[Bug target/42601] New: Simplify code to address function static variables with option -fpic

[carrot at google dot com]

Compile following code with options -mthumb -fpic -Os,

int foo(int j)
{
  static int i;
  int t = i;
  i = j;
  return t;
}

GCC generates:

foo:
        ldr     r3, .L2       // A
        ldr     r2, .L2+4     // B
.LPIC0:
        add     r3, pc        // C
        add     r3, r3, r2    // D
        ldr     r2, [r3]
        @ sp needed for prologue
        str     r0, [r3]
        mov     r0, r2
        bx      lr
.L3:
        .align  2
.L2:
        .word   _GLOBAL_OFFSET_TABLE_-(.LPIC0+4)
        .word   .LANCHOR0(GOTOFF)

".word   _GLOBAL_OFFSET_TABLE_-(.LPIC0+4)" is the GOT table address relative to
instruction C, ".word   .LANCHOR0(GOTOFF)" is offset of symbol .LANCHOR0
relative to GOT table. So firstly instruction AC computes the GOT table
address, then instruciton BD add the offset of variable i relative to GOT
table. Actually it is not necessary to compute the GOT address. The better code
can be:

foo:
        ldr     r3, .L2     // E
.LPIC0:
        add     r3, pc      // F
        ldr     r2, [r3]
        @ sp needed for prologue
        str     r0, [r3]
        mov     r0, r2
        bx      lr
.L3:
        .align  2
.L2:
        .word   .LANCHOR0-(.LPIC0+4)     //offset of i relative to instruction
F

Now it has two less instructions, 1 less memory access and smaller constant
pool.


-- 
           Summary: Simplify code to address function static variables with
                    option -fpic
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        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=42601