[Bug optimization/12395] New: Suboptimal code with global variables

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

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12395

           Summary: Suboptimal code with global variables
           Product: gcc
           Version: 3.4
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: steven at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org

Consider this tiny function:

void foo() {
        extern int a;
        if(++a) ++a;
}

It produces the following assembly with 3.4 (experimental) and 3.2 (Red Hat 9)
at -O2 and better:

        .file   "t.c"
        .text
.globl foo
        .type   foo, @function
foo:
        movl    a, %eax
        incl    %eax
        testl   %eax, %eax
        movl    %eax, a
        je      .L1
        incl    %eax
        movl    %eax, a
.L1:
        ret
        .size   foo, .-foo
        .section        .note.GNU-stack,"",@progbits
        .ident  "GCC: (GNU) 3.5-tree-ssa 20030924 (merged 20030817)"

This is subobtimal. The optimal code should be:

foo:
        addl    $1,a
        je      .L1
        addl    $1,a
.L1:
        ret