[Bug target/40657] New: allocate local variables with fewer instructions

[carrot at google dot com]

Compile following code with options -Os -mthumb -march=armv5te

extern void bar(int*);
int foo()
{
  int x;
  bar(&x);
  return x;
}

Gcc generates:

        push    {lr}
        sub     sp, sp, #12
        add     r0, sp, #4
        bl      bar
        ldr     r0, [sp, #4]
        add     sp, sp, #12
        @ sp needed for prologue
        pop     {pc}

A better code sequence could be:

        push    {r1-r3,lr}
        add     r0, sp, #4
        bl      bar
        ldr     r0, [sp, #4]
        @ sp needed for prologue
        pop     {r1-r3, pc}

The local variable allocation and deallocation can be merged into the push/pop
instruction, so we can avoid the extra sub/add instructions and reduce two
instructions.


-- 
           Summary: allocate local variables with fewer instructions
           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=40657