysdeps/powerpc/fpu/fenv_libc.h (fegetenv_status_ISA300):  Delete.
	(fegetenv_status):  Generate 'mffsl' unconditionally.
---
 sysdeps/powerpc/fpu/fenv_libc.h | 14 +-------------
 1 file changed, 1 insertion(+), 13 deletions(-)
Paul E Murphy - Aug. 21, 2019, 9:25 p.m.
On 8/20/19 4:19 PM, Paul A. Clarke wrote:
> From: "Paul A. Clarke" <pc@us.ibm.com>
> 
> fegetenv_status() wants to use the lighter weight instruction 'mffsl'
> for reading the Floating-Point Status and Control Register (FPSCR).
> It currently will use it directly if compiled '-mcpu=power9', and will
> perform a runtime check (cpu_supports("arch_3_00")) otherwise.
> 
> Nicely, it turns out that the 'mffsl' instruction will decode to
> 'mffs' on architectures older than "arch_3_00" because the additional
> bits set for 'mffsl' are "don't care" for 'mffs'.  'mffs' is a superset
> of 'mffsl'.

That is a pretty neat trick. I would recommend inlining the above 
comment. Otherwise, LGTM.

> -#define fegetenv_status_ISA300()					\
> +#define fegetenv_status()					\
>     ({register double __fr;						\
>       __asm__ __volatile__ (						\
>         ".machine push; .machine \"power9\"; mffsl %0; .machine pop"	\
> @@ -45,18 +45,6 @@ extern const fenv_t *__fe_mask_env (void) attribute_hidden;
>       __fr;								\
>     })

Patch

diff --git a/sysdeps/powerpc/fpu/fenv_libc.h b/sysdeps/powerpc/fpu/fenv_libc.h
index 8ba4832..186612b 100644
--- a/sysdeps/powerpc/fpu/fenv_libc.h
+++ b/sysdeps/powerpc/fpu/fenv_libc.h
@@ -37,7 +37,7 @@  extern const fenv_t *__fe_mask_env (void) attribute_hidden;
 /* Equivalent to fegetenv_register, but only returns bits for
    status, exception enables, and mode.  */
 
-#define fegetenv_status_ISA300()					\
+#define fegetenv_status()					\
   ({register double __fr;						\
     __asm__ __volatile__ (						\
       ".machine push; .machine \"power9\"; mffsl %0; .machine pop"	\
@@ -45,18 +45,6 @@  extern const fenv_t *__fe_mask_env (void) attribute_hidden;
     __fr;								\
   })
 
-#ifdef _ARCH_PWR9
-# define fegetenv_status() fegetenv_status_ISA300()
-#elif defined __BUILTIN_CPU_SUPPORTS__
-# define fegetenv_status()						\
-  (__glibc_likely (__builtin_cpu_supports ("arch_3_00"))		\
-   ? fegetenv_status_ISA300()						\
-   : fegetenv_register()						\
-  )
-#else
-# define fegetenv_status() fegetenv_register ()
-#endif
-
 /* Equivalent to fesetenv, but takes a fenv_t instead of a pointer.  */
 #define fesetenv_register(env) \
 	do { \