On 05/01/2024 19:46, Segher Boessenkool wrote:
On Fri, Jan 05, 2024 at 05:30:51PM +0100, David Brown via Gcc-help wrote:However, when I use just "asm ("" : "+X" (x));", I get an error message "error: inconsistent operand constraints in an 'asm'". I have no idea why this is an issue.The C constraint means "Any operand whatsoever is allowed." Here you are saying to use it both as input and as output, and GCC does not know how to reload wherever it chose to put it.
It doesn't need to reload it - the "+" says it will be read, then possibly changed and returned in the same place. It is geared towards inline assembly that modifies something - you'd use it for the "x" in assembly doing "x += a;", unlike for "x = a + b;". No reloads are needed because the same register or memory address is used for input and output.
Using "X" for outputs is strange already, fwiw, but tieing it to an input is, hrm, let's call it "creative" :-)
Sure, this is a little "creative". After all, the assembly template itself is intentionally blank. It is similar in principal to the commonly used asm("" ::: "memory") barrier. It is an example of fully portable assembly!
I'd imagine that the "X" operand doesn't see much use in real inline assemblyIt is used rather often. But usually for inputs, and (as you found out) trying to use if for an input and for an output at the same time does not work reliably. "+" really creates two operands, and ties them together. Writing asm("oink" : "+X"(bla)); is shorthand for asm("oink" : "=X"(bla) : "0"(bla));
I don't know if these are /exactly/ the same, but they are certainly similar.
I can't see any reason for there to be a special issue with "X" here. I have not seen any complications with "g", meaning "general-purpose register or memory". But sometimes data can be in a register that is not general-purpose - such as a floating point register, or a SIMD register - and then I'd rather not have extra instructions added to force it to fit a "g" operand.
