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Pure attributes

Karl Williamson
November 25, 2016 03:31
Pure attributes
Message ID:
Karl and I have been having some back and forth on function attributes that I think would be better public.  Here he's discussing his search for functions that can be flagged as __attribute_pure__.

--- start Karl ---
One thing I realized is that it is easy to make a mistake and claim that something is pure that really isn't.  This can be the source of very subtle bugs that vary between platforms, as they can optimize differently.

I decided to not mark any static functions as pure, as the compiler is better at figuring that out than me.  So, I limited myself to looking at ones that are linked to externally.  I came up with this short possible list to look at sometime.  I don't remember how I got it, whether it was just the name, or something else:

: Candidates for pure
: ApdR	|I32	|looks_like_number|NN SV *const sv
: Apd	|int	|grok_infnan	|NN const char** sp|NN const char *send
: Apd	|int	|grok_number	|NN const char *pv|STRLEN len|NULLOK UV *valuep
: Apd	|int	|grok_number_flags|NN const char *pv|STRLEN len|NULLOK UV *valuep|U32 flags
: ApdR	|bool	|grok_numeric_radix|NN const char **sp|NN const char *send
: ApM	|bool	|_is_in_locale_category|const bool compiling|const int category
: Amd	|I32	|sv_eq		|NULLOK SV* sv1|NULLOK SV* sv2
: Apd	|I32	|sv_eq_flags	|NULLOK SV* sv1|NULLOK SV* sv2|const U32 flags
: Apd	|STRLEN	|sv_len		|NULLOK SV *const sv
: Apd	|STRLEN	|sv_len_utf8	|NULLOK SV *const sv
: p	|STRLEN	|sv_len_utf8_nomg|NN SV *const sv
: Apdn	|bool	|isinfnan	|NV nv
: p	|bool	|isinfnansv	|NN SV *sv
--- cut Karl ---

Karl, it sounds like you're saying that the compiler is better at figuring out pureness of static functions than non-static functions.  That's surprising to me, because it seems like the purity of a function wouldn't depend on its visibility.   Can you say more?

Wouldn't grok_number and grok_number_flags not be pure because they set *valuep?

FYI, here's my list.  Looks like we have exactly zero in common. :-)

+inRP   |bool   |should_warn_nl|NN const char *pv
+sP     |bool   |looks_like_bool|NN const OP* o
+snRP   |bool   |num_overflow   |NV value|I32 fldsize|I32 frcsize
+iRnP   |bool   |path_is_searchable|NN const char *name
+snP    |NV|mulexp10    |NV value|I32 exponent
+sP     |bool   |ckwarn_common  |U32 w
+ApoP   |bool   |ckwarn         |U32 w
+ApoP   |bool   |ckwarn_d       |U32 w Perl Programming lists via nntp and http.
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