register used as both FP and GP register when -Os switch used

[Paul S]

I'm porting gcc 4.6.2 to a 16 bit CPU that has four GP registers. I've 
chosen to allocate R3 as the frame pointer when one is needed.

In line with GCC Internals info on FIXED_REGISTERS ("except on machines 
where that can be used as a general register when no frame pointer is 
needed") I have not marked R3 as fixed. The problem I'm describing below 
doesn't occur when FP (R3) is marked as FIXED.

#define FIXED_REGISTERS
  {
 /* r0  r1  r2  r3  sp  pc  cc .....*/
    0,  0,  0,  0,  1,  1,  1,  1
  }

and FP is marked as ELIMINABLE

#define ELIMINABLE_REGS \
  { \
    { ARG_POINTER_REGNUM,   STACK_POINTER_REGNUM}, \
    { ARG_POINTER_REGNUM,   FRAME_POINTER_REGNUM}, \
    { FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}  \
  }

TARGET_FUNCTION_ARG only allows parameter passing in R0 & R1.

This functions perfectly for all optimizations (-O0...-O3) and both 
-fomit-frame-pointer and -fno-omit-frame-pointer excepting "-Os 
-fno-omit-frame-pointer" where the following test snippet

struct ts
{
    int x,z,y;
};

int f( struct ts *s)
{
    while( --s->y )
    {
        s->x *= s->z/s->x;
    }
    return s->x;
}

produces ..

...
.L3:
    mov        r1,r2    ;# 13    *movhihi/1    [length = 2]
--> mov        r3,[r3+2]    ;# 51    *movhihi/3    [length = 4]
    mov        r0,[r3+2]    ;# 14    *movhihi/3    [length = 4]
    call    ___divhi3    ;# 15    call_value_label/1    [length = 4]
    mul        r2,r0    ;# 17    mulhi3/2    [length = 2]
    mov        r0,[r3+2]    ;# 52    *movhihi/3    [length = 4]
    str        r2,[r0]    ;# 18    *movhihi/4    [length = 4]
.L2:
    inc        [r3],#-1    ;# 65    inchi3_cc    [length = 2]
--> mov        r3,[r3+2]    ;# 55    *movhihi/3    [length = 4]
    mov        r2,[r3]    ;# 41    *movhihi/3    [length = 4]
    bne        .L3    ;# 24    cbranchcc4    [length = 6]
...

I have marked the lines that uses r3 as a GP register and clobbers it 
with -->. The corresponding RTL is ...

(insn 51 13 14 3 (set (reg:HIreg:3)
        (mem/c:HI (plus:HI (reg/f:HIreg:3)
                (const_int 2 [0x2])) [4 %sfp+2 S2 A16])) t.c:14 88 
{*movhihi}
     (nil))
(insn 14 51 15 3 (set (reg:HIreg:0)
        (mem/s:HI (plus:HI (reg:HIreg:3)
                (const_int 2 [0x2])) [2 s_1(D)->z+0 S2 A16])) t.c:14 88 
{*movhihi}
     (nil))
(call_insn/u 15 14 16 3 (set (reg:HIreg:0)
        (call (mem:HI (symbol_ref:HI ("__divhi3") [flags 0x41]) [0 S2 A8])

when -O2 is used the offending fragment is correct ...


.L4:
    mov        r1,[r3]    ;# 23    *movhihi/3    [length = 4]
    mov        r0,[r3+2]    ;# 24    *movhihi/3    [length = 4]
    call    ___divhi3    ;# 25    call_value_label/1    [length = 4]
    mov        r1,[r3]    ;# 64    *movhihi/3    [length = 4]
    mul        r1,r0    ;# 28    mulhi3/2    [length = 2]
    str        r1,[r3]    ;# 65    *movhihi/4    [length = 4]
    add        r2,#-1    ;# 29    addhi3_cc/3    [length = 2]
    bne        .L4    ;# 32    cbranchcc4    [length = 6]


I wonder if anyone can provide some hints before I waste time hunting 
down an optimisation bug that is really a problem with my configuration ?

Cheers, Paul