[Bug target/11824] New: [ARM] Parameter passing via stack could be improved

[alga at rgai dot hu]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11824

           Summary: [ARM] Parameter passing via stack could be improved
           Product: gcc
           Version: 3.4
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: alga at rgai dot hu
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: arm-unknown-elf

For each function that uses the stack for parameter passing GCC generally
first stores lr and decrements sp, and before returning it increments
sp and loads lr into pc.
If the number of the parameters is not too much, then GCC could perform the
parameter passing in a tricky way (as far as code size is concerned),
i.e. save some arbitrary registers with lr on the stack
(so the sp is modified implicitly).

--- c example ---
// arm-elf-gcc -S -g0 -Os -o param-stack.s param-stack.c
void func (int* a, int* b);
void foo ()
{
   int a=6, b=7;
   func(&a, &b);
}

--- asm code ---
foo:
 mov ip, sp
 stmfd sp!, {fp, ip, lr, pc} <- OLD
 mov r3, #6
 sub fp, ip, #4
 sub sp, sp, #8 <- OLD
 sub r0, fp, #16
 str r3, [fp, #-16]
 sub r1, fp, #20
 add r3, r3, #1
 str r3, [fp, #-20]
 bl func
 ldmea fp, {fp, sp, pc}

--- possible solution ---
foo:
 mov ip, sp
 stmfd sp!, {r1, r2, fp, ip, lr, pc} <-NEW
 mov r3, #6
 sub fp, ip, #4
 sub r0, fp, #16
 str r3, [fp, #-16]
 sub r1, fp, #20
 add r3, r3, #1
 str r3, [fp, #-20]
 bl func
 ldmea fp, {fp, sp, pc}