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Re: Request for discussion: Rewrite of inline assembler docs

Sorry for the slow response.

Thanks for getting back to me. I was pretty sure I didn't have this all quite right yet.

   asm ("" : "=m" (*x), "=r" (y));

you have to assume that the address in %0 might use the same register as %1

Ok, now I'm getting there. It helps that I've compiled some examples and can see what is happening. This one is subtle. I'm going to have to go back and review my code to see if I've ever done this.

So, the existing text (which only talks about overlaps with input parameters) reads:

"Unless an output operand has the '&' constraint modifier (see Modifiers), GCC may allocate it in the same register as an unrelated input operand, on the assumption that the assembler code will consume its inputs before producing outputs. This assumption may be false if the assembler code actually consists of more than one instruction. In this case, use '&' for each output operand that must not overlap an input."

I'm thinking about adding something like this after it:

"The same problem can occur if one of the output parameters allows a register constraint and contains an address. In this case, GCC may use the same register for this parameter as it does for other output parameters that allow a memory constraint. This can produce inconsistent results if the register address is updated before updating the memory address. Combining the '&' constraint with the register constraint prevents this overlap and resolves the inconsistency."

That's as clear as I can come up with.  Better?


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