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Date: Tue, 23 Mar 2010 14:21:00 -0000
Subject: [Bug rtl-optimization/43491]  New: Unnecessary temporary for global register variable
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gcc sometimes allocates a temporary register for a variable that is global and
has a fixed register. This happens when:
 a. the global is a register-fixed variable
 b. global is a pointer to structure
 c. an address of structure's field is passed as argument to inlined function
 d. the global is marked as constant


code:

struct b {
        unsigned g,h,j;
};

register struct b *const reg asm("r4");

static inline int diff(unsigned *p)
{
        return *p;
}

void c(void);

void d(void)
{
        while (diff(&reg->j))
                c();
}


generates:

d:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        ldr     r3, [r4, #8]    @ first ok
        push    {r5, lr}
        mov     r5, r4          @ an temporary even when r4 is marked const
        cbz     r3, .L1
.L4:   
        bl      c
        ldr     r3, [r5, #8]    @ accesses via temporary
        cmp     r3, #0
        bne     .L4
.L1:   
        pop     {r5, pc}


-- 
           Summary: Unnecessary temporary for global register variable
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mirq-gccboogs at rere dot qmqm dot pl
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: arm-none-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43491