[Bug target/86673] New: inline asm sometimes ignores 'register asm("reg")' declarations

[arnd at linaro dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86673

            Bug ID: 86673
           Summary: inline asm sometimes ignores 'register asm("reg")'
                    declarations
           Product: gcc
           Version: 8.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arnd at linaro dot org
  Target Milestone: ---

Created attachment 44438
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44438&action=edit
linux/net/core/scm.o, preprocessed

Building older linux kernels for ARM with a gcc-8.1 compiler has triggered a
check for broken compiler versions, which compares the register number that is
used in an inline assembly statement with the expected value, for an argument
that was declared with the 'register foo asm ("reg")' syntax described in the
gcc manual under "Specifying Registers for Local Variables".

The diagnostic from the assembler is

$ arm-linux-gnueabi-gcc -Wall -O2 scm.i -c -Wno-pointer-sign
-fno-strict-aliasing
/tmp/ccCGMQmS.s:648: Error: .err encountered
/tmp/ccCGMQmS.s:679: Error: .err encountered

Unfortunately, a change made to the kernel a few years ago had made this go
unnoticed as everyone was testing gcc-8.1 only on more recent kernels that did
not run into the particular check, but may have run into the bug without
triggering the check. Architectures other than arm may also be affected, but
nothing else has this check.

I tested gcc-8.1.0 and today's gcc-8.1.1 (r262956), both with the same result.
I attached one of the files that showed the problem, and reduced this using
creduce to:

int a, c, d, e;
long b;
void fn1() {
  int f = ({
    ({
      long g = b, j = g;
      register const typeof(c) h asm("r2") = 1, i = d;
      __asm__(".ifnc %2,r2; .err; .endif\n\t"
                "bl     __put_user_4"
              : "=&r"(e)
              : ""(i), ""(h), ""(j));
      e;
    });
  });
  a = f;
}