CODEWITHSUNDEEP
cgccoptimization
Fuad
Fuad

Reputation: 1509

Can you do all gcc optimizations (-O2, -O3) manually in your c source code?

In my class project, my project is set to use gcc's optimization level of -O0 (no optimizations) and we are not allowed to change it for the final submission.

I tested my code using -O2 and got around a 2x speedup of my entire program. So I was wondering, is it possible to go through each optimization that -O2 does, and manually do those optimizations in my code? Or are some of the -O2 optimizations internal to the stack, frame, machine/assembly, etc, thus disallowing me, the programmer, from manually making those optimizations in my source code (If that makes sense)

Upvotes: 2

Views: 255

Answers (2)

user149341
user149341

Reputation:

Is it possible to go through each optimization that -O2 does, and manually do those optimizations in my code?

No. Many of the optimizations performed by the compiler cannot be represented in C. Some of these include:

  • Disabling the frame pointer
  • Removing unnecessary register saves/restores at the beginning and end of a function
  • "Peephole" optimizations on the assembly, such as removing redundant moves, loads, or stores
  • Inserting no-ops to align loops to specific address boundaries (typically 16 bytes)

This isn't to say that all of the optimizations performed by the compiler are untranslatable, of course -- merely that some of them are.

Upvotes: 6

Dúthomhas
Dúthomhas

Reputation: 10083

Yes, but that's the same as building your own 8086-class microprocessor in Minecraft — not worth your time and effort. And yes, many of those optimizations involve stuff below the language level of abstraction. Your professor might have unknown-to-you reasons for wanting an unoptimized executable.

Upvotes: 1

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