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[Bug c/20972] New: Compiler-generated code produces an assembler warning

Test gcc.c-torture/compile/930210-1.c, when compiled with:

    -O2 -c

produces the warning:

    /var/tmp//ccSVOi5X.s: Assembler messages:
    /var/tmp//ccSVOi5X.s:23: Warning: source register same as write-back base

Because of the following instruction generated in the assembly:

    strb    r2, [r2], #1

The assembler warning is produced because the instruction is unpredictable when the source register 
and the base destination register are the same and post-increment addressing is used.

Note that while this test doesn't fail for the arm-none-elf target, a similar test case does demonstrate 
the same issue:

void f(void)
  char  c1, c2;
  char *p1, *p2;

  c1 = c2 = *p1++;
  while (c1--)
    *p2++ = *p1++;

This behavior is present in gcc-4.1.0-20050405, and gcc-4.0.0-20050410 (RC1).
Configuration options: --target=arm-unknown-elf --program-prefix=arm-elf- --disable-nls --
enable-languages=c,c++ --with-newlib --enable-multilib --disable-shared

           Summary: Compiler-generated code produces an assembler warning
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jconner at apple dot com
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: powerpc-apple-darwin7.8.0
GCC target triplet: arm-unknown-elf

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