Problem with __gmp_expr

Marc Glisse marc.glisse at
Thu Apr 19 14:43:04 UTC 2018

On Sat, 18 Jan 2014, Marc Glisse wrote:

> On Fri, 17 Jan 2014, Marc Glisse wrote:
>> On Fri, 17 Jan 2014, Marc Glisse wrote:
>>> In this particular case (I haven't checked closely yet), it may be that 
>>> things "work" with other expression template libraries whose expressions 
>>> use copies instead of references. In gmp, -m*a is a neg expr that stores a 
>>> ref to a mult expr that stores references to m and a. Since you managed to 
>>> postpone the evaluation of the expression, the mult expr went out of scope 
>>> (older versions of gcc don't crash because they are not good at reusing 
>>> space from dead objects). Other libraries would have a big neg expr that 
>>> embeds a copy of the mult expr, and only mp?_class is spared the copying. 
>>> It has advantages. It also seems (didn't try it myself) to make it harder 
>>> for the compiler to eliminate all this meta-stuff. It may be worth 
>>> re-evaluating (I don't have time to do it myself soon).
>> The following patch (only tested on this example) handles your example with 
>> and without LAMBDA.
> Surprisingly, that patch, which passes the testsuite, actually seems to help 
> the compiler quite a bit. On this testcase (random operations to build a 
> large enough expression):
> mpq_class f(mpz_class z,mpq_class q1,mpq_class q2,mpq_class q3){
>  return (q1*q1+2)/-z+(q3-q2-1)*2*(q2-z);
> }
> With the patch I get a .s file of size 3855 (-O2 or -O3, doesn't matter), 
> while with the old gmpxx.h I get 4315 at -O3 and 4513 at -O2. The difference 
> is mostly a bunch of useless mov+lea that disappear. The patch also lets the 
> compiler notice one more __builtin_constant_p for the multiplication by 2, 
> and it notices that 2 (in +2) is positive at -O2 instead of -O3.

That was then. Now, with gcc-8, the patch causes gcc to generate much 
worse code ( ). So I am 
not pushing the patch right now.

Note that std::valarray in libstdc++ is likely to get a similar patch for 
gcc-9 ( ).

Marc Glisse

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