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[perl #123616] Weed out needless PERL_UNUSED_ARG

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From:
bulk88 via RT
Date:
January 17, 2015 20:33
Subject:
[perl #123616] Weed out needless PERL_UNUSED_ARG
Message ID:
rt-4.0.18-22087-1421526813-188.123616-15-0@perl.org
On Sat Jan 17 09:00:57 2015, jasmine.ngan@outlook.com wrote:
> Hi there,
> 
> This is my first time contributing a patch so I'm looking for a little
> guidance. As suggested by the subject and with assistance from Hugo at
> the London Perl Hackday, I've weeded out some obvious PERL_UNUSED_ARG
> statements inside static functions as requested in the Perl 5 todo.pod
> file.
> 
> The remaining PERL_UNUSED_ARG statements appear to be for variables
> that are (1) contained in non-static functions, (2) conditionally used
> depending on pre-processor macros  or (3) placeholders so that the
> static function prototypes match particular function pointers.

Reading about dquote_static.c and grok_bslash_x brings up a couple questions how its implementation.

grok_bslash_x is 0x133 bytes of VC 2003 -O1 32 bit machine code. It is declared as PERL_STATIC_INLINE in dquote_static.c and in "EMiR" embed.fnc. Something that fat in machine code and pagefuls of C code should be inlined in exactly one scenario, it has only 1 caller, and it is unconditionally (basically not in a "if" branch) executed in that caller. VC optimizer refused to inline grok_bslash_x and instead it is called from 3 callers in VC machine code and in Perl C source code, S_regatom, S_regclass, and S_scan_const.

The inline came from http://perl5.git.perl.org/perl.git/commitdiff/a04812933fd16241c831d020a4c64870d7ca8624 by khw from 5.17.1.

If I didn't have VC's WPO/LTO/LTCG mode turned on, there would be 2 copies of S_grok_bslash_x in the perl binary (VC 6 doesn't have LTCG, too old, so it suffers too), since a normal C linker can't dedup nameless, symbol-less static functions. Here is a symbol list from HPUX HP C blead perl showing that HPUX ld didn't dedup the functions.

name              addr     length
S_grok_bslash_c   00097224 00000148
S_grok_bslash_c_1 000BFBE0 00000148
S_grok_bslash_o   0009736C 000001CC
S_grok_bslash_o_1 000BFD28 000001CC
S_grok_bslash_x   00097538 00000268
S_grok_bslash_x_1 000BFEF4 00000268

I assume that all 3 S_grok_blash* functions are used in re::* XS module. 

There are 2 things that should have been done.

Either S_grok* becomes Perl_grok*_*p and Xp or stays as S_grok* and becomes X in embed.fnc, so they are exported to re::* XS SO.

or 

dquote_static.c becomes a formal translation unit for core but its functions are not exported in embed.fnc. Then on restricted visibility platforms (IE Win32) it is then #include-ed in re::* again but this time S_grok_bslash* are PERL_STATIC_INLINE. dquote_static.c and embed.fnc has an

#if PERL_EXT && RESTRICTED_SYM_VIS
iR	|bool	|grok_bslash_xp	|NN char** s|NN UV* uv           \
				|NN const char** error_msg       \
				|const bool strict               \
				|const bool silence_non_portable \
				|const bool utf8
#elif PERL_CORE
pR	|bool	|grok_bslash_xp	|NN char** s|NN UV* uv           \
				|NN const char** error_msg       \
				|const bool strict               \
				|const bool silence_non_portable \
				|const bool utf8
#endif


#if PERL_EXT && RESTRICTED_SYM_VIS
PERL_STATIC_INLINE bool
#elif PERL_CORE
bool
#else
#  error What are you doing?
#endif
S_grok_bslash_x(pTHX_ char **s, UV *uv, const char** error_msg,
                      const bool strict,
                      const bool silence_non_portable,
                      const bool UTF)
{

This way core's S_grok_bslash can get WPO optimized (random register calling conventions, removing unused params, merging params, etc) but re:: XS SO/DLL gets its own machine code copy that also was hopefully WPO-ed.

-- 
bulk88 ~ bulk88 at hotmail.com

---
via perlbug:  queue: perl5 status: new
https://rt.perl.org/Ticket/Display.html?id=123616

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