Inspite of specifying -march=native when using Low Temporal Write(OPCH_LTW),
the compiler generates 'prefetchw' instruction instead of 'prefetchwt1'
instruction available on processor as in 'Case B'. To make the compiler emit
prefetchwt1 instruction, -mprefetchwt1 needs to be passed to configure
explicitly.
[Problem]
Case A:
OVS_PREFETCH_CACHE(addr, OPCH_HTW) [__builtin_prefetch(addr, 1, 3)]
[Assembly]
leaq -112(%rbp), %rax
prefetchw (%rax)
Case B:
OVS_PREFETCH_CACHE(addr, OPCH_LTW) [__builtin_prefetch(addr, 1, 1)]
[Assembly]
leaq -112(%rbp), %rax
prefetchw (%rax) <***problem***>
[Solution]
./configure CFLAGS="-g -O2 -mprefetchwt1"
Case B:
OVS_PREFETCH_CACHE(addr, OPCH_LTW) [__builtin_prefetch(addr, 1, 1)]
[Assembly]
leaq -112(%rbp), %rax
prefetchwt1 (%rax)
See also:
https://mail.openvswitch.org/pipermail/ovs-dev/2017-December/341591.html
Signed-off-by: Bhanuprakash Bodireddy <bhanuprakash.bodire...@intel.com>
---
Documentation/intro/install/general.rst | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/Documentation/intro/install/general.rst
b/Documentation/intro/install/general.rst
index 718e5c2..4d2db45 100644
--- a/Documentation/intro/install/general.rst
+++ b/Documentation/intro/install/general.rst
@@ -280,6 +280,19 @@ With this, GCC will detect the processor and automatically
set appropriate
flags for it. This should not be used if you are compiling OVS outside the
target machine.
+Compilers(gcc) won't emit prefetchwt1 instruction even with '-march=native'
+specified. In such case, -mprefetchwt1 needs to be explicitly passed during
+configuration.
+
+For example inspite of specifying -march=native when using Low Temporal Write
+i.e OVS_PREFETCH_CACHE(addr, OPCH_LTW), the compiler generates 'prefetchw'
+instruction instead of 'prefetchwt1' instruction available on processor.
+
+To make the compiler generate the appropriate instruction, it is recommended
+to pass ``-mprefetchwt1`` settings::
+
+ $ ./configure CFLAGS="-g -O2 -march=native -mprefetchwt1"
+
.. note::
CFLAGS are not applied when building the Linux kernel module. Custom CFLAGS
for the kernel module are supplied using the ``EXTRA_CFLAGS`` variable when
--
2.4.11
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