[Bug target/17565] New: asms in delay slots

["rsandifo at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

The MIPS port allows asms to be put into delay slots if the
compiler guesses they are only one instruction long.  This is
wrong because of the possibiltiy of it containing macros.

The problem can be reproduced as an assembler warning
in the following testcase:

int foo (int n)
{
  register int k asm ("$16") = n;
  if (k > 0)
    {
      bar ();
      asm ("li %0,0x12345678" : "=r" (k));
    }
  return k;
}

because the multi-instruction asm statement goes into the delay
slot of the call to bar().

This is reduced from a much more serious linux problem.
Linux is fond of using empty asm statements, and since gcc estimates
empty asms to be one instruction long, they too might be put into
delay slots.  This actually has the effect of putting the following
instruction into the delay slot instead.  Since there's no assembler
warning, the problem was only detected as a run-time failure.

This problem goes back to at least 2.95.

Richard

-- 
           Summary: asms in delay slots
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P2
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rsandifo at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: mips64-elf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17565