Thread: Inline assembly and compiler optimizations?

I read several times that using inline assembly is considered as a bad practice, since it will interfere with the compiler optimization.

And recently, I read this:

Q: How can I reference C variables from my inline assembly code?

A: GCC has extensive inline assembly facilities. They allow you to specify everything other compilers let you (like the registers where GCC will put specific results), but in a way that doesn't interfere with the compiler's optimizations of the C code that includes inline assembly. Because of this flexibility, the syntax of the inline assembly code is very different from the other DOS-based compilers.

DJGPP FAQ -- Inline Assembly code with GCC

How it's possible?

Google is really cool. I entered "gcc inline assembly" and got this link as the first result.

Come on, search FIRST then ask questions.

The compiler takes your specific requests for optimizations, "under advisement".

Then it does it the way it see's as best, regardless of your requests.

Originally Posted by bernt

Perhaps you should have read his question first and then suggested a site?

You may have noted that the question "how it's possible" is QUITE ambiguous. I took the reference of "How is this possible to access a C Variable in an asm() call. The link provided gives that info.

Now, the real deal is that the OP should not be so vague in the questions asked.

The usage of inline assembly in C source code is discouraged due to the associated side effects and compiler/assembler interference. The asm statements are translated only by the assembler while the C source is translated only by the compiler and it is extremely hard to weed out their mutual interference. For example, the compiler may optimize certain variables and store them in registers for speeding things up, while the asm statements may clear up those very registers causing the program to behave weirdly and even crash. Traditionally inline assembly was used so that the programmer could access hardware features inaccessible to C, but nowadays most compilers provide intrinsics for that very purpose. Intrinsics is the preferred way to produce assembly language code because intrinsics are function like APIs that are only for consumption by the compiler.