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[PATCH] x86-64: Improve branch predication in _dl_runtime_resolve_avx512_opt [BZ #21258]
- From: "H.J. Lu" <hongjiu dot lu at intel dot com>
- To: GNU C Library <libc-alpha at sourceware dot org>
- Date: Thu, 16 Mar 2017 13:39:11 -0700
- Subject: [PATCH] x86-64: Improve branch predication in _dl_runtime_resolve_avx512_opt [BZ #21258]
- Authentication-results: sourceware.org; auth=none
- Reply-to: "H.J. Lu" <hjl dot tools at gmail dot com>
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.
Any comments?
H.J.
---
[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.
---
sysdeps/x86_64/dl-trampoline.S | 3 +--
sysdeps/x86_64/dl-trampoline.h | 9 +++++----
2 files changed, 6 insertions(+), 6 deletions(-)
diff --git a/sysdeps/x86_64/dl-trampoline.S b/sysdeps/x86_64/dl-trampoline.S
index 33d7fcf..c14c61a 100644
--- a/sysdeps/x86_64/dl-trampoline.S
+++ b/sysdeps/x86_64/dl-trampoline.S
@@ -87,11 +87,9 @@
#endif
#define VEC(i) zmm##i
#define _dl_runtime_resolve _dl_runtime_resolve_avx512
-#define _dl_runtime_resolve_opt _dl_runtime_resolve_avx512_opt
#define _dl_runtime_profile _dl_runtime_profile_avx512
#include "dl-trampoline.h"
#undef _dl_runtime_resolve
-#undef _dl_runtime_resolve_opt
#undef _dl_runtime_profile
#undef VEC
#undef VMOV
@@ -145,4 +143,5 @@
# define VMOV vmovdqu
#endif
#define _dl_runtime_resolve _dl_runtime_resolve_sse_vex
+#define _dl_runtime_resolve_opt _dl_runtime_resolve_avx512_opt
#include "dl-trampoline.h"
diff --git a/sysdeps/x86_64/dl-trampoline.h b/sysdeps/x86_64/dl-trampoline.h
index b27fa06..8db24c1 100644
--- a/sysdeps/x86_64/dl-trampoline.h
+++ b/sysdeps/x86_64/dl-trampoline.h
@@ -129,19 +129,20 @@ _dl_runtime_resolve_opt:
# YMM state isn't in use.
PRESERVE_BND_REGS_PREFIX
jz _dl_runtime_resolve_sse_vex
-# elif VEC_SIZE == 64
+# elif VEC_SIZE == 16
# For ZMM registers, check if YMM state and ZMM state are in
# use.
andl $(bit_YMM_state | bit_ZMM0_15_state), %r11d
cmpl $bit_YMM_state, %r11d
- # Preserve %xmm0 - %xmm7 registers with the zero upper 384 bits if
- # neither YMM state nor ZMM state are in use.
+ # Preserve %zmm0 - %zmm7 registers if ZMM state is in use.
PRESERVE_BND_REGS_PREFIX
- jl _dl_runtime_resolve_sse_vex
+ jg _dl_runtime_resolve_avx512
# Preserve %ymm0 - %ymm7 registers with the zero upper 256 bits if
# ZMM state isn't in use.
PRESERVE_BND_REGS_PREFIX
je _dl_runtime_resolve_avx
+ # Preserve %xmm0 - %xmm7 registers with the zero upper 384 bits if
+ # neither YMM state nor ZMM state are in use.
# else
# error Unsupported VEC_SIZE!
# endif
--
2.9.3