Thursday, February 3, 2022

[SOLVED] Extended asm with goto, including an example from the gcc docs, fails to compile

Issue

Some extended assembly statements using the goto qualifier fail to compile with GCC 10.1.0. Specifically,

int foo(int count)
{
  asm goto ("dec %0; jb %l[stop]"
            : "+r" (count)
            :
            :
            : stop);
  return count;
stop:
  return 0;
}

(which is an example in the GCC extended asm docs) fails to compile with the message expected ‘:’ before string constant. Removing the "+r" (count) and the dec %0 allows it to compile successfully, but regardless of what I try whenever an output operand is supplied in the same asm statement as a goto label, it errors in this same way.


Solution

asm goto doesn't allow output operands.

It is a gnu decision. in the function c_parser_for_statement from c-parser.c you can find :

/* For asm goto, we don't allow output operands, but reserve
the slot for a future extension that does allow them.  */

https://github.com/gcc-mirror/gcc/blob/releases/gcc-10/gcc/c/c-parser.c

However may be this situation will change since in the master branch this comment is not present anymore.



Answered By - yflelion
Answer Checked By - Mary Flores (WPSolving Volunteer)
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