protected alloca class for malloc fallback

[Richard Biener]

On August 5, 2016 4:42:48 PM GMT+02:00, Aldy Hernandez <aldyh@redhat.com> wrote:
>
>> I was surprised by the always_inline trick.  I suppose it makes
>> sense but I wouldn't have expected to be able to rely on it.  Out
>> of curiosity I tested it with other compilers.  It turns out that
>> it only works with some like GCC and IBM XLC++, but not others
>> like Clang or Sun CC.  In recursive calls, they don't seem to
>> hold on to the memory allocated via alloca by the class ctor in
>> the caller.
>
>Well, it was surprising to me as well, hence the comment.  I expected
>it 
>to just work, and when it didn't I had to hunt in the inliner code to 
>find out why it was selectively inlining:
>
>     case GIMPLE_CALL:
>    /* Refuse to inline alloca call unless user explicitly forced so as
>	 this may change program's memory overhead drastically when the
>	 function using alloca is called in loop.  In GCC present in
>	 SPEC2000 inlining into schedule_block cause it to require 2GB of
>	 RAM instead of 256MB.  Don't do so for alloca calls emitted for
>	 VLA objects as those can't cause unbounded growth (they're always
>	 wrapped inside stack_save/stack_restore regions.  */
>
>As Richi pointed out, if the constructor doesn't get inlined (as you're
>
>seeing in Clang and Sun CC), we could potentially clobber freed memory.
>
>  So much for that idea...
>
>>
>> FWIW, rather than creating a new class around alloca and putting
>> effort into changing code to use it I think that investing time
>> into replacing the function's uses with an existing C++ container
>> class (either GCC's own or one from the STL) might be a better
>> way to go.
>
>Yes, I suggested a combination of auto_vec<> (gcc's) and std::string 
>down thread.

Please don't use std::string.  For string building you can use obstacks.

Richard.

>Thanks for checking out the result from other compilers.  Much
>appreciated.
>
>Aldy