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

[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

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

[rguenth at gcc dot gnu dot org]

-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
           Keywords|                            |missed-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42601

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

[ramana at gcc dot gnu dot org]

------- Comment #1 from ramana at gcc dot gnu dot org  2010-01-05 18:20 -------
Confirmed but definitely an enhancement. 


-- 

ramana at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2010-01-05 18:20:29
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42601

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

[carrot at gcc dot gnu dot org]

------- Comment #2 from carrot at gcc dot gnu dot org  2010-04-10 13:14 -------
Subject: Bug 42601

Author: carrot
Date: Sat Apr 10 13:13:47 2010
New Revision: 158189

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=158189
Log:
        PR target/42601
        * config/arm/arm.c (arm_pic_static_addr): New function.
        (legitimize_pic_address): Call arm_pic_static_addr when it detects
        a static symbol.
        (arm_output_addr_const_extra): Output expression for new pattern.
        * config/arm/arm.md (UNSPEC_SYMBOL_OFFSET): New unspec symbol.


Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/arm/arm.c
    trunk/gcc/config/arm/arm.md


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42601

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

[carrot at google dot com]

------- Comment #3 from carrot at google dot com  2010-04-10 13:17 -------
Fixed by patch http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=158189.


-- 

carrot at google dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42601