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authorAvi Kivity <avi@redhat.com>2010-02-13 10:33:12 +0200
committerH. Peter Anvin <hpa@zytor.com>2010-02-13 13:37:56 -0800
commit0d1622d7f526311d87d7da2ee7dd14b73e45d3fc (patch)
treeeb97e7b70d96faabbbd32cfea8fa34ac5e12eef5 /arch/sh/configs/ecovec24-romimage_defconfig
parent1838ef1d782f7527e6defe87e180598622d2d071 (diff)
x86-64, rwsem: Avoid store forwarding hazard in __downgrade_write
The Intel Architecture Optimization Reference Manual states that a short load that follows a long store to the same object will suffer a store forwading penalty, particularly if the two accesses use different addresses. Trivially, a long load that follows a short store will also suffer a penalty. __downgrade_write() in rwsem incurs both penalties: the increment operation will not be able to reuse a recently-loaded rwsem value, and its result will not be reused by any recently-following rwsem operation. A comment in the code states that this is because 64-bit immediates are special and expensive; but while they are slightly special (only a single instruction allows them), they aren't expensive: a test shows that two loops, one loading a 32-bit immediate and one loading a 64-bit immediate, both take 1.5 cycles per iteration. Fix this by changing __downgrade_write to use the same add instruction on i386 and on x86_64, so that it uses the same operand size as all the other rwsem functions. Signed-off-by: Avi Kivity <avi@redhat.com> LKML-Reference: <1266049992-17419-1-git-send-email-avi@redhat.com> Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Diffstat (limited to 'arch/sh/configs/ecovec24-romimage_defconfig')
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