openjdk/zero: use double code for atomic64 on powerpc (original) (raw)

Sebastian Andrzej Siewior openjdk at ml.breakpoint.cc
Wed Jun 30 08:30:51 PDT 2010

this cute C code does the same thing on powerpc as the assembly code that was here before. If the compiler was built with the SPE extensions instead of traditional FPU and double operations are performed in HW then we are one step further: The compiler turns this into evldd & evstdd. Voila :)

This C code could also be activated on s390. The compiler turns this into a single mvc instruction which does the copy operation. I don't know if mvc's copy ability is atomic or not and therefore I leave it as it.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>

Index: openjdk-6-6b18-1.8/ports/hotspot/src/os_cpu/linux_zero/vm/os_linux_zero.hpp

--- openjdk-6-6b18-1.8.orig/ports/hotspot/src/os_cpu/linux_zero/vm/os_linux_zero.hpp 2010-06-30 16:55:28.000000000 +0200 +++ openjdk-6-6b18-1.8/ports/hotspot/src/os_cpu/linux_zero/vm/os_linux_zero.hpp 2010-06-30 17:08:49.000000000 +0200 @@ -22,6 +22,15 @@

+#define ATOMIC64_COPY_THROUGH_DOUBLE 1 + +#endif +#endif

static void setup_fpu() {}

@@ -33,12 +42,23 @@

// Atomically copy 64 bits of data static void atomic_copy64(volatile void *src, volatile void *dst) { -#if defined(PPC) && !defined(_LP64)

+#if ATOMIC64_COPY_THROUGH_DOUBLE + /* + * In order to copy 8 bytes atomicly we rely on the trick that some + * architectures can load and store a double as a single operation. + * gcc picks the correct opcode here and with optimization turned on + * all temporary assignments are gone. - bigeasy + */ + union { + double *d; + volatile void *v; + } s, d; + + s.v = src; + d.v = dst; + + *d.d = *s.d; + #elif defined(S390) && !defined(_LP64) double tmp; asm volatile ("ld %0, 0(%1)\n"

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