x86-64: Improve branch predication in _dl_runtime_resolve_avx512_opt [BZ #21258]

On Skylake server, _dl_runtime_resolve_avx512_opt is used to preserve
the first 8 vector registers.  The code layout is

  if only %xmm0 - %xmm7 registers are used
     preserve %xmm0 - %xmm7 registers
  if only %ymm0 - %ymm7 registers are used
     preserve %ymm0 - %ymm7 registers
  preserve %zmm0 - %zmm7 registers

Branch predication always executes the fallthrough code path to preserve
%zmm0 - %zmm7 registers speculatively, even though only %xmm0 - %xmm7
registers are used.  This leads to lower CPU frequency on Skylake
server.  This patch changes the fallthrough code path to preserve
%xmm0 - %xmm7 registers instead:

  if whole %zmm0 - %zmm7 registers are used
    preserve %zmm0 - %zmm7 registers
  if only %ymm0 - %ymm7 registers are used
     preserve %ymm0 - %ymm7 registers
  preserve %xmm0 - %xmm7 registers

Tested on Skylake server.

	[BZ #21258]
	* sysdeps/x86_64/dl-trampoline.S (_dl_runtime_resolve_opt):
	Define only if _dl_runtime_resolve is defined to
	_dl_runtime_resolve_sse_vex.
	* sysdeps/x86_64/dl-trampoline.h (_dl_runtime_resolve_opt):
	Fallthrough to _dl_runtime_resolve_sse_vex.

1 files changed, 9 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 3ed2df8..1367e8e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2017-03-21  H.J. Lu  <hongjiu.lu@intel.com>
+
+	[BZ #21258]
+	* sysdeps/x86_64/dl-trampoline.S (_dl_runtime_resolve_opt):
+	Define only if _dl_runtime_resolve is defined to
+	_dl_runtime_resolve_sse_vex.
+	* sysdeps/x86_64/dl-trampoline.h (_dl_runtime_resolve_opt):
+	Fallthrough to _dl_runtime_resolve_sse_vex.
+
 2017-03-21  Joseph Myers  <joseph@codesourcery.com>
 
 	* INSTALL: Regenerated.