pp_reverse: don't COW buffer just to then un-COW it #22729
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PS Doing a full copy also seems like not the best idea, since it read-writes the entire string (albeit between buffers), only for the function to then read-write the whole buffer at least once. I have a branch in progress to separate out the "swipe" case from the "copy" case, where in the "copy" case the buffer isn't actually copied wholesale, instead the reverse operation reads from the source buffer and writes to the destination buffer. Seems worth making this PR in case the WIP branch never lands. |
tonycoz
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Nov 12, 2024
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LGTM except for the "scaler" in the commit message. |
When it has a single argument, scalar-mode `pp_reverse` copies the
source PV buffer to the destination PV buffer via the `sv_setsv()`
macro. This expands to:
sv_setsv_flags(dsv, ssv, SV_GMAGIC|SV_DO_COW_SVSETSV)
The `SV_DO_COW_SVSETSV` flag means that Perl_sv_setsv_flags will
try to achieve the "copy" via these mechanisms:
* Swipe the source buffer (if possible)
* COW the buffer (if possible)
* Copy the buffer
However, using COW makes no sense because pp_reverse is going to
then modify the buffer. Almost immediately after the `sv_setsv`
call, any COWing will be reversed by the call to `SvPV_force` and
a full copy of the buffer taken. So this commit expands the
`sv_setsv` call and drops the COW flag.
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When it has a single argument, scaler-mode
pp_reversecopies the source PV buffer to the destination PV buffer via thesv_setsv()macro. This expands to:The
SV_DO_COW_SVSETSVflag means that Perl_sv_setsv_flags will try to achieve the "copy" via these mechanisms:However, using COW makes no sense because pp_reverse is going to then modify the buffer. Almost immediately after the
sv_setsvcall, any COWing will be reversed by the call toSvPV_forceand a full copy of the buffer taken. So this commit expands thesv_setsvcall and drops the COW flag.