[Bug target/50106] New: [ARM] Wrong code with -march=armv5t -mthumb -Os

[sebastian.hu...@embedded-brains.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50106

             Bug #: 50106
           Summary: [ARM] Wrong code with -march=armv5t -mthumb -Os
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: sebastian.hu...@embedded-brains.de
            Target: arm-rtemseabi4.11


Created attachment 25028
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25028
Sample code.

Command line:

arm-rtemseabi4.11-g++ -march=armv5t -mthumb -Os -S compiler1.test.ii -o
compiler1.test.eabi.Os.s
arm-rtemseabi4.11-g++ -march=armv5t -mthumb -O2 -S compiler1.test.ii -o
compiler1.test.eabi.O2.s

Relevant function:

extern _TestStruct _GetIDS(unsigned int Index)
{
  if (Index < _LIST_SIZE) {
    return _List[Index];
  } else {
    return _List[0];
  }
}

Partial content of compiler1.test.eabi.Os.s with comments:

        .text
        .align  1
        .global _Z7_GetIDSj
        .code   16
        .thumb_func
        .type   _Z7_GetIDSj, %function
_Z7_GetIDSj:
        .fnstart
.LFB0:
        .save   {r0, r1, r2, lr}
        push    {r0, r1, r2, lr}

Why do we save the volatile registers r0, r1, r2 here?

        ldr     r3, .L4
        ldr     r1, .L4+4
        ldr     r3, [r3]
        cmp     r0, r3
        bcs     .L2
        lsl     r0, r0, #1
        add     r1, r1, r0
.L2:
        mov     r2, #2
        add     r0, sp, #4
        bl      memcpy

Here we copy two bytes from the table into the stack.

        add     r3, sp, #4
        ldrb    r0, [r3, #1]
        ldrb    r2, [r3]

Here we copy two bytes from the stack into two registers.

        lsl     r0, r0, #8
        orr     r0, r2

Here we combine the two registers into one.

        @ sp needed for prologue
        pop     {r0, r1, r2, pc}

Here we overwrite our result (r0) with the value of r0 at the function entry.

.L5:
        .align  2
.L4:
        .word   .LANCHOR0
        .word   .LANCHOR1

With -O2 the problem vanishes (because we don't use memcpy()).