[tree-ssa] copy propagation and the abstraction penalty

[Andrew Pinski]

Also it will fix PR9566, which is the same problem.
It will also speed up mostly likely any C++ problem ever made.

Thanks,
Andrew Pinski

On Wednesday, May 14, 2003, at 18:44 US/Eastern, Joe Buck wrote:

> Consider the code
>
> --------------------------------
> struct complex {
>     double re, im;
>     complex(double r, double i) : re(r), im(i) {}
> };
>
> inline complex operator+(const complex& a, const complex& b) {
>     return complex(a.re+b.re, a.im+b.im);
> }
>
> complex addone(const complex& arg) {
>     return arg + complex(1,0);
> }
> -------------------------------
>
> We get really lousy code for this, in all gcc versions, including 
> tree-ssa.
> The reason is that we build a temporary struct to hold the 1, 0 and 
> don't
> get rid of it, so we take no advantage of the zero.
>
> Calling this foo.C, foo.C.t09.ssa gives
>
>
> ;; Function complex addone(const complex&) (_Z6addoneRK7complex)
>
> complex addone(const complex&) (arg)
> {
>   struct complex retval.12;
>   struct complex <UVa150>;
>   struct complex * T.8;
>   struct complex * T.9;
>   struct complex & T.10;
>   struct complex T.11;
>
>   {
>     T.8_2 = &<UVa150>;
>     {
>       double i;
>       double r;
>       struct complex * const this;
>
>       this_3 = (struct complex * const)T.8_2;
>       r_5 = 1.0e+0;
>       i_6 = 0.0;
>       {
>         this->re = 1.0e+0;
>         this->im = 0.0;
>         {
>           (void)0
>         }
>       }
>     };
>     T.9_9 = &<UVa150>;
>     T.10_10 = (struct complex &)T.9_9;
>     {
>       struct complex & b;
>       struct complex <UVa770>;
>
>       b_11 = T.10_10;
>       {
>         struct complex * T.1;
>         double T.2;
>         double T.3;
>         double T.4;
>         double T.5;
>         double T.6;
>         double T.7;
>
>         {
>           T.1_13 = &<UVa770>;
>           T.2_15 = arg->re;
>           T.3_16 = b->re;
>           T.4_17 = T.2_15 + T.3_16;
>           T.5_18 = arg->im;
>           T.6_19 = b->im;
>           T.7_20 = T.5_18 + T.6_19;
>           {
>             double i;
>             double r;
>             struct complex * const this;
>
>             this_21 = (struct complex * const)T.1_13;
>             r_23 = T.4_17;
>             i_24 = T.7_20;
>             {
>               this->re = T.4_17;
>               this->im = T.7_20;
>               {
>                 (void)0
>               }
>             }
>           };
>           {
>             (void)0;
>             goto <ULa700>;
>           }
>         }
>       };
>       <ULa700>:;;
>       retval.12_27 = <UVa770>
>     };
>     T.11_28 = retval.12_27;
>     return retval.12_27;
>   }
> }
>
> ------------------------------------------------------------
>
> It would seem simple enough to eliminate all uses of the temporary 
> struct
> <UVa150> by doing copy propagation: what we would have left is only the
> initialization of the struct itself; all uses of the struct would get 
> the
> 1.0 and 0.0 values from its re and im fields.  At this point the 
> temporary
> struct should be eligible for killing.
>
> If we could do this alone, we would greatly improve C++ performance, 
> especially
> on things like the Boost graph library.  It seems that we have most of 
> what we
> need in place, right?
>
>
>