[Bug rtl-optimization/52025] New: caller-saves triggers a frame without need

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52025

             Bug #: 52025
           Summary: caller-saves triggers a frame without need
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Keywords: missed-optimization, ra
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: g...@gcc.gnu.org
                CC: eric.wedding...@atmel.com
            Target: avr


The following code triggers a frame even though there is just one local
variable and there are plenty of call-saved registers.

Apart from the optimization regression, this PR might be related to PR50925.

Notice that the code is a minimized test case from the top-level code of a
real-world embedded application. Therefore, the code runs in an infinite loop.

extern void putch (char);
extern char getch (void);
extern void verify (void);

void main_loop (void)
{
    unsigned char c;

    for (;;)
    {
        getch();

        c = getch();
        verify();
        putch (c);
    }
}

The generated code with -Os -dp -save-temps is

main_loop:
    push r29     ;  36    *pushhi/1    [length = 2]
    push r28
    push __tmp_reg__     ;  40    *addhi3_sp_R_pc2    [length = 1]
    in r28,__SP_L__     ;  41    *movhi_sp/2    [length = 2]
    in r29,__SP_H__
.L2:
    rcall getch     ;  7    call_value_insn/3    [length = 1]
    rcall getch     ;  9    call_value_insn/3    [length = 1]
    std Y+1,r24     ;  34    *movqi/3    [length = 1]
    rcall verify     ;  12    call_insn/3    [length = 1]
    ldd r24,Y+1     ;  35    *movqi/4    [length = 1]
    rcall putch     ;  15    call_insn/3    [length = 1]
    rjmp .L2     ;  44    jump    [length = 1]


The insns triggering the frame are 34 and 35.

-fno-caller-saves resolves the issue and the code then then uses r17 to backup
the value from getch.

main_loop:
    push r17     ;  30    *pushqi/1    [length = 1]
.L2:
    rcall getch     ;  7    call_value_insn/3    [length = 1]
    rcall getch     ;  9    call_value_insn/3    [length = 1]
    mov r17,r24     ;  10    *movqi/1    [length = 1]
    rcall verify     ;  12    call_insn/3    [length = 1]
    mov r24,r17     ;  14    *movqi/1    [length = 1]
    rcall putch     ;  15    call_insn/3    [length = 1]
    rjmp .L2     ;  33    jump    [length = 1]

Target: avr
Configured with: ../../gcc.gnu.org/trunk/configure --target=avr
--prefix=/home/georg/install/gcc-4.7 --enable-languages=c,c++ --disable-nls
--disable-shared --disable-lto --disable-checking --disable-libquadmath
--with-dwarf2
Thread model: single
gcc version 4.7.0 20120102 (experimental) (GCC)