On 07/01/2024 19:24, Segher Boessenkool wrote:
On Sun, Jan 07, 2024 at 04:10:21PM +0100, David Brown wrote:
On 05/01/2024 19:46, Segher Boessenkool 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 error message is emitted during reloading.
"+" 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.
They are the same thing. The "+" syntactic sugar is modified during
gimplification already, almost nothing in the compiler has to deal with
it.
See "gimplify_asm_expr", see the code after
/* An input/output operand. To give the optimizers more
flexibility, split it into separate input and output
operands. */
(Constraints that allow registers get a "0" (or "1" etc.) matching
constraint for the input duplicate, constraints that do not (like "m")
are unmodified. The output duplicate just gets the "+" changed to "=").
Thanks for that explanation - I've learned a bit more now.
So what then does this mean?
asm("" : "+X" (x) : "0" (x));
On x86-64, I get the error with this :
typedef double T;
T test(T a, T b) {
T x = a + b;
asm ("" : "+X" (x));
return x - b;
}
But not with this:
typedef double T;
T test(T a, T b) {
T x = a + b;
asm ("" : "+X" (x) : "0" (x));
return x - b;
}
(But that still gives an error on ARM-32!)
I'm sure there's a logical pattern here somewhere, but I can't see it yet.
David
