sv_setsv_cow: only succeed if sv_setsv() would also COW #22120
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This will actually fail in some cases in the next commit.
Previously if you had a successful match against an SV with a
SvLEN() large relative to the SvCUR() the regexp engine would
use sv_setsv_cow() to make a COW copy of the matched SV,
extending the life of the large allocation buffer.
A normal sv_setsv() normally didn't do such a COW copy, but the above
also marked the source SV as COW, so further copies of the SV
could even further extend the lifetime of the buffer, eg:
while (<>) { # readline tends to make large SvLEN()
/something/; # some sort of match
push @save, $_; # with a successful match, the large $_ buffer
# survives until @save is released
}
Fixes part of Perl#21877
tonycoz
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iabyn
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Apr 8, 2024
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This improved performance of the test case I was using from:
to
on cygwin.
The problem that is fixed here is that sv_setsv_cow() would COW SVs even though they had a very short string in a very large buffer, which had some sort of unclear to me interaction with the Win32 virtual memory system that had a painful performance impact when the large SV was made COW.
It may also improve memory handling on non-Win32 systems, since the SV was made COW, other SV copying functions wouldn't check if it was suitable for COW, so further copies would retain a COW reference for the large buffer. In the example test case from #21877 if we pushed the SV to an array after a successful match memory use would balloon out, running even a system with a generous amount of memory, out of memory.
Fixes part of #21877