[Bug tree-optimization/66664] New: gcc misses optimization emits subtraction where relocation arithmetic would suffice

[fuz at fuz dot su]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66664

            Bug ID: 66664
           Summary: gcc misses optimization emits subtraction where
                    relocation arithmetic would suffice
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fuz at fuz dot su
  Target Milestone: ---

For the following C code:

    int foo(int x)  {
        extern int bar[];
        return bar[x - 1];
    }

gcc emits the following code (with -O3) on amd64 Linux:

    foo:
        subl    $1, %edi
        movslq  %edi, %rdi
        movl    bar(,%rdi,4), %eax
        ret

        .ident  "GCC: (GNU) 5.0.0 20150314 (experimental)"

I expected gcc to emit the following (as signed underflow is undefined):

    foo:
        movslq  %edi, %rdi
        movl    bar-4(,%rdi,4), %eax
        ret

or even this (as the memory model places global variables in the first 2^32
byte of RAM):

    foo:
        movl    bar-4(,%edi,4), %eax
        ret