value size does not match register size specified by the constraint and modifier

Question

In arm64, compiling the inline assembly below causes a warning below:

value size does not match register size specified by the constraint and modifier [-Wasm-operand-widths].

This is because the arm64's register is 64bit.

int a = 1;
int b = 2;
asm volatile(
   "add %[a], %[a], %[b]  \n"
   :
   [a] "+r"(a)
   :
   [b] "r"(b)
   :
    );

And clang give the fix advice, use constraint modifier "w".

Answer 1

value size does not match register size specified by the constraint and modifier

means:

your variable is 32bit not match with ARM64's 64bit register

you could:

• change your variable to 64bit

long a = 1;
long b = 2;
asm volatile(
   "add %[a], %[a], %[b]  \n"
   : [a] "+r"(a)
   : [b] "r"(b)
   :);

or

• use 32bit=Word register, by add prefix w
  • which is clang suggested

int a = 1;
int b = 2;
asm volatile(
   "add %w[a], %w[a], %w[b]  \n"
   : [a] "+r"(a)
   : [b] "r"(b)
   :);