Messages in this thread | | | Date | Sun, 6 Jun 2021 09:45:32 -0400 | From | Alan Stern <> | Subject | Re: [RFC] LKMM: Add volatile_if() |
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On Sun, Jun 06, 2021 at 06:53:36AM -0500, Segher Boessenkool wrote: > On Sat, Jun 05, 2021 at 09:29:03PM -0400, Alan Stern wrote: > > Interesting. And changing one of the branches from barrier() to __asm__ > > __volatile__("nop": : :"memory") also causes a branch to be emitted. So > > even though the compiler doesn't "look inside" assembly code, it does > > compare two pieces at least textually and apparently assumes if they are > > identical then they do the same thing. > > And that is a simple fact, since the same assembler code (at the same > spot in the program) will do the same thing no matter how that ended up > there.
Sure. But the same assembler code at two different spots in the program might not do the same thing. (Think of code that stores the current EIP register's value into a variable.)
So while de-duplicating such code may be allowed, it will give rise to observable results at execution time.
Alan
> And the compiler always is allowed to duplicate, join, delete, you name > it, inline assembler code. The only thing that it cares about is > semantics of the code, just like for any other code. > > > Segher
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