https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113927
Bug ID: 113927
Summary: [avr-tiny] Sets up a stack-frame even for trivial code
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: gjl at gcc dot gnu.org
Target Milestone: ---
Code like
char func (char c)
{
return c;
}
compiles as expected to
func:
/* prologue: function */
/* frame size = 0 */
/* stack size = 0 */
.L__stack_usage = 0
/* epilogue start */
ret
with avr-gcc -S -Os -mmcu=attiny26 -da , but for attiny40 (Reduced Tiny with
16 GPRs only) the result is:
func:
push r28
push r29
push __tmp_reg__
in r28,__SP_L__
in r29,__SP_H__
/* prologue: function */
/* frame size = 1 */
/* stack size = 3 */
.L__stack_usage = 3
/* epilogue start */
pop __tmp_reg__
pop r29
pop r28
ret
In .asmcons, i.e. just prior to register allocation, the code reads:
(insn 13 4 2 2 (set (reg:QI 46)
(reg:QI 24 r24 [ c ])) "main.c":2:1 86 {movqi_insn_split}
(expr_list:REG_DEAD (reg:QI 24 r24 [ c ])
(nil)))
(insn 2 13 3 2 (set (reg/v:QI 44 [ c ])
(reg:QI 46)) "main.c":2:1 86 {movqi_insn_split}
(expr_list:REG_DEAD (reg:QI 46)
(nil)))
(note 3 2 10 2 NOTE_INSN_FUNCTION_BEG)
(insn 10 3 11 2 (set (reg/i:QI 24 r24)
(reg/v:QI 44 [ c ])) "main.c":4:1 86 {movqi_insn_split}
(expr_list:REG_DEAD (reg/v:QI 44 [ c ])
(nil)))
(insn 11 10 0 2 (use (reg/i:QI 24 r24)) "main.c":4:1 -1
(nil))
so everything is fine and this PR is not a dup of PR110093. According to
Vladimir Makarov, PR110093 is because DFA cannot handle subregs, but the RTL
code above does not have subregs. What's the case is that IRA has very high
register costs, for example in .ira:
Pass 0 for finding pseudo/allocno costs
a1 (r46,l0) best NO_REGS, allocno NO_REGS
a0 (r44,l0) best NO_REGS, allocno NO_REGS
a0(r44,l0) costs: POINTER_X_REGS:65535000 POINTER_Y_REGS:65535000
POINTER_Z_REGS:65535000 BASE_POINTER_REGS:65535000 POINTER_REGS:65535000
SIMPLE_LD_REGS:65535000 GENERAL_REGS:65535000 MEM:3000
whereas the .ira for attiny26 (ordinary core with 32 GPRs):
Pass 0 for finding pseudo/allocno costs
a0 (r46,l0) best GENERAL_REGS, allocno GENERAL_REGS
a0(r46,l0) costs: POINTER_X_REGS:4000 POINTER_Y_REGS:4000 POINTER_Z_REGS:4000
BASE_POINTER_REGS:4000 POINTER_REGS:4000 ADDW_REGS:4000 SIMPLE_LD_REGS:4000
LD_REGS:4000 NO_LD_REGS:4000 GENERAL_REGS:4000 MEM:4000
../../source/gcc-master/configure --target=avr --disable-nls --with-dwarf2
--with-gnu-as --with-gnu-ld --disable-shared --enable-languages=c,c++
