[glibc] [powerpc] No need to enter "Ignore Exceptions Mode"

[Paul Clarke <pc at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=7b8481b330720d28c019a2e5994492a1923d5daa

commit 7b8481b330720d28c019a2e5994492a1923d5daa
Author: Paul A. Clarke <pc@us.ibm.com>
Date:   Thu Sep 19 11:11:04 2019 -0500

    [powerpc] No need to enter "Ignore Exceptions Mode"
    
    Since at least POWER8, there is no performance advantage to entering
    "Ignore Exceptions Mode", and doing so conditionally requires
     - the conditional logic, and
     - a system call.
    
    Make it a no-op for uses within glibc.

Diff:
---
 sysdeps/powerpc/fpu/fenv_private.h | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/sysdeps/powerpc/fpu/fenv_private.h b/sysdeps/powerpc/fpu/fenv_private.h
index 5eedc3b..4af767b 100644
--- a/sysdeps/powerpc/fpu/fenv_private.h
+++ b/sysdeps/powerpc/fpu/fenv_private.h
@@ -23,6 +23,17 @@
 #include <fenv_libc.h>
 #include <fpu_control.h>
 
+#ifdef _ARCH_PWR8
+/* There is no performance advantage to non-stop mode.  */
+/* The odd syntax here is to innocuously reference the given variables
+   to prevent warnings about unused variables.  */
+#define __TEST_AND_BEGIN_NON_STOP(old, new) do {} while ((old) * (new) * 0 != 0)
+#define __TEST_AND_END_NON_STOP(old, new) do {} while ((old) * (new) * 0 != 0)
+#else
+#define __TEST_AND_BEGIN_NON_STOP __TEST_AND_ENTER_NON_STOP
+#define __TEST_AND_END_NON_STOP __TEST_AND_EXIT_NON_STOP
+#endif
+
 static __always_inline void
 libc_feholdexcept_setround_ppc (fenv_t *envp, int r)
 {
@@ -30,7 +41,7 @@ libc_feholdexcept_setround_ppc (fenv_t *envp, int r)
 
   old.fenv = *envp = fegetenv_register ();
 
-  __TEST_AND_ENTER_NON_STOP (old.l, 0ULL);
+  __TEST_AND_BEGIN_NON_STOP (old.l, 0ULL);
 
   /* Clear everything and set the rounding mode.  */
   new.l = r;
@@ -49,8 +60,8 @@ __libc_femergeenv_ppc (const fenv_t *envp, unsigned long long old_mask,
   /* Merge bits while masking unwanted bits from new and old env.  */
   new.l = (old.l & old_mask) | (new.l & new_mask);
 
-  __TEST_AND_EXIT_NON_STOP (old.l, new.l);
-  __TEST_AND_ENTER_NON_STOP (old.l, new.l);
+  __TEST_AND_END_NON_STOP (old.l, new.l);
+  __TEST_AND_BEGIN_NON_STOP (old.l, new.l);
 
   /* If requesting to keep status, replace control, and merge exceptions,
      and exceptions haven't changed, we can just set new control instead
@@ -141,7 +152,7 @@ libc_feholdsetround_noex_ppc_ctx (struct rm_ctx *ctx, int r)
   ctx->env = old.fenv;
   if (__glibc_unlikely (new.l != old.l))
     {
-      __TEST_AND_ENTER_NON_STOP (old.l, 0ULL);
+      __TEST_AND_BEGIN_NON_STOP (old.l, 0ULL);
       fesetenv_control (new.fenv);
       ctx->updated_status = true;
     }