https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109910
Bug ID: 109910
Summary: GCC prologue/epilogue saves/restores callee-saved
registers that are never changed
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: gjl at gcc dot gnu.org
Target Milestone: ---
Created attachment 55120
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55120&action=edit
args.c: C test case.
There are targets like AVR that can pass values in callee-saved registers.
There is no need to save / restore them if a caller never changes them.
Example:
$ avr-gcc args.c -dp -mmcu=atmega128 -S -Os
typedef __UINT64_TYPE__ uint64_t;
void callee_ld (long double, long double);
void callee_ull (uint64_t, uint64_t);
void noop_ld (long double x, long double y)
{
callee_ld (x, y);
}
void noop_ull (uint64_t x, uint64_t y)
{
callee_ull (x, y);
}
y is passed in R10...R17 which are callee-saved:
noop_ld:
push r10 ; 73 [c=4 l=1] pushqi1/0
push r11 ; 74 [c=4 l=1] pushqi1/0
push r12 ; 75 [c=4 l=1] pushqi1/0
push r13 ; 76 [c=4 l=1] pushqi1/0
push r14 ; 77 [c=4 l=1] pushqi1/0
push r15 ; 78 [c=4 l=1] pushqi1/0
push r16 ; 79 [c=4 l=1] pushqi1/0
push r17 ; 80 [c=4 l=1] pushqi1/0
call callee_ld ; 39 [c=0 l=2] call_insn/1
pop r17 ; 83 [c=4 l=1] popqi
pop r16 ; 84 [c=4 l=1] popqi
pop r15 ; 85 [c=4 l=1] popqi
pop r14 ; 86 [c=4 l=1] popqi
pop r13 ; 87 [c=4 l=1] popqi
pop r12 ; 88 [c=4 l=1] popqi
pop r11 ; 89 [c=4 l=1] popqi
pop r10 ; 90 [c=4 l=1] popqi
ret ; 91 [c=0 l=1] return_from_epilogue
There is no need to save R10..R17 because they are never changed. If any of
these regs is changed by callee_ld, that function will take care of it because
these regs are callee-saved.
No need to mention that this adds considerably to code size, compute and stack
usage.
The AVR backend uses df_regs_ever_live_p to determine whether a register must
be saved / restored in lack of a better alternative like non-existing
df_regs_ever_changed_p.
Configured with: ../../source/gcc-master/configure --target=avr --disable-nls
--with-dwarf2 --with-gnu-as --with-gnu-ld --disable-shared
--enable-languages=c,c++
gcc version 14.0.0 20230518 (experimental) (GCC)
