[Bug rtl-optimization/42226] New: [missed optimization] inefficient byte access when -Os is specified

["carrot at google dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Compile the attached test case with options -Os -mthumb, gcc generates:

        add     r1, r1, #40
        mov     r3, r0
        ldrb    r2, [r1]
        add     r3, r3, #40
        strb    r2, [r3]
        @ sp needed for prologue
        bx      lr

When change the options to -O2 -mthumb, gcc generates:

        mov     r3, #40
        ldrb    r2, [r1, r3]
        strb    r2, [r0, r3]
        @ sp needed for prologue
        bx      lr

It is both smaller and faster.

Compare the dumped IL with different options, all TREE expressions are
identical. The first difference occurs after rtl expanding.


-- 
           Summary: [missed optimization]  inefficient byte access when -Os
                    is specified
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: carrot at google dot com
 GCC build triplet: i686-linux
  GCC host triplet: i686-linux
GCC target triplet: arm-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42226