Patch to inline os::SpinPause() for X86 on non-Windows OS

[Man Cao]

Hello,

The Java platform team at Google has maintained a local patch to inline
os::SpinPause() since 2014. We would like to upstream this patch to
OpenJDK. Could someone sponsor this patch?

It is difficult to demonstrate performance improvement in Java benchmarks.
It is more of a code refactoring to better utilize modern GCC. It partly
addresses the comment about inlining SpinPause() above its declaration in
os.hpp.
I found an interesting discussion about PAUSE and a microbenchmark in:
http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2012-August/004352.html
However, the microbenchmark has a large variance in our experiment, making
it difficult to tell if there's any benefit from inlining PAUSE. Inlining
PAUSE does seem to reduce the variance a bit.

The patch is inlined and attached below:

diff --git a/src/hotspot/os_cpu/bsd_x86/bsd_x86_32.s
b/src/hotspot/os_cpu/bsd_x86/bsd_x86_32.s
--- a/src/hotspot/os_cpu/bsd_x86/bsd_x86_32.s
+++ b/src/hotspot/os_cpu/bsd_x86/bsd_x86_32.s
@@ -63,15 +63,6 @@
         popl     %eax
         ret

-        .globl  SYMBOL(SpinPause)
-        ELF_TYPE(SpinPause, at function)
-        .p2align 4,,15
-SYMBOL(SpinPause):
-        rep
-        nop
-        movl    $1, %eax
-        ret
-
         # Support for void Copy::conjoint_bytes(void* from,
         #                                       void* to,
         #                                       size_t count)
diff --git a/src/hotspot/os_cpu/bsd_x86/bsd_x86_64.s
b/src/hotspot/os_cpu/bsd_x86/bsd_x86_64.s
--- a/src/hotspot/os_cpu/bsd_x86/bsd_x86_64.s
+++ b/src/hotspot/os_cpu/bsd_x86/bsd_x86_64.s
@@ -46,15 +46,6 @@

  .text

-        .globl SYMBOL(SpinPause)
-        .p2align 4,,15
-        ELF_TYPE(SpinPause, at function)
-SYMBOL(SpinPause):
-        rep
-        nop
-        movq   $1, %rax
-        ret
-
         # Support for void Copy::arrayof_conjoint_bytes(void* from,
         #                                               void* to,
         #                                               size_t count)
diff --git a/src/hotspot/os_cpu/linux_x86/linux_x86_32.s
b/src/hotspot/os_cpu/linux_x86/linux_x86_32.s
--- a/src/hotspot/os_cpu/linux_x86/linux_x86_32.s
+++ b/src/hotspot/os_cpu/linux_x86/linux_x86_32.s
@@ -42,15 +42,6 @@

  .text

-        .globl  SpinPause
- .type   SpinPause, at function
-        .p2align 4,,15
-SpinPause:
-        rep
-        nop
-        movl    $1, %eax
-        ret
-
         # Support for void Copy::conjoint_bytes(void* from,
         #                                       void* to,
         #                                       size_t count)
diff --git a/src/hotspot/os_cpu/linux_x86/linux_x86_64.s
b/src/hotspot/os_cpu/linux_x86/linux_x86_64.s
--- a/src/hotspot/os_cpu/linux_x86/linux_x86_64.s
+++ b/src/hotspot/os_cpu/linux_x86/linux_x86_64.s
@@ -38,15 +38,6 @@

  .text

-        .globl SpinPause
-        .align 16
-        .type  SpinPause, at function
-SpinPause:
-        rep
-        nop
-        movq   $1, %rax
-        ret
-
         # Support for void Copy::arrayof_conjoint_bytes(void* from,
         #                                               void* to,
         #                                               size_t count)
diff --git a/src/hotspot/os_cpu/solaris_x86/solaris_x86_64.s
b/src/hotspot/os_cpu/solaris_x86/solaris_x86_64.s
--- a/src/hotspot/os_cpu/solaris_x86/solaris_x86_64.s
+++ b/src/hotspot/os_cpu/solaris_x86/solaris_x86_64.s
@@ -51,15 +51,6 @@
         movq %fs:0x0,%rax
         ret

-        .globl  SpinPause
-        .align  16
-SpinPause:
-        rep
-        nop
-        movq    $1, %rax
-        ret
-
-
         / Support for void Copy::arrayof_conjoint_bytes(void* from,
         /                                               void* to,
         /                                               size_t count)
diff --git a/src/hotspot/share/runtime/os.hpp
b/src/hotspot/share/runtime/os.hpp
--- a/src/hotspot/share/runtime/os.hpp
+++ b/src/hotspot/share/runtime/os.hpp
@@ -1031,6 +1031,13 @@
 // of the global SpinPause() with C linkage.
 // It'd also be eligible for inlining on many platforms.

+#if defined(X86) && !defined(_WINDOWS)
+extern "C" int inline SpinPause() {
+  __asm__ __volatile__ ("pause");
+  return 1;
+}
+#else
 extern "C" int SpinPause();
+#endif

 #endif // SHARE_VM_RUNTIME_OS_HPP

-Man
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