Warum ändert sich das Makefile, um die Leistung zu erhöhen?

Question

Ich habe vor kurzem ein Leistungslabor in der Klasse abgeschlossen, aber es gab eine Sache, die mein Kumpel mir zeigte, dass ich nicht herausfinden kann warum. Im ursprünglichen Make-Datei wir hatten:

## 
## 

CXX =g++ 
CXXFLAGS= -m32 -static 

Aber ich änderte den CXXFLAGS auf:

## 
## 

CXX =g++ 
CXXFLAGS= -m32 -static -funroll-loops -O3 

Was genau macht die -funroll-Schleifen-O3 tun, dass das Original nicht?

Answer 1

Versuchen Sie dies: man gcc!

-funroll-loops:

-funroll-loops 
      Unroll loops whose number of iterations can be determined at 
      compile time or upon entry to the loop. -funroll-loops implies 
      -frerun-cse-after-loop, -fweb and -frename-registers. It also 
      turns on complete loop peeling (i.e. complete removal of loops with 
      a small constant number of iterations). This option makes code 
      larger, and may or may not make it run faster. 

-O1:

 -O1 Optimize. Optimizing compilation takes somewhat more time, and a 
       lot more memory for a large function. 

       With -O, the compiler tries to reduce code size and execution time, 
       without performing any optimizations that take a great deal of 
       compilation time. 

-O2

-O2 Optimize even more. GCC performs nearly all supported 
      optimizations that do not involve a space-speed tradeoff. As 
      compared to -O, this option increases both compilation time and the 
      performance of the generated code. 

-O3

-O3 Optimize yet more.

-O0

-O0 Reduce compilation time and make debugging produce the expected 
      results. This is the default. 

-0s

-Os Optimize for size. -Os enables all -O2 optimizations that do not 
     typically increase code size. It also performs further 
     optimizations designed to reduce code size. 

-Ofast

-Ofast 
     Disregard strict standards compliance. -Ofast enables all -O3 
     optimizations. It also enables optimizations that are not valid 
     for all standard-compliant programs. It turns on -ffast-math and 
     the Fortran-specific -fno-protect-parens and -fstack-arrays. 

hoffe, es hilft!