[Bug rtl-optimization/45235] New: const volatile read moved out of order

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

This example is reduced from hardware-specific code that uses memory-mapped
registers to control and sample a logical signal.  The read of the input signal
is moved to follow the clear of the output signal, in violation of the
requirements for volatile memory access.

Reproduce with:  gcc -S -O2 ira-bug.c

Examine the generated assembly code to verify the read of in has moved to
follow the second write of out within the loop body.

The presence of the const qualifier on the in variable enables the bug; if the
qualifier is removed the original order is retained.


volatile const short int in;
volatile short int out;

void func () {
  short int value;
  do {
    out |= 2;
    value = in;
    out &= ~2;
  } while (value & 1);
}


-- 
           Summary: const volatile read moved out of order
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bigotp at acm dot org
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45235